The SQIID User's Manual

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1. Introduction

The NOAO SQIID Infrared Camera (Simultaneous Quad Infrared Imaging Device) produces simultaneous images of the same field in the J, H, K, and narrowband L filters, using individual 512X512 quadrants of ALADDIN InSb arrays and is designed for use at the f/15 Cassegrain foci of the KPNO 2.1-m and 4-m telescopes. The observations are generally background (photon statistics) limited. The designated array for each channel is selected for characteristics (read noise, settling time, and dark signal) appropriate to background limited operation under actual observing conditions for its single filter. SQIID, which serves as its own acquisition camera, is a good match to "point and shoot" observing at the 2.1-m without a telescope operator. The filters are fixed in place and dark slide and window covers are the only moving parts. SQIID employs closed cycle refrigeration instead of liquid cryogens and in its prior configuration operated flawlessly for periods as long as 40 days, providing an unparalleled degree of system stability. Typical observing programs include:

Each detector is a Raytheon Infrared Operations (nee SBRC) ALADDIN 1024X1024 indium antimonide (InSb) hybrid focal plane array with 27 micron pixels (90% geometric and 100% optical fill factors) produced under contract to the ALADDIN Consortium. The ALADDIN Consortium, consisting of NOAO/KPNO and the US Naval Observatory, Flagstaff, under the engineering guidance of Al Fowler, has designed, developed, characterized and optimized the performance of the ALADDIN array for the wider community. Since each SQIID channel illuminates a single 512X512 quadrant, devices need only one otherwise excellent operable quadrant to be acceptable. The optics for each channel are independently optimized with a relatively narrow bandpass (apart from the entrance window, which is shared in common), permitting high efficiency AR coatings to deliver high instrumental throughput. The four separate channels are co-focused and co-aligned with a minimum overlap region of roughly 500X500 pixels common to all channels and are physically edge-masked to stop stray light from entering (or leaving) the readout. The detectors are of sufficiently high quality and uniformity that the dark/sky subtracted raw data are useful in assessing data quality in near real-time.

Detector area, stability, uniformity, quantum efficiency, low read noise and dark signal combine to make SQIID the system of choice for any observation which requires multi-color (JHK) imaging over large areas of the sky, quick look at transient targets, and for deep observations of selected regions. IR observations of necessity consist of sequences of frames with interspersed telescope motions. Each set includes a subset of small telescope motions (dithering) to dodge bad pixels and provide better image sampling and may require equal time spent off target (especially when observing extended sources and/or crowded fields). Typically the minimum time spent on taking a complete set of frames at a given field (within detector FOV) can be on the order of 10 minutes or more. With a single channel imager, the same set of pointings needs to be repeated for each filter. Since variations in observing conditions - seeing, airmass, sky transparency, and atmospheric background - ultimately limit the cohesiveness of a data set, multi-color observations are necessarily limited in depth and/or areal extent by the mechanics of taking the observations and the systematic effects of combining them into a single coherent data set. The advantage of SQIID is self-evident. During the time necessary to take the observations at the most time consuming wavelength, one in effect gets the other channels for free under the same observing conditions. Systematic effects are limited and determinate. This is why SQIID competes favorably with larger format arrays for multi-color applications. In addition, wide field L band imaging is possible (albeit with restricted bandpass) with sufficient sensitivity to detect and accurately locate red sources in the field. This feature is particularly important for detecting and indentifying for further study sources with IR excess, highly reddened sources, and intrinsically cold sources near the galactic plane where the star formation process is still active.

Details of the prior SQIID system are discussed by Ellis et al. 1992 ("The Simultaneous Quad-color Infrared Imaging Device (SQIID): A Leap Forward in Infrared Cameras for Astronomy" by T. Ellis, R. Drake, A. M. Fowler, I. Gatley, J. Heim, R. Luce, K. M. Merrill, R. Probst, & N. Buchholz, 1992, in Proc. SPIE, 1765, pp. 94-106.) and the ALADDIN detectors are discussed by Fowler et al. 1996 ("ALADDIN, The 1024x1024 InSb Array: Design, Description, and Results", by A. M. Fowler, Ian Gatley, P. McIntyre, F. J. Vrba, & A. Hoffman, 1996, in Proc. SPIE,2816, pp 150-160).

2. Instrument Description

Optical Description

The SQIID optical system, which is matched to f/15, has a demagnification of 4. The pupil masks are fixed (i.e., not selectable), but a reasonable compromise for the 2.1-m and the 4-m telescopes has been implemented (n.b., by design, the KPNO IR secondaries handle the central obscuration without requiring a separate cold stop within the pupil mask.) According to Charles Harmer (who has the current design available), the worst case blur circle for SQIID is about 75 microns with 90% encircled energy. The optical layout of SQIID is shown in Fig. 1. The f/15 telescope focal plane is located inside the entrance window near the top of the instrument. A series of dichroics and flat mirrors separates the incoming beam into four separate wavelength channels, each with its own camera (Lyot stop, filter, optics) and focal plane array. Dichroic #1 passes L and reflects JHK. Dichroic #2 passes H and reflects JK. Dichroic #3 passes J and reflects K. The telescope exit pupil is imaged at the Lyot stops which are sized to the exit pupil image diameter to serve as an optical cold stop. Standard astronomical passband filters located at the Lyot stops restrict the range in wavelength passed to the array.

The system of dichroics is matched to natural atmospheric windows, adequately spaced in wavelength so that their inband transmission and outband reflectivity is very high. Since each camera is designed for optimum performance over a limited wavelength range, the AR coatings provide unusally high transmission. The opportunity for optimizing each channel for operation over a restricted wavelength compensates for transmission losses within the dichroic system and has yielded high throughput in all channels, comparable to that of a single camera system.

Fig. 1 - Optical layout of SQIID. The elements are identified in the text.

SQIID, which was built in an era where 58X62 was standard and initially deployed (1990-1995) with 256X256 Platinum Silicide arrays, was designed to smoothly accommodate larger devices. The optics was designed to illuminate a 512X512 array of 25 micron pixels. Since the ALADDIN array has 27 micron pixels, some vignetting is inevitable. In addition, upon seeing the full FOV for the first time, we discovered some unintended vignetting in RA within the instrument that would require a substantial effort to rework. The pixel scale and the unvignetted spatial coverage at the two telescopes are summarized in Table 1.

Table 1. Pixel Scale and FOV
Telescope f/ratio arcsec/pixel # pixels RA X DEC field (arcsec)
2.1-m 15 0.69 440 X 460 304 X 317
4-m 15 0.39 440 X 460 172 X 179


Each channel of SQIID has its own fixed filter. We are unable to accommodate other filters tailored to specific programs. The deployed filter complement as of February, 2002 is summarized in Table 2. Note: Prior to February 2002 the K channel contained a Barr K filter.

Table 2. SQIID Filters
channel vendor ID midpoint (microns) FWHM bandpass (microns) HP short (microns) HP long (microns)
J Barr J 1.267 0.271 1.131 1.402
H Barr H 1.672 0.274 1.535 1.809
K pre 02/2002 Barr K 2.224 0.394 2.027 2.421
K post 02/2002 OCLI Ks 2.225 0.35 1.95 2.30
L Barr PAH 3.299 0.074 3.262 3.336

J filter data are estimated from the manufacturer's warm tracing by applying 1.63% shortward shift. Other data are from manufacturer's 77K tracings. Dichroic transparency has not been applied.

The geometric distortion is comfortably small within the unvignetted area, so that the data from the different channels can be brought to a common basis using a superposition of linear transforms (position shift, rotation and magnification) and a modest amount of pincushion/barrel distortion. (Note: J has pincushion distortion and HK have nearly identical barrel distortion.) The geometric distortion appears to be radially symmetric and is well represented by the addition of a cubic term. This simplifies the data reduction enormously, as spatial registration at any channel can be directly translated into registration at all channels.

Since the individual channels of SQIID illumine a single quadrant of a four quadrant array (which is has to be physically mounted within a small volume so as to not interfere with its neighbors) and the best quadrant of each device is selected, the relative orientation on the sky on the array is channel specific. From the point of view of the array, which is read from the outside corner (see Fig. 13, rows are oriented along the horizontal and columns along the vertical), the sky is "natively" seen oriented as follows:

            J           H           K          L

            W           N           S          E
          S   N       W   E       E   W      N   S
            E           S           N          W

Initially, SQIID data where displayed and saved oriented as shown. As of September 2000, the saver task correctly handles the orientation issue during the saver process. Images saved to disk (and automatically displayed) are properly oriented with North up and East to the left.

Mechanical Description

Located at the telescope focal plane are the imager cryostat, the mechanical interface, and the associated warm electronics in two boxes mounted to the instrument. Fig. 2 shows a side view of the instrument and identifies the important parts. SQIID communicates with a remotely located instrument computer and ultimately with the user in a remote observing room (Fig. 3). Even though the f/15 focal plane at the 4-m is well back from the "nominal" focal plane of the telescope, a re-imaging lens in each guide probe assembly permits them to be used for guiding and precision offsetting.

Fig. 2 - Side View of SQIID

SQIIDs temperature is maintained by a pair of Closed Cycle Cryogenic Coolers that employ pressurized helium gas as a refrigerant. Most of the internal parts, including the dichroics, optics, and filters, are cooled below 70 K with the first stage of the Closed Cycle Cryogenic Coolers. Each array is operated at ~30 K by a thermal strap to the second stage of the Closed Cycle Cryogenic Coolers plus a closed loop heater circuit.

Refer to Fig. 2 to identify the external features of the instrument. SQIID will be cabled upon installation, and should not be uncabled for any reason without contacting KPNO staff first. The only necessary user contacts with the focal plane instrument are the instrument power switch located on the electronics box and the mechanisms for inserting the internal and external dark slides.

3. Command, Communication, and Control

SQIID operates through a distributed computer network that has slowly evolved to the meet changing needs of computer support. Historically SQIID was operated by the user from a SUN workstation in the telescope control room through the WILDFIRE system, a transputer based system that communicates over optical fibers. WILDFIRE supports fast co-adding in place, movie mode, and data transfer directly to the SUN. Since WILDFIRE is tied to Sbus based hardware and a SunOS host system there is little room for growth in the core machine. The system was hard pressed by the upgrade from four 256X256 arrays to four 512X512 arrays during the SQIID upgrade. Movie mode was dropped owing to I/O delays associated with this upgrade.

The current configuration has SQIID remotely operated from a Linux-based Workstation (physically located in the control room) that connects to the SQIID host Sun Workstation (now completely in the computer room) through a VNC server.

The WILDFIRE system uses transputers and transputer links to control and acquire data from SQIID. A transputer is a single-chip microcomputer with its own local memory and communication links, which can operate either by itself or in conjunction with other elements linked to form computing arrays and networks. The WILDFIRE system consists of three main hardware components:

Communications between SQIID and the DSP take place over transputer links implemented on an optical fiber cable. The B016 interconnects the transputer DSP to the SUN SparcStation computer via a VME to SBUS converter within the Heurikon box.

The WILDFIRE user interface on the SUN is implemented within the TCL (tool command language) environment. On startup, one can configure the image save to produce either FITS images or IRAF images (via IMFORT routines) so that they can be manipulated and archived to tape within IRAF. The image data are generated in IEEE 32-bit floating point format. It is important to note that these images are NOT PROTECTED in any way and can be overwritten if the full path names of existing and new images are the same. (Currently the saver task attempts to manage conflicting filenames by appending ".nnn" to the incoming conflicting filename.) The data may be written to Exabyte or DAT tapes on local tape drives or sent via 'ftp' to one's home institution. Depending on the amount of header information, a single FITS file of a 512 X 512 image is about 1.057MB and a single 4 color exposure is 4 images (4.2MB).

Under the present version of WILDFIRE data acquisition:

It is interesting to note that the current arrangement allows SQIID operations to be viewed remotely. However, since hosting multiple VNC connections places an undue burden on the Sun systems, this is not recommended for continuous useage.

A separate Linux based system serves as the telescope control, with a terminal at the LTO station; a hardwire link between the TCS and instrument control computers is used to send TCS commands to the telescope (singly, or within TCL scripts) and to retrieve telescope information for the image header. A schematic depiction of this arrangement is shown in Fig. 3.

Fig. 3 - Schematic configuration of electronics and computers used with SQIID/WILDFIRE

Important Note: The disks within the primary workstations khaki and lapis are designated /data1. At the 2.1-m, WILDFIRE is run on the secondary workstation royal, whose partition is /data2. The disks are cross-mounted so that access to both is possible from either machine. However, such cross-access (e.g., /data1 from royal) is significantly slower than accessing the disk resident in the workstation. Therefore, it is imperative that the partition used for storing data taken by WILDFIRE be /data2 on the 2.1-m telescope! While it is possible to designate /data1 as the WILDFIRE data partition, operation will be much slower and subject to crashes, so don't do it. At the 4-m, one may designate either /data1 or /data2 as the data partition.

WARNING: Do not let the designated data disk fill up. You will lose data! The filenames may appear, but they will have zero length. (The WILDFIRE housekeeping screen reports available space.) See Appendix VI.

4. The InSb Detector Array

The four detectors in SQIID are ALADDIN 1024X1024 hybrid focal plane InSb arrays produced by Raytheon Vision Systems (nee Santa Barbara Research Center). They consist of a photovoltaic InSb detector array mated to a silicon direct readout multiplexer via indium bumps. The readout is a p-channel MOSFET device. The ALADDIN array was cooperatively designed and developed at RIO/SBRC with the ALADDIN Consortium, consisting of NOAO and the US Naval Observatory, Flagstaff.

The device is presently operated in a non-destructive readout mode providing double correlated sampling. A representation of the voltage on a single pixel during an integration and readout is shown in Fig. 4. An address cycle consists of a "reset" to the canonical detector bias voltage, a non-destructive "read", followed by a second "read". During the reset operation, the voltage on each pixel is set to the value VR. When the reset switch is opened, the voltage left on the sense node will differ slightly from VR, due to charge spillback from the reset gate and from "kTC" noise. After a time 'fdly', the voltage on the pixel is sampled nondestructively (i.e., without resetting), yielding V1. After a second time interval, defined as the integration time, the voltage is again sampled, yielding V2. The "signal" is the difference between the two reads. Note that this technique, known as "double correlated sampling" eliminates the effect of the transient following the reset operation. The intervals indicated (not to scale) at the bottom of Fig. 4 represent the time required to carry out each operation on the entire array; thus, on an absolute frame, the time at which a given pixel is reset and read depends on its location in the array.

Fig. 4 Schematic representation of the voltage on a single pixel as a function of time. The intervals labeled "reset" and "readout" suggest the time when these events occur, and are not to scale.

The operating microcode for ALADDIN arrays includes a provision for "multiple correlated sampling" (frequently referred to as Fowler sampling in deferrence to its discovery, Al Fowler of NOAO) in which the "reads" consist of a series of N nondestructive reads coadded to yield the values V1 and V2. This greatly reduces (by approximately N0.5) the array read noise on long, low-background integrations. Since SQIID observations are generally background limited (specifically, they are not detector limited), we only use "Fowler 1" which corresponds to double correlated sampling. At the shortest integrations (within the SQIID environment) selecting "coadds >= 2" also improves S/N (by approximately N0.5).

Table 3 summarizes the device characteristics and measured performance levels (according to standard lab protocol) of the ALADDIN arrays assigned to SQIID. SQIID was designed for background limited performance at low to moderate backgrounds. The ALADDIN array has 27 X 27 micron pixels with 90% geometric fill factor (100% optical fill factor) and a 1024 X 1024 format comprised of 4 electrically independent quadrants. The ALADDIN2/ALADDIN3 readout designation corresponds to improvements in the readout design. Note: Table 3 reflects the array assignments within SQIID as of February 2002. The prior configuration was SCA49484_Q3 at H channel, SCA41375_Q3 at K channel and SCA414107_Q3 at L channel. The bias values listed are effective as of January 2005.

Table 3. SQIID Arrays
Characteristic J H K L Comments
SBRC Indentification (Quadrant) SCA 45986 (Q3) SCA 46888 (Q4) SCA 414375 (Q2) SCA 415412 (Q3) ALADDIN InSb
Response uniformity +/-5% +/-5% +/-5% +/-5% within FOV
Quantum efficiency 95% 95% 95% 85% in band
Bias 700 700 700 900 mv
Full well >200000 >160000 >200000 >300000 electrons
lab read noise 40 45 35 35 electrons rms
lab dark signal @ 300mv bias 0.3 0.4 0.6 0.1 electrons/sec
Conversion gain 10 10 11 11 electrons/ADU
Cosmetics isolated regions isolated pixels reduced QE region isolated pixels primary defects

It is useful to note that "dark current" is a function of applied bias and because both "dark current" and read noise are temperature dependent in opposite senses (below 40K "dark current" decreases with decreasing temperature to a minimum near 30K and read noise increases with decreasing temperature from a minimum near 40K) they cannot both be minimized. Hence the array operating temperature is generally selected to provide an application (background) specific compromise. Further information on the array design and operation may be found in Fowler et al., Proc. SPIE, 2268,340-345 (1994), and in Fowler et al., Proc. SPIE, 2816,150-160 (1996). The multiple correlated sampling technique used for read noise reduction is described in Fowler and Gatley, Ap. J. (Letters), 353, L33 (1990).

Telescope performance is covered in Section 5. The relatively high dark signals listed in Table 4 - which do not compromise performance - are indicative of compromises in the instrument design (not intended for low backgrounds), the need to operate at high bias, and the freedom to deploy specific arrays that might be unsuited to more demanding applications.

5. Observing Run Preparation

The limiting performance of SQIID depends on a number of factors.

System Responsivity

Typical observed fluxes for a 10.0 magnitude star at the 2.1-m telescope are listed in Table 4. By direct measurement the fluxes are a factor of 3.21 higher at the 4-m telescope and the sky background per pixel is essential the same. The conversion gain is a detector-specific 10-11 electrons/ADU. Note: the biases listed are not necessarily the current bias values.

Table 4. SQIID Performance on 2.1-m
channel J H K L comments
10 mag star within aperture 3.25e4 2.90e4 1.78e4 ? ADU/sec
Sky brightness level 98 421 387 ??? ADU/sec/pixel
Sky brightness origin OH airglow, scattered light OH airglow, scattered light OH airglow, thermal emission thermal emission Note: components vary in intensity
Conversion gain 10 10 11 11 electrons/ADU
Bias 600 600 700 800 mv
Full well >200000 >200000 >200000 >250000 electrons
in situ read noise 40 40 35 35 electrons rms
in situ dark signal 21 32 197 26 electrons/sec
Minimum integration time 0.84 0.84 0.84 0.11 seconds

The estimated limiting magnitude for S/N = 3 in 60 sec integration time for a point-source (pt: mag) and a diffuse-source (diff: mag/square_arcsec) under average conditions (temperature = 50F; 3mm PWV; midrange OH background) sans sky subtraction is summarized in Table 5. For the purposes of this calculation, a 10 pixel collecting area on the sky is assumed. In practice, observing limits are dependent on the mode of sky subtraction employed. When mapping an extended region, the number of frames available for producing sky frames significantly exceeds the depth at a given position, virtually eliminating the sky subtraction "penalty". However, for the case where sky frames are exposed for an aggregate time comparable to that of the source frames, sensitivity is reduced by 0.376*mag (2.5*(log(sqrt(2))) from the values listed in Table 5 . Such is often the case for deep integrations of individual fields.

Table 5. SQIID limiting magnitude for S/N = 3 in 60 sec
Channel 2.1-m pt 2.1-m diff 4-m pt 4-m diff
J 19.8 20.2 21.1 21.3
H 18.9 19.7 20.2 20.4
K 18.3 19.2 19.6 19.8
PAH 12.3 13.1 13.5 13.7

Sky Background

There are two predominant sources of sky background, which are essentially independent, both physically and spectrally. The transition between the two occurs at approximately 2.3 microns. Short of this wavelength, the sky background is dominated by emission lines from OH in the upper atmosphere (typically 90 km altitude). The strength of these lines can vary over the course of the night with a timescale on the order of an hour; in addition, upper level winds generate inhomogenieties and overall motion of the airglow. As a result, the intensity of the background emission can vary unpredictably during the night. Beyond 2.3 microns, thermal emission from the telescope optics and sky is the predominant background. This roughly follows a blackbody at ~300K temperature and increases very rapidly with increasing wavelength. To further complicate matters, atmospheric lines (primarily H2O, HDO, CH4, and N2O), which show up as absorption features in spectra, appear as emission features in the sky background, so that the loss of signal and the increase in background are temperature and wavelength dependent. For imaging one generally ignores the finer points and treats the longer wavelength filters like their shorter wavelength brethren when processing data.

One must be prepared for the simple fact that IR observations are subject to a wider range of sensitivity variations linked to changing environmental conditions (OH airglow and temperature) than the optical, where phase of the moon and sky transparency predominate. Table 6 estimates the relative sensitivity for SQIID for three levels of OH airglow.

Table 6. Estimated SQIID performance variation with OH airglow
Airglow J H K L Comments
low +0.25 +0.25 +0.08 +0.00 magnitudes improvement
medium +0.00 +0.00 +0.00 +0.00 magnitudes improvement
high -0.24 -0.20 -0.10 +0.00 magnitudes improvement

Estimates of the relative background and sensitivity variation for SQIID for three different ambient temperatures and 4 values of Precipitable Water Vapor are summarized in Table 7 and Table 8 respectively. At KPNO, 1mm PWV is a winter rarity and 9mm PWV is a Monsoon Season staple. It is useful to remember that the thermal background comes from both the atmosphere (which varies with airmass) and the telescope (which does not vary with airmass) in comparable quantities. Although observations at JHK are onscale under all conditions, at high temperature the L channel can saturate and become unusable. The primary symptom of saturation is an East/West gradient in the L background at the shortest integration times. If one desperately needs to perform L channel observations under higher temperature conditions, insertion of the cold internal polarizer element (it is the third choice in the open/dark/polarizer position controlled by the hand crank on the instrument) drops the background (and the signal!) by a factor of two, with edge vignetting (because it is too small to service the full FOV). Although one can obtain useful data at JHKL within the central field with the polarizer in place, the vignetting near the edge (which is variable, since we do not put a close tolerance on the indication when the polarizer is in position) is problematic. It is also worth remembering that for polarized sources your signal loss will be larger.

Table 7. Estimated SQIID performance variation: ambient temperature
Ambient Temperature J H K L comments
30 F +0.00 +0.00 +0.21 (0.7) +0.35 (0.5) magnitudes improvement (background relative to 50F)
50 F +0.00 +0.00 +0.00 (1.0) +0.00 (1.0) magnitudes improvement (background relative to 50F)
70 F +0.00 +0.00 -0.30 (1.7) -0.31 (1.8) magnitudes improvement (background relative to 50F)

Table 8. Estimated SQIID performance variation: Precipitable Water Vapor
PWV J H K L comments
9mm +0.00 +0.00 -0.02 (1.02) -0.19 (1.12) magnitudes improvement (background relative to 6mm)
6mm +0.00 +0.00 +0.00 (1.00) +0.00 (1.00) magnitudes improvement (background relative to 6mm)
3mm +0.00 +0.00 +0.03 (0.98) +0.26 (0.84) magnitudes improvement (background relative to 6mm)
1mm +0.00 +0.00 +0.06 (0.95) +0.54 (0.67) magnitudes improvement (background relative to 6mm)

System Overheads

To optimize observing efficiency, it is important to keep the two distinctly different system overheads in mind, related to:

It takes time to move the data from the instrument through the distributed hardware out into storage on the Sun Workstation. During this time, critical resources are involved which do not lend themselves to overlap/reuse, so that the start of a new integration must be held off until the pipeline is clear. For an externally realized integration, it currently takes 40 seconds following the last readout to safely deliver the results. Since the actual disk write time is but a fraction of this total, little time is to be gained, for example, by electing to not save undesired channels to disk.

The preferred method for improving observing efficiency involves internal co-addition, since co-addition of JHK frames entails an overhead of only 10 millisec. By co-adding integrations to produce the equivalent of a 1 to 3 minute integration, the effect of data pipeline flow overhead is minimized.

The mode of array operation also has a significant impact on observing efficiency. In particular, it is important to understand the role of minimum JHK integration time in the JHK observing efficiency. SQIID currently employs global reset and double correlated sampling to produce an image: each image is the difference between two reads of the array and the minimum integration time is roughly the time to read the array once. Consequently, at the shortest integration times (approaching the minimum integration time of oreder 1 sec) the observing efficiency declines to 50% and for integration times of order 10 seconds, the JHK observing efficiency approaches 90%.

It is equally important to understand the relationship between JHK integration time and L integration time that affects L observing efficiency. During the course of an integration, SQIID simultaneously resets the JHK arrays, then simultaneously performs a non-destructive read (equivalent to a CCD bias frame) on the JHK arrays, waits roughly an integration time then reads out the JHK arrays again. Between JHK reads, the L band array is read out (in pairs and differenced) as often as will fit between the JHK read pair and the results co-added. Since there is some dead time before running L and since the minimum integration time is roughly the readout time, the maximum time spent integrating at L within a JHKL cycle is less than half the JHK integration time. One could use a different technique to read L (e.g., row reset) to improve efficiency, but we have yet to produce the complex code required to read L differently than JHK and will most likely produce a code employing row reset for all channels. The situation is summarized in Table 9 for the case of the shortest L time:

Table 9. L channel observing efficiency
L time per coadd (sec) JHK time (sec) L coadds total L time (sec) % time (L/JHK)
0.11 1.00 1 0.11 11.0%
0.11 2.00 6 0.66 33.0%
0.11 3.00 10 1.10 36.6%
0.11 5.00 19 2.09 41.8%
0.11 10.00 42 4.62 46.2%
0.11 15.00 65 7.15 47.7%

Channel Specific Characteristics

Channel specific characteristics, such as scans of the filters, geometric distortion, and representative bad pixel masks can be found at Supplement 1: Channel Specific Characteristics.

Other Preparations

Object Coordinates for any epoch can be entered into the telescope computer for use during the run. Although this task could be done by the telescope operator during the course of the night, lengthy observing lists are best entered by electronic submission (see below). These may include objects, standards, offset and guide stars, etc. Acquisition of optically faint or invisible objects might require initial acquisition and coordinate updating on a nearby bright star, so advance selection of these offset stars can save considerable time while observing. SQIID does not use a guider.

Conscientious observers may send coordinate lists via email (two weeks or more before the run) to Files should be ASCII text, no longer than 2000 lines. Start the file with your name, a cache name, telescope, and dates of the observing run. Coordinates will be checked for format, loaded into the appropriate telescope computer, and acknowledgement will be sent. Each object should be one line of text. The format is object name, RA (starting column 16 or greater, delimited by first blank after col 15; hours, minutes, seconds), DEC (degrees, minutes, seconds), and epoch. Each field should be separated by one or more spaces (NO TABS); the delimiter in the RA and DEC fields may be spaces or colons. Example:

Further details may be found in the June 1992 NOAO Newsletter or the new Observers Handbook.

Standards are a subject of continuing discussion, and probably will remain so for some time. For the purposes of determining and removing the effects of telluric absorption and throughput in the instrument, it is desirable to observe a calibration star as near as possible to the object in both space and time. Owing to its high sensitivity and relative coarse pixel scale, SQIID must be calibrated using standard stars fainter than JHK=9mag. Recent compendia of faint standards are extremely useful in this regard:

Visitors should arrive on the mountain at least by early afternoon of the first night. This will allow time to become familiar with the instrument, create and test observing parameter sets, and enter object coordinates into a cache. First-time users of SQIID may wish to arrive a day early and spend some time in the evening looking over the shoulder of the previous observer, with his/her prior permission.

6. The IR Instrument Control System -- WILDFIRE

Note: This contains a SQIID-specific synopsis of the WILDFIRE manual written by Nick Buchholz.

Interim notes for operation at the 2.1-m telescope

As of December 2006, the SQIID operating environment at the 2.1-m telescope has changed with the incorporation of a Linux-based workstation as the console for operations in the control room:

Initializing the Environment with OBSINIT

The optical CCD (ICE) and infrared (WILDFIRE) environments are both operated from the same account on the 2.1-m (2meter) and 4-m (4meter) telescopes. The all-important obsinit command performs a number of functions relevant to this operating procedure.

First Night of SQIID Block

On the first night of an IR block, the ICE environment might still be active (the presence of the "CCD Acquisition" and "CCD Reduction" windows will verify this). It will be necessary to run obsinit as detailed below to change to the WILDFIRE environment; since the hardware may be in an unknown state, it is also recommended to run through a complete hardware initialization on the first night of an IR block as part of the obsinit process. This will involve rebooting the observer's SUN workstation with the DSP (in the computer room) powered on and the SQIID instrument power off.

The "First Night" procedure is detailed within Appendix IX. Installation Issues. Since this procedure differs both in complexity and detail from the situation normally comforting the observer, we will not recount it here.

New Observer

The following assumes that the change from ICE to Wildfire has already occurred and that Wildfire is already running properly.

On subsequent SQIID runs, obsinit is run only to enter the new observer and proposal ID information. It is NOT necessary to power down SQIID or reboot the computer. After typing exit in the Instrument Control window, logging out of all IRAF processes and running obsinit, simply exit OpenWindows from the desktop menu and log back in when the login window appears.

At the 2.1meter telescope, PROPID and OBSERVER can be modified without running obsinit. One can either run the newobserver command within the Wildfire Instrument Window or one can change the appropriate environmental variables within the ".cshrc2" file used by Royal on /data1/2meter/.

Note: The SQIID instrument power supply is located on the instrument itself. The (rocker) switch is on the right (South) side (near the top by the power cord) of the electronics box mounted on SQIID.

After a few seconds, OpenWindows will automatically load and present the login window shown below:

Login as [telescope] (where telescope is "2meter" or "4meter" as appropriate) with the current password posted on the workstation terminal. The WILDFIRE system will then load automatically, resulting in a terminal screen layout approximately like Fig. 5 below; the dashed window labeled Instrument Status will appear in the approximate position shown only after the instrument microcode has been loaded.

Normal WILDFIRE Startup

The Windows

Once the environment has been set to WILDFIRE by obsinit, it will remain in that state, even if it is necessary to reboot the instrument computer for any reason. There should be no reason to execute obsinit more than once during a run. If a reboot is required, the login procedure in the window displayed above will automatically bring up the WILDFIRE windows.

Fig. 5 - Windows layout after initiation of WILDFIRE-left screen

A brief description of the windows follows:

Fig. 5 - Windows layout after initiation of WILDFIRE-left screen

Bringing up WILDFIRE

There are three basic steps in the complete startup of WILDFIRE: hardware initialization; starting WILDFIRE; instrument initialization. The procedure below will go through all three steps, as would be necessary on the first night the instrument is on the telescope.

Hardware Initialization

This procedure establishes the link between the DSP box and the computer, by rebooting the observer's SUN workstation with the SQIID power off. The obsinit procedure for the first night of an SQIID block (described above) includes these steps.

HISTORIC NOTE: The startup script for WILDFIRE was simplified significantly in 1999. The microcode will be loaded automatically and the bias for SQIID set to the default values. The dialog during a typical WILDFIRE/SQIID initialization is recorded in Appendix VIII.


At this point, the windows should be present as in Fig. 5. Go to the Instrument Control Window and enter:


This will lead you through an interactive startup procedure. READ THE QUESTIONS CAREFULLY; simply entering [cr] will return the default, which may not be appropriate. For the full startup, the replies are:

At this point, the transputer nodes will bootstrap, and four .tld files will load. Eventually (when the startup script automatically executes "setup sqiid") you will see messages regarding the downloading of the microcode, setting of 4 values of VddCl1 (-1.3), VddCl2 (-3.5), VggCl1 (-4.9), VggCl2 (-2.8), and Vset (-1.8). When this is completed, the final message will appear:

You will see messages reporting 4 biases being set, followed by:

If you want to use a different parameter file than the default "sqiid" parameter file that was executed by the startup script, you can enter "puse parameter_filename" at this time.

SQIID is now ready for operation.


If difficulties are encountered in startup, entering trouble in any of the windows (except the Instrument Control) will open a troubleshooting diagnostic, listing symptoms and possible solutions. However, most problems occur during the initial installation, and are often hardware related. The most common problems are listed below:

Note: Detailed instructions for recovering WILDFIRE operation are contained in Appendix III.

red LED(s) in DCU Bad fiber connection. With the instrument power on, the green LED in the DCU should be on, and the two red LEDs off. If either or both red LEDs is lit, there is a fiber problem which must be repaired. A similar set of LEDs in the DSP box can diagnose fiber problems at that end.
halt after "Configuring C004" Bad fiber optic connection (see above). Even if red LEDs are off, one or more fibers may have poor throughput, which must be measured. Power supplies may be connected improperly. Check that the analog connector goes to "CCD Power" and not "PS-10 Power" on the telescope.
halt after "bootstrapping node 100" Bad fiber optic connection (see above). C004 may not be configured and a full startup may be necessary (DSP cycle, reboot, startwf).
"error #16 (cannot open link)" System stuck in funny state. Full startup may be required. If that does not help, check for proper power connection and fiber throughput.
"cannot read telescope status" Link to TCP computer is down. This is usually solved by rebooting the TCP computer. WILDFIRE will still work, but cannot move telescope or retrieve telescope status information for header.

In addition, comments, suggestions, and descriptions of persistent problems should be emailed to wfire@lemming, which has been set up as an equivalent to service for WILDFIRE instrumentation.

Parameter Sets

"parameter sets" are used to control the attributes of data acquisition. A listing of the parameters is given below. Because the data are saved directly as IRAF images, note that parameters include not only observation-specific items such as integration time, but archiving items such as the IRAF filename and the header and pixel directories.

Observing Parameters
coadds *
integration_time *
IRAF header title
number of coadded integrations per image
number of low noise reads (1 for SQIID)
number of pictures per observation
integration time (seconds)
IRAF filename
image header directory
pixel file directory
process mode [stare, sep, hphot]
picture index
channels to display (j, h, k, or l for SQIID)
RA of object #
DEC of object #
epoch of object #
observation offset
type of observation [object,dark,flat..]
airmass of object #
filename of image list
channels to be saved to disk ([j h k l] or subset for SQIID)
channels to be archived ([j h k l] or subset for SQIID)

* WARNING: Use 'set-time' to set coadds and integration time rather than responding to the individual 'coadds' and 'integration_time' queries from 'ask'.

In general, the parameters fall into three categories:

The command ped will open an editing session on the current parameter set, listing each parameter in turn and prompting for new entry ([cr] returns the present value). At the beginning of a run, one should execute ped and set up those parameters falling into categories 2 and 3 above. NOTE: One cannot specify a non-existent header or pixel directory in ped; it is necessary to go to the IRAF XTERM window and create those directories first! Since it is cumbersome to go through the entire parameter list for each observation, there is a command eask, which runs through the entire parameter list, permitting the observer to specify which parameters should be queried at the beginning of each observation. Entering "la" for a parameter selects it for the "observation menu"; entering "l" excludes it. NOTE: The "up arrow" key may be used to back up through the ped list if one wishes to change a previously entered parameter.

When this is complete, save the parameter set with the command psave [filename]. This will save both the edited parameter set and the menu selected by eask in the file '[filename].par'. Should the system crash, this information may be retrieved by the command puse [filename]. Should major changes be made to the parameter file, such as change of header or pixel directory (say on another night of the run), it is a good idea to psave the updated file so it, and not the previous version, will be recovered by puse.

Observing Words

The basic observation is initiated by the command observe. The system will print on the screen, one at time, those parameters selected by eask, and the current value [], prompting for entry of a new value or [cr], which will enter the current value. The command go will begin an observation, but will use the current values for the parameters (except the picture index, which will be automatically incremented). The command movie will begin a loop consisting of an observation (using the current parameters!) and a display; this may be terminated with end at any time. The observation in progress will be completed and displayed. Movie observations are stored on disk! This is unfortunately necessary to prevent orphaned pixel files from filling up the disk. A recommended procedure is to include the 'filename' parameter in the ask menu and change to a dummy filename at the beginning of a movie. When returning to data taking, one may reset the filename to that used for the data. If one wishes to retain continuity in the index number, it is also necessary to reset 'nextpic' to the value before the movie observations. Keep good logs!!

The ask command will cycle through the selected parameters, prompting for changes, just as with obs, but will NOT begin an observation. This command is useful for checking parameters, and is essential before executing movie, which will use the parameters for the previous observation, even if it were 600s in length. The combination of ask and go is a perhaps preferable alternative to observe.

One may abort an observation (such as an unintentional 600s movie) by entering abort in the Instrument Control window; the system should respond by acknowledging the abort and the observation should terminate gracefully in a few seconds. This can sometimes turn off the display and save operations, so it is advisable to re-enter save j h k l (or whichever subset you have been using) and display k (or whichever channel you have been using after an abort.

TCL Dithering Scripts and IRAF Tasks

The user interface is written in the Tool Command Language (tcl), which is well-suited to the construction of scripts for data taking. Scripts are a powerful tool for executing a sequence of tcl commands, including telescope motions, instrument motor commands, and observations, as a single executable program. Even for those who are not veteran programmers (most of us), simple scripts are fairly easy to construct. Scripts are highly recommended for spatial sampling (dithering) and linearity calibrations. The best recipe for starting out is to copy an existing script to a new file and then edit that file as desired. The first line of the script file contains the basename of the script file ("proc "), and must be edited to reflect the new name of a script created in this manner. Before the initial use of a script (or after a system restart), it must be identified as an executable in the Instrument Control window, using the full path name of the file; e.g.,

source  /data2/2meter/tclSamples/[scriptname].tcl

To execute the script, enter the basename [scriptname] as a command in the Instrument Control window. A sample script is given in Appendix IV.

Scripts may be found in directory "tclSamples" under the "[telescope]" directory, as in the path above, and also in /usr/wfire/tcl. This latter path is the system response to query pwd in the Instrument Control window. When creating a custom script, please copy a system script into an observer directory and then rename and modify it, to avoid confusion.

Scripts copied into the user home script directory (/data2/2meter/wfire at the 2.1-m and /data2/4meter/wfire at the 4-m) and sourced can be marked for automatic inclusion in subsequent invocations of "startwf" by entering "mkIndx".

For the more sophisticated (or daring) observer, a TCL manual is available. WILDFIRE presently uses TCL version 6.7 and properly written code should run with no special limitations. Please note we will not debug or otherwise support user code, nor will user supplied TCL routines be saved within WILDFIRE from one observing run to the next.

The following WILDFIRE default scripts are useful for various observing programs, and as templates for user-constructed modification. They are initiated by entering the script name as a command, and going through a series of interactive queries to set internal parameters. Alternatively, several have command line versions for faster use. These are default scripts which do not require sourcing.

Refer to the Appendices for listings of WILDFIRE and SQIID commands (Appendix II) and troubleshooting procedures (Appendix III).


Michael Merrill's provisional IRAF script tmove may be used for centering stars on the array, using an image displayed in the ximtool window. Because this is not yet a standard IRAF task, it will probably have to be manually installed for an observing run.


Michael Merrill's provisional IRAF script idisp may be used to display sky-subtracted images in the ximtool window. Because this is not yet a standard IRAF task, it will probably have to be manually installed for an observing run. Note: type upsqiid at the cl prompt if idisp is undefined.

7. Observing Practices

The installation of the instrument and cables will be handled before the beginning of the run by the mountain technical staff and are not of concern to the user. The SQIID Reference Manual provides coverage of the details of installation and setup for those who are interested. SQIID remains on the telescope (with power on, under normal circumstances) for the entire observing run.

Getting Started

After SQIID is installed on the telescope, go through the WILDFIRE startup procedure outlined previously. Once the system is operational and the detector activated, check the detector and temperature status with status s and compare with the nominal values below:

status s

                        SQIID Base Status Display        18:11:29

                      H detector    K detector    J detector    L detector
Integ time (secs) =      0.855         0.855         0.855         0.185
CoAdds            =      1             1             1             1x1
Lnrs              =      1             1             1              1
Number of Pics    =    1

Detector Temp     =      29.81K        30.68K        29.81K        30.68K    

Det Heat Pwr (mw) =     129.39         56.15        146.48         75.68     

Observation Settings
Mode              =   stare stare stare stare
File name  = "test%03d%s" Index =  95               Space Available
Header Dir = "/data2/2meter"                    3046308kb
Pixel Dir  = "/data2/2meter/pixels"                     3046308kb

Housekeeping information is reported via status screens. Additional status screens for SQIID are:

status t

                        SQIID Temperature Display

                  H detector    K detector    J detector    L detector
Detector Temp     =     29.81K        30.68K        29.81K        30.68K    

Det Heat Pwr (mw) =    136.72         61.03        153.81         78.12     

Bench 0 (North)   =    40.69K       Bench 1 (South)    =    39.61K 
Wheel             =    46.29K       Sieve Plate        =    18.15K 
Cold Hd A 1st St  =    32.17K       Cold Hd B 1st St   =    32.34K 
Cold Hd A 2nd St  =    11.53K       Cold Hd B 2nd St   =    13.47K 

status v

                SQIID PRCD Voltage Status Display
                   H detector    K detector    J detector    L detector
VSet              =    -1.783        -1.790        -1.779        -1.803
ISet              =     0.266         0.297         0.257         0.322
VDet              =    -2.897        -2.815        -2.926        -2.701
VDesUR            =    -5.579        -5.589        -5.583        -5.587
BOK               =    -2.896        -2.815        -2.925        -2.698
VddUC             =    -3.475        -3.497        -3.488        -3.489
VddOut            =    -0.986        -0.992        -0.989        -0.991
VddCl1            =    -1.315        -1.319        -1.303        -1.303
VddCl2            =    -3.545        -3.560        -3.517        -3.514
VggCl1            =    -4.867        -4.886        -4.861        -4.908
VggCl2            =    -2.773        -2.786        -2.769        -2.791
Vp                =    -0.489        -0.494        -0.494        -0.493
VnRow             =    -6.207        -6.186        -6.179        -6.177
VnCol             =    -3.988        -3.993        -3.987        -3.987
VDesLr            =    -0.494        -0.489        -0.497        -0.494
VRowOn            =    -5.985        -5.983        -5.985        -6.002
VRowOff           =    -0.685        -0.694        -0.695        -0.688
VRstOn            =    -5.971        -5.989        -5.973        -5.970
VRstOff           =    -2.991        -2.991        -2.989        -2.991

status 3

                SQIID Power supply Voltage Display

Gnd Ref H/K Box   =    -0.097        Gnd Ref J/L Box   =    -0.105
Vcc1 H/K Box      =     4.740        Vcc1 J/L Box      =     4.699
Vcc2 H/K Box      =     4.750        Vcc2 J/L Box      =     4.706

PRCD Card        H detector    K detector    J detector    L detector
+5v Sup           =     4.951         4.951         4.945         4.944
+15v Sup          =    14.866        14.906        14.893        14.922
-15v Sup          =   -15.100       -15.198       -15.078       -15.250
+7v VRef          =     6.914         6.921         6.911         6.906
-7v VRef          =    -6.909        -6.914        -6.897        -6.897

PA Cards         H detector    K detector    J detector    L detector
+5v Sup           =     4.952         4.950         4.944         4.947
+15v Sup          =    14.968        14.923        14.960        14.804
-15v Sup          =   -15.188       -15.062       -15.295       -15.131
+5v VRef          =     5.002         5.001         4.992         4.992
-5v VRef          =    -4.986        -4.986        -4.980        -4.978
                   H detector    K detector    J detector    L detector
VOff1             =     0.675         0.675         0.669         0.669
VOff2             =     0.678         0.677         0.668         0.672



SQIID acts as its own acquisition camera. Open up and acquire a star in SQIID. Stars with K magnitude fainter than 6 are OK for initial acquisition, but stars fainter than K = 9 are necessary to avoid saturation for final focus adjustments. [Note: When SQIID is first installed, it is best to start with unambiguous stars to verify initial telescope pointing. Stars brighter than K = 3 are best for this purpose.] Use individual 'go' exposures (because of the 40 second delay, movie can be confusing), to get the star within the central region of the array. Choose a wavelength channel for determining focus (preferably K which suffers least from seeing) and stick with it through the run to avoid confusion. The four SQIID channels are near parfocal, but differences in sensitivity to seeing (which improves with increasing wavelength) and slight differences in image quality away from focus can be confusing if you switch back and forth. Once the star is found, move the telescope until it is centered and focus the telescope, resetting display limits and integration time as necessary, until a tight image is obtained. For optimizing focus, it is best to obtain single images, using observe or go, and analyze the image quality with the IRAF task imexam using the 'r' command. Remember that it may be necessary to relocate the beam when moving to a new object.

An example of excellent focus (K channel, 1.0 sec) can be seen in Figure 6. Note that the star appears to be positioned at the center of a pixel. Since the geometric quality of the optics on the order of 80% flux within two pixels radius and the profile fitting algoritms stall near 1.5 pixels, this is as good as it gets. FWHM of 1.8-2.0 is more typical. (The rightmost three values at the bottom of the profile image are the FWM from different profile fitting algorithms.) Naturally, longer exposures will have somewhat broader profiles.

Fig. 6 - Radial image profile for excellent short exposure focus: 1 sec at K

Stars which are too bright will tend to be either flat topped or possibly even contain a central void as seen in Figure 7.

Fig. 7 - Radial image profile of a star that is too bright

NOTE: It is likely that the single pixel events (that occur at the roughly once per second and are visible even in dark frames) are in response to alpha particles from the anti-reflection coating of the last lens surface. We have since discovered that thorium fluoride is the coating of choice for producing durable wide bandpass coatings. The magnitude of these single pixel events is typically 3000-5000 ADU with of order 5% leakage into the four nearest neighbors. Figure 8 is the radial image profile for a typical single pixel event.

Fig. 8 - Radial image profile of a single pixel event

At the 2.1m telescope focus is a simple function of temperature. Best focus as a function of temperature shifts as deltaF/deltaT = 0.025 per degree K with a focus at 5.10 for 8K. After the temperature of the telescope structure stabilizes (it varies rapidly for roughly an hour after opening), the relationship provides an accurate estimate for best focus, which generally shifts towards smaller values as the night progresses. Pick a temperature (such as the secondary or the front surface of the primary) and monitor that temperature to adjust focus. It appears that best focus was a tolerance of at least +/- 0.01 focus units. It is useful to note that there appears to be no backlash in the IR secondary. At the 4m telescope, nominal focus is -7300 with an as yet undetermined temperature dependence.

Fig. 9 - Focus at the 2.1m telescope

Performance Checks

SQIID will be installed and checked out at the start of each observing run by a competent and cheerful support scientist. Users may confirm continued proper operation during their run with software interrogation and by comparing dark and flatfield frames against "standard" frames.

Detector status and temperature information is displayed with the word status s; it is also automatically updated at the beginning of an observation. Standard values for the default SQIID configuration are displayed above. The cryogen temperature readouts are displayed as temperature based on a generic relationship between voltage at constant current and temperature.\.

The heater power may vary somewhat around the typical value given. However, a significant and persistent departure from this value may indicate the dewar is losing its vacuum (going "soft") or that the Closed Cycle Cooler System is losing efficiency. If this is suspected, contact the instrument support scientist.

Finally, sky- or dark-subtracted frames may occasionally show a dark (or light) potato-shaped artifact about 30 pixels wide. This "Phobos effect" (see Fig. 10) results from a region of lower signal in one of the frames, and can appear as a positive or negative image anywhere on the array. This occurs very infrequently, and is apparently a crystal relaxation phenomenon in response to the array being warmed after having been too cold (below 25K). This can happen over a span of a few hours when the SQIID electronics have just been powered on after they have been off for an extended period. (The array heater power is controlled by the same power supply as the electronics.)

Fig. 10 - "Phobos effect" (dark circular patch at lower right)

8. Calibration

Responsivity Calibration

Atmospheric extinction must be calibrated by observations of either a standard star as close as possible to the same zenith distance used for the object or a series of stars that span the range in zenith distance for the observation. Typical extinction for SQIID is summarized in Table 9.

Table 9. Estimated SQIID Extinction in mag/airmass
0.15 +0.06 +0.08 TBD

Flatfield exposures are necessary to calibrate the pixel-to-pixel gain variations in the array and the effects of the illumination of the array by the internal optics. The system response is stable and is very flat across the arrays, with a slight intrinsic column to column modulation of +/- a few percent (owing to relative array orientations, columns may have either NS or EW orientations in the saved and displayed images). Consequently, the flatfield for each channel should be stable at the percent level under normal illumination and global flatfields can be constructed which are viable for extended periods of time. Since direct illumination of the array is possible (remember that the secondary mirror is undersized), observations near bright sources, such as the moon, which have atypical illumination, should not be use to determine global flatfields. Observations during twilight will also have illumination atypical of nighttime observations.

Because sky flats provide the same array illumination as real observations, they are preferable in principle to dome flats using the White Spot. It is, nonetheless, a good idea to obtain dome flats as a backup. If one is observing in a sufficiently sparse star field, one may use the same set of observations for the object, sky, and flatfield. Because the sky flats will include the array dark current, it is necessary to obtain separate "dark" observations for subtraction from the sky observations. Unfortunately, what constitutes a "dark" frame for creating flats is ill-determined due to the "memory" effect which accumulates in time. For example, a series of observations in the dark filter immediately following sky (or dome) flat observations will show a monotonic decrease in mean value as the "memory" of the relatively bright preceding observations decays over 5 to 10 integration times. By the same token, a series of dome flats following dark or low-background observations will show a monotonic increase in mean value as the "memory" of the higher flux observations accumulates. One possible approach is to take a larger number of dome flat or dark observations and reject those early in the series, when the change in value from one frame to the next is the greatest.

Should one decide to obtain "dome flats", it is recommended that one take exposures when the dome is dark, to minimize the ambient radiation at J and H. At the 2.1-m and 4-m telescopes, the dome screen is illuminated by lamps mounted on the telescope top ring. (The 4-m telescope is positioned by mountain technical staff.) Darkening the dome may require waiting until visitor hours end at 1600 hr. The lamp controls at these telescopes are on the LTO console. One should obtain a relatively large number (7 or more) of flatfield images for post-processing within IRAF, where floating point arithmetic and sigma-clipping or median combining are possible (the latter to eliminate noise artifacts which may appear in a single observation). Since the illumination level differs markedly at J, H, K and PAH, one should anticipate taking data at several settings of the lamps. Within the K and PAH bands, one will be dominated by the thermal emission from the dome screen, and at the PAH band, the lamps are not necessary. Dark current and residual illumination subtraction require obtaining an identical series of observations with the flatfield lights off (for J,H,K) or the dark slide in the beam (PAH). After subtraction of the "off" or "dark" frame, normalization (using, for example the IRAF 'response' task) and median combining of these observations should eliminate noise spikes or systematic features in the spectra. To stay within the relatively linear portion of the array response, it is preferable to turn down the lamp intensity and use exposure times relatively long in comparison to the readout time of 0.84s (e.g., 3 - 5 s). Keeping the flux at a modest signal level (< 5000 ADU) may also help in this regard; with a read noise ~ 35 e, one is completely background limited by signals > 500 ADU.

The time-dependent dark signal is generally small compared to the sky background for all four arrays. However, higher-order spatial features, such as the gradient on one side of the H channel, are significantly brighter and cannot be neglected when contructing flatfields. "Dark" images contain both static and time variable components with diverse causes that obviate simple scaling of dark frames to alternate integration times. Consequently darks corresponding to the integration times for data that will be used for defining flatfield and linearity issues should be taken. A sequence of 9 exposures should be sufficient. Since the dark current is so low, you will be able to see transient events, such as those due to cosmic rays that can be removed by median filtering. Darks should be taken in the afternoon and/or in the morning. Closing the cold internal darkslide (hand crank on the side of the instrument) is both necessary and sufficient for taking darks. The time dependence of the median dark current within [100:400,100:400] is shown in Figure 11. Note that the H channel exhibits an apparently negative dark current at short integration times.

Fig. 11 - SQIID dark current for H (green), K (red), and J (blue) Channels at 700mv bias. Conversion gain is approximately 11 electrons per ADU.

Sky Subtraction

While the detectors are stable, the sky is not necessarily so. Under good conditions, sky flux varies only with airmass and at a given position, the sky can be stable over an hour or more. Naturally sky flux increases when you get near the moon. When airglow is high and variable, the sky at H can vary by a factor of two over the course of an hour, while the sky at J varies by 40%. Consequently it is prudent to monitor the sky (use imstat inside IRAF) to be sure that you have sufficient data to perform sky subtraction on your targets.

In the IR (even at JHK) observations of all but the brightest stars (9 mag) are background limited so that typical diffuse targets (such as galaxies) are seen at very low contrast in individual frames. As noted above, the sky background is constantly changing (slowly with airmass and on slow to moderately rapid timescales with sky emission) so that the contrast varies as a function of time. Since observations are ultimately limited by inadequacies in the flatfield, one cannot simply co-add such observations and obtain a meaningful result; median filtering such data merely selects the middle frame. (For example, as the contrast varies the illumination pattern on the array also varies since the relative mix of radiation seen through the telescope optics and that seen directly past the secondary changes.)

One must devote a comparable amount of time off-source, intermixed throughout the on-source observations, to provide sky frames; subtracting sky frames on a pixel by pixel basis reduces the contrast problem and allows one to successfully co-add data taken over a long interval of time. When the targets are much smaller than the array, one can accomplish this sky subtraction by moving the target around the array and in effect taking the sky and source data at the same time.

GHOSTING - Each channel has its ghost, whose position is opposite the optic axis from its parent bright star. Only saturated stars are bright enough to clearly show their ghost, which is out-of-focus, covering about 25% of the chip. The out-of-focus ghosts that lie opposite the optic axis from the primary have the following characteristics:

            J       H       K   
            90     108      56  pixels diameter
           5.5     6.3     5.5  difference in integrated flux in magnitudes
In addition, the J channel has an in-focus ghost located 36.5 pixels east of its parent and 5.4 magnitudes fainter.


When a pixel is reset, the voltage difference (bias) between the pixel and detector substrate creates a depletion region that acts as a potential well for the collection of (mostly) photogenerated carriers. Electrically, one may consider this potential well as a capacitor. As charge accumulates in the pixel, the depletion region fills in, increasing its capacitance and that of the entire pixel node. This changes the electrical gain of the system, resulting in a sub-linear voltage-charge relationship, which quickly rolls off (saturates) when the pixel voltage reaches that of the detector substrate (zero bias). Technically, a pixel will continue to accumulate charge even into forward bias, but its response by that time will be significantly nonlinear. As a direct consequence, pixel response departs from linearity in a predictable fashion for accumulated signals above a device-specific level. Since the total capacitance (which determines system gain) is the sum of the distributed (non-varying) capacitance of the system and the (variable) capacitance at each detector node, the degree of non-linearity is a function of the ratio of nodal to total capacitance. Since key pixel parameters such as quantum efficiency are very uniform, the linearity appears to be a global property of the array rather than pixel specific.

Increasing the bias on a pixel will not only increase the depth of the potential well, but by decreasing the capacitance of the depletion region in relation to parallel components of the node capacitance, will result in a more linear voltage-charge relation. As a result, a bias increase from 0.6 to 0.8 volts will effectively double the charge capacity of the pixel. This comes, however, at a significant cost in dark current, which increases dramatically with bias. Therefore, we recommended a bias of 0.6v for JH, 0.7 for K, and 0.9v for high-background (L band) observations, where the increased dark current is not important. These were the default bias values for SQIID until January, 2005 whrn we adopted the current bias values. To further improve linearity, we currently use a bias 0.7v for jhk and 0.9v for L

In the following discussion, observed (output) values have not been corrected for the unseen charge collected during delay between biasing the detectors (reset) and the first (non-destructive) read of the bias. A second (non-destructive) read is taken an integration time later. Only the difference between reads is output. In detail, the global reset mode used to operate SQIID resets all the pixels at the same time and then performs a non-destructive read of the array. The first pixel is read out shortly after the global reset and the last pixel is read out 0.61 sec after the global reset. The charge accumulated at the time of the position dependent first read is "lost", since its value does not get reflected in the difference. Hence, the reported signal is always less than the total signal on the array at the time of the second read. At short integration times and high rates of photon arrival (bright stars and/or high background), this unreported charge can lead to a significant underestimate of the signal outside the scope of the usual linearity correction. The unseen charge can be accurately estimated (apart from noise issues) on the basis of the timing, but since the amount of unseen charge is position dependent, one is better off not "going there"! Ideally, the data fit are always within the regime where this correction for unseen charge was small. This is not the case, for example, when doing 0.84 sec integrations on standards!

One normally expresses the departure from linearity in terms of the relative error (input-observed) or the normalized relative error (input-observed)/observed for each array and fits these data to an appropriate mathematical model. For SQIID, the model fitted function for the relative error is:

  ym[x] = a(1) * x**2 + a(2) * x**3

     where ym = relative error = (input - observed)
           x  = observed (adu)  
The fit is valid for values < Xmax. For values beyond Xmax, the array is rapidly approaching saturation. The fitted parameters for each channel are summarized in Table 10 and the fitted linearity is plotted in Figure 12. A new array was installed in the H channel in February 2002.

Table 10. Fitted Linearity Coefficients
Valid from Jun 2000 Jan 2005 Jan 2005 Mar 2002 Jun 2000 Jun 2000
Channel (bias) Ks (700mv) H (700mv) J (700mv) H (600mv) J (600mv) H (600mv)
a(1) 6.4864e-7 -1.3456e-8 1.2626e-6 8.4783e-7 1.2164e-6 4.5326e-6
a(2) 4.4790e-10 6.1010e-10 2.9498e-10 6.8416e-10 4.7829e-10 4.8759e-10
Xmax (ADU) 16000 16000 16000 16000 16000 14000
Data date Apr 2003 Apr 2003 Apr 2003 Apr 2003 Apr 2003 Apr 2001

Fig. 12 - Linearity Correction for SQIID. Relative error for H (green), K (red), and J (blue) Channels for 600mv and 700mv bias. Conversion gain is approximately 11 electrons per ADU. Prior to 2005, the JH bias was 600mv. As of January 2005, the JHK bias is 700mv.


Eventually, a pixel will accumulate sufficient charge to forward bias it to the point where no more is collected, resulting in a condition known as saturation. The reset-read-read address cycle used in double correlated sampling (Fig. 4) results in a characteristic, but unusual, saturation behavior. For a given integration time, the effect of increased flux is a steeper slope of the voltage-time curve. As the flux increases, the voltage V2 will eventually saturate, while the voltage V1 will continue to increase because of the time interval between the reset and the first read; the difference signal will thus decrease with increasing flux. Finally, the detector will saturate in the short interval between the reset and first read, resulting in identical values for V1 and V2, or a difference value of zero. Thus, the double-correlated signal relation with flux is initially nearly linear, then increasingly nonlinear, and eventually decreasing to an ultimate value of zero as one saturates. The A/D channels (2 for J,H,K and 8 for L) will saturate at slightly different levels, resulting in an evident odd-even column pattern. These effects are evident in a Saturation Image of the L band, in which the flux increases rapidly from the bottom to the top of the array.

Pixel Masks

Representative pixel masks for the J, H, K channels of SQIID are contained in Supplement 1: Channel Specific Characteristics. These masks include the combined effects of intrinsically bad pixels and instrumental vignetting.

9. Appendices

Appendix I: Telescope Checklist

Observing Platform / Cassegrain Cage Setup

At the 4-meter the telescope and the dome are operated by the Telescope Operator (nights) or by the Technical Assistant (days). Arrange schedules with them in advance for positioning for dome flats, and opening the telescope and dome at the start of the night. The observer should:

Computer Room Setup

Fig. 16 Heuricon DSP Box

Control Room Setup

Inside the control room there are a number of tasks to attend to:

SQIID is now ready to take data.

Calibration Data

Although sky flats are generally adequate for reducing SQIID observations, dome flats may be useful. Dome flatfields may be done either in the afternoon before observing or in the morning. Remember that you will need dark frames taken the same way (integration time and number of coadds) as your sky/dome exposures to properly produce your sky flats. Since the temperature and operating conditions for SQIID are highly stable by design, one should not have to repeat flatfields once they have been verified. In any case, it may be desirable to take some short and long (30s) dark frames before the first night of observing to verify performance.

Final Setup at Start of Night

Telescope Operator will do final dome and telescope checks and acquire stars, offset, etc. SQIID acts as its own acquisition camera.

Observing Miscellany

Telescope motions are generally commanded from LTO terminal, although they can also be entered from the observer's terminal. For programs demanding frequent motions, agree on a mutually compatible protocol with the LTO. In the event of multiple observers, only one should serve as the communication link to the LTO.

Control Room Shutdown

Observing Platform / Cassegrain Cage Shutdown

Computer Room Shutdown

Appendix II: WILDFIRE Command List for SQIID

SQIID image names follow the naming convention:

where "filename" is an observer controlled parameter, "ch_id" is the approripate designator from the set "j h k l", the number "XXX" is sequentially numbered (being automatically incremented for each exposure until a new value is declared, either at a new object or new night) and "image_extension" is either ".fits" or ".imh" (depending on the saver setting declared at startup). If you correctly set 'oldirafname = yes' when reading the data from tape, the image names will be restored to their original names and match the log sheets.

A list of available commands within the WILDFIRE instrument control window is:
! commands execute the commands in csh or run csh
? command give help on a command
ed name call up an editor on a proc
help name display help for one of these topics
man name display a man page for a given topic
progress 0 minimize diagnostic output during integrations (**node...)
source program temporarily include tcl program within recognized system; need to source again after powerup or go (full path name required)
activate activate the detector
deactivate deactivate the detector
setup sqiid set up the default SQIID voltages and prompt for activation. Alternative parameter named parameter files can also be invoked via setup filename
?ucode list array microcode currently in use (also in image header)
dl microcode_name download specific array microcode
startwf initiate bootstrapping and downloading of the WILDFIRE system
exit deactivate the array and exit the WILDFIRE controller
trouble open troubleshooting session (do NOT enter in Instrument Control window)
hung attempts to complete link protocol; used as part of the restart procedure when WILDFIRE is hung (INSTRUMENT CONTROL window unresponsive and data collection stalled); must be entered in Console window
newobserver enter observer name and proposal ID for image header
status [|s|v|t|f] display a status screen; (general status |s|; voltages|v|; temperatures|t|; filters |f|)
longheaders [on|off] will disable/enable house keeping data in the header
tcp_on enable link to TCP for telescope status info and offsetting
tcp_off disable link to TCP for telescope status info and offsetting
Note: a parameter has two attributes, its value and flags indicating whether the parameter and its value should be displayed and/or queried when the ask or observe tasks are run.
lpar list the names of the available parameter files
plist list all the current parameters
psave filename save the current parameter set (values and ask/display flags) to the named parameter file
puse filename load the named parameter file
ped edit the current parameter file selected by puse asking all questions regardless of query status
ask prompt for the eask selected subset of parameters within the current parameter set
eask iterate through all the known parameters, allowing the user to specify which parameters are queried and which are displayed. After each question an "l" signifies display only; "a" signifies query; "la" will list the current value (which may be selected by [cr]) or accept a new entry
Note: many SQIID parameters have 4 values (one per channel) which are entered on the same line separated by whitespace. The last argument in a series is adopted for the rest of the arguments: "1" yields "1 1 1 1", "1 2" yields "1 2 2 2"; "1 2 3" yields "1 2 3 3". When no argument is given, task prompts with current value.
?coadds returns the number of coadds for next image
pics [n] sets the number of pictures to be taken at each observe/go
set-time [f f2 n] prompts and sets the JHKL coadds and integration time (in seconds to millisec level) sets JHK time to f sec, L time to f2 sec, and JHK coadds to n (task computes L coadds). This is the preferred method to set integration time and/or coadds
setIntegration [f f2 n] sets JHK time to f sec, L time to f2 sec, and JHK coadds to n (task computes L coadds)
nextpic [n] sets the picture index appended to filename to [n]
header_dir sets path for FITS image or IRAF image headers
pixel_dir sets path for IRAF image pixel files (unused for FITS images)
mode sets operational mode for array readout (stare/sep); sep is for observing
title sets title field for IRAF/FITS image header
comment sets comment line within IRAF/FITS image header
offset relative telescope (RA,DEC) position parameter updated by TCL commanded motions
resetoffset reset the position "offset" values in the IRAF/FITS image header to "0,0"
filename filename sets the IRAF/FITS image "filename". The path is is not included in "filename"; if no argument given, will prompt with current value. For SQIID, a "%d" or "%03d" should be inserted where the picture number should be placed. If no field is given, "%03d" will be appended. The format will be: "filename"//"nextpic"//"ch_id"//"image_extension" (Note: when filenames conflict, the saver task attempts to create a unique name by appending .nnn to conflicting filename.)
ask prompting for pre-selected observing parameters
go initiate an observation using the previously set parameters
observe perform one observation using current parameter set, prompting for key parameters
abort abort an observation (enter in Instrument Control window); follow with save
save [ch_id ...] include all the channels you wish to save, eg.,"j h k"; be sure to re-issue after an abort
display [ch_id ...] display channel n after each coadded integration
east [n] move telescope n arcseconds east
west [n] move telescope n arcseconds west
north [n] move telescope n arcseconds north
south [n] move telescope n arcseconds south
toffset [e] [n] move telescope e arcsecs east and n arcsecs north: + for north/east; - for south/west
zs [z1] [z2] set zscale values [z1] and [z2] for the image display
zs 0 0 enables autoscaling for the image display
tcp_on enable link to TCP for telescope status info and offsetting
tcp_off disable link to TCP for telescope status info and offsetting
movie begin observe/display loop. NOTE: parameters (filename, running number, integration time, coadds, etc.) will be those of previous observation or 'ask' routine unless specifically reset!!!! Movie frames are saved to disk and should be deleted periodically. It helps to use a filename like "junk" when using movie. Terminate movie with end [CR] in Instrument Control window. Pay careful attention to the nextpic numbers, as movie increments nextpic for only the selected channel and after movie terminates, nextpic for the channels will no longer match.

Appendix III: Troubleshooting

As with all Kitt Peak instrumentation, nothing is ever supposed to malfunction. On the rare occasions when something seems to go wrong, either by pilot error, exquisite software gotchas, or hardware failures, recovery can in many cases be fairly simple. In particular, hang-ups in the instrumentation software can usually be corrected without resorting to rebooting the computer, which should be considered a last resort.

The following tables cover situations that may arise with the Instrument Computer or SQIID itself. Some situations are not covered in this manual, since the recommended recovery could involve procedures that are potentially harmful if done incorrectly. In these cases, the user is requested to call for technical assistance from the Observatory staff.

Instrument Hardware

no signal Internal cold dark slide closed. Check status s for proper temperatures, voltages. Check that green LED in analog electronics box is lit. If the detector has been accidentally deactivated, program will not sense this; observe will work, but return pixel values near zero in image.
minimal signal at J/H; large signal at K Telescope mirror covers, dust slide (top, South), and/or external dark slide (pull knob at NorthEast) closed. Check status s for proper temperatures, voltages.
apparent vignetting External darkslide may be partially in place or internal aperture wheel may not be set to open position. Vignetting by external darkslide or dust cover will elevate K background. If the internal polarization anaylzer is in position, the image will be vignetted on all sides with half the signal. Vignetting also results in higher backgrounds in the K and L channels.
bootstrap failure If the startwf procedure fails during the "bootstrapping node ..." process, a likely culprit is a bad (or incorrect, if the failure occurs on the initial setup) fiber optic connection. Check the three status LEDs visible through sight holes on the SQIID electronics. The left LED should be green if there is power to the instrument; the middle and right LEDs should be off. If either the middle (channel 1) or right (channel 2) LED is red, there is a fiber continuity problem in that channel. There is a duplicate set of LEDs in the Heurikon DSP box in the computer room (reporting on the other two fibers); it is necessary to remove the front cover to view them. A bad fiber channel will require the substitution of one of the spare fibers. Call for assistance.


After a reboot, type 'dmesg' and you will find these lines if the PTVME and the B011 are found.

If the PTVME cable is disconnected or the PTVME hardware is not responding, you will see these 2 lines:

'dmesg' prints out the most recent system messages, which are stored in /usr/adm/messages*, with message the most recent, and message.1, message.2, etc. the older logs. In the files, the lines are time stamped:


The following procedures are intended as a guide for restoring the WILDFIRE system following various levels of system failure. Re-booting the computer and cycling power to the instrument or DSP Heurikon box in the computer room are not normal WILDFIRE operations and should not be done without proper consultation, or unless the specific conditions below are valid. These procedures are listed roughly in order of increasing severity, so unless a specific condition has occurred (e.g., DSP power cycling), try the less dramatic procedures first.

An extensive troubleshooting library may be consulted by entering trouble in any active window (except the Instrument Control window). The resulting interactive session can be used to diagnose and correct problems.


If WILDFIRE has been exited normally and the system is intact (neither instrument power nor Heurikon DSP power has been interrupted), one can simply type the following within the Instrument Control window to restore operation:

The startup script will run automatically to the point where the question about array activation is presented.


If the Instrument Status window has vanished, first check to see if it has simply been closed. Type checkfire (or !checkfire if necessary) in an active Unix (xgterm) window on either royal or lapis at the 2.1m or khaki at the 4m.

If the "hkserv" process is present, the window has been closed, and it will be necessary to locate and open it. If the icon is not visible, it may be hiding behind one of the open windows. In OpenWindows, one can check the "windows" item in the menu for the status of all operating windows; if the Instrument Status window is present, open it and continue observing.

If the Instrument Status window has died, perform the SIMPLE RESTART procedure below.


If WILDFIRE has crashed (Instrument Status window has vanished and could not be found by above procedures), and/or the "[hostcomputer]" prompt has returned to the Instrument Control window, the following steps within the Instrument Control window should restore operation:

[NOTE: If the power to the instrument and/or the Heurikon DSP box in the computer room has been interrupted or the computer has been rebooted, this procedure may not be sufficient. See below for more specific procedures]


If WILDFIRE is hung (Instrument Control window unresponsive - won't respond with a system prompt after issuing a "CR" - and data collection stalled):


Note: This Totally Stalled System Quick Restart procedure can be tried in situations where one does not want to take the time to recycle of the instrument power, since that would require returning to zenith. However, if this procedure fails to recover operations, you will need to re-cycle instrument power and follow the Totally Stalled System Restart procedure below.

If the STALLED SYSTEM procedure fails to return the UNIX prompt, or an examination of the operating processes by entering ps ax in the Console window reveals a process which cannot be halted via the kill -9 [process number] command, it will be necessary to reboot the instrument computer.

In detail, type checkfire (or !checkfile if necessary) in an active Unix (xgterm) window on either royal or lapis. If the "checkfire" task is unavailable, type:

in a Unix (xgterm) window logged into either lapis or royal at the 2.1-m or khaki at the 4m (if necessary, use the menu to start one). Look for a "control" process in state "D". ". Should you find one, you need to reboot the instrument computer to kill the process.

The startup script will run automatically to the point where the question about array activation is presented - answer 'y' to question about detector activation


If the STALLED SYSTEM procedure fails to return the UNIX prompt, or an examination of the operating processes by entering ps ax in the Console window reveals a process that cannot be halted via the kill -9 [process number] command, it will be necessary to reboot the instrument computer. In detail, type checkfire (or !checkfire if necessary) in an active Unix (xgterm) window on either royal or lapis. If the "checkfire" task is unavailable, type:


If the power to the instrument was interrupted but the black Heurikon DSP box in the computer room remained powered up and the computer was not rebooted:


If the black Heurikon DSP box in the computer room has been powered down, then it is necessary to do the following.

NOTE: The order of these steps is important. If the dsp box is powered down, rebooting the instrument computer is necessary. Make sure no one else is using the instrument computer at the time. If only the power to the instrument has been interrupted, perform the procedure above. Whenever the instrument computer is rebooted, the instrument power must be off and the Heurikon DSP power on!