3. The Mosaic Hardware

3.1. The Mosaic CCDs

The Mosaic Imager features eight 2048 (serial or pixels/row) x 4096 (parallel or pixels/column) 15 mm pixel CCDs arranged as an 8192 x 8192 pixel detector. The Mosaic-1 CCDs are read out through a single amplifier per chip simultaneously to 8 controller inputs (on 4 Arcon controllers). For Mosaic-2 (at CTIO) 16 amplifiers, 2 per CCD, are working and can be readout simultaneously. Unfortunately, two of the CCDs in Mosaic-1 only have one working amplifier, limiting Mosaic-1 to work in 8 channel mode. The resulting mosaic array is a square about 5 inches on an edge. The gaps between CCDs are about 0.7 mm in the row direction and 0.5 mm in the column direction. [See Figure 3.1.1 showing an image labeled with chip numbers.] The Mosaic Imager is populated with thinned, AR coated SITe CCDs. These chips have only minor flaws that have little effect on their scientific performance, but they do require careful calibration to attain excellent flat-fielded images.

Figure 3.1.1: A Flat-field (R band) map of the CCDs in the Mosaic with their 'extensions' (im1, im2, im3, ?) as used in the IRAF nomenclature (see section 5).

Figure 3.1.2 shows the average QE for the 8 CCDs. Individual CCDs deviate from this curve as shown in Figure 3.1.3.

Figure 3.1.2: The average QE for the 8 SITe CCDs in Mosaic I.

Figure 3.1.3: The QE differences relative to the average for the 8 SITe CCDs in Mosaic I. Also shown are the readnoise (RN) and gain (GN) values for each CCD.

3.2. The Dewar

The Mosaic dewar is a large (6.3 liter) vacuum vessel radiatively coupled to the CCD mount. It is the large round cylindrical object in the center of Figure 3.2.1. The hold time of the dewar is 17 hours. It is filled by an observing technician at the start and end of each night.

Several temperatures within the dewar are monitored and displayed in the Mosaic Graphical User Interface (GUI).The CCDs should be between -83 and -98 C. The dewar tank is cooled to -166 C or cooler. A good way to detect the exhaustion of LN2 is the warming of the 'fill neck' temperature, which is normally near 0 C. If this temperature begins to rise much above zero or any of the temperature boxes turn and stay red, call for assistance to have the dewar filled. Mosaic automatically e-mails mountain personnel if various temperatures rise to levels that indicate a warm-up.

Figure 3.2.1: The Mosaic system mounted at the back of the 0.9-m telescope. The dewar containing the 8 CCDs is the silvery, cylindrical object in the middle. It is surrounded by the filter track, which is housed in the large black oval that extends horizontally across most of the picture.

3.3. The Arcon Controllers

The eight CCDs are read out through four Arcon controllers. These controllers run at 100 kpix/sec per CCD, yielding a readout time, including all overheads, of~2min34sec for Mosaic-1 (only a single amplifier is used). Data values are stored as 16-bit unsigned integers. For further details about the Arcon controller systems, see the technical paper by Roger Smith (http://www.ctio.noao.edu/instruments/arcon/arcon.html).

The data taking computer (rush or rust) is a 125 Mhz Sun Sparcstation 10 running SunOS. It has sufficient resources to manage the data acquisition, but not much more. The large data volume is handed off to the reduction computer (tan (KP4m) or emerald (W0.9m)) via Fast Ethernet for all analysis and reductions, thereby relieving rush/rust of unnecessary loads. These are fast Linux boxes with >100 Gbytes of disk and >1 Gbyte of RAM.

Figure 3.3.1: A view of the Mosaic system on the 0.9-m telescope indicating 2 of the 4 Arcon controllers. Also the south guide TV housing can be seen, as well as the Mosaic dewar (now appearing in black).

3.4. The CCD Shutter

The shutter consists of a pair of opposing sliding blades, one of which has rectangular slots open for the guide field. The blades are attached to pneumatically driven cylinders to provide very fast control of the shutter. This design allows the TV guide fields to be shuttered independently of the science field. In the guide mode, the closed shutter still allows the TV guide cameras to see the sky. In the dark mode, these fields are closed as well. The acquisition software controls which mode the shutter remains in between exposures. For object observations, the shutter goes to the guide mode before the exposure begins. For requested observation types of dark, flat, or zero, the shutter goes to the dark mode before the exposure begins (and remains in this mode after the exposure and readout are completed).Note that the TV fields are always open when the shutter is open; the different shutter modes only control the TV fields when the science shutter is closed. If you have been taking darks, flats, or zeros, you may need to set the shutter mode to guide in order to get light to the TV guide camera.

The time for the blades to move completely across the field is 23 msec. The motion of the blades during both opening and closing are in the same direction so that the exposure level is nearly constant over the array. The motion of the shutter blades is along columns. The accuracy of the shutter has been measured to be ~3% in a 1-second exposure (that is, the exposure is really 0.97 seconds; see also Section 8).

3.5. The Filter Track

The filter track holds 14 filters. For each filter position, there is a filter for the CCD field, and two separate filters for the two TV fields. Separate filters are used so that a narrow bandpass science filter does not constrain the observer to find very bright guide stars. Normally, one would use clear (BK7) filters for the TV, but one can use a red filter to minimize moonlight or match the science filter more accurately. One might want to match the science filter, at least approximately, to minimize a guider drift. Even at the 4-m, residuals after the correction from the ADC are of order 0.1-0.2 arcsec. This, and all filter decisions, must be made ahead of time, as the filters can only be changed during the day by a qualified observing technician.

Adapters exist to allow the use of 4-inch-square (1 adaptor) and 2-inch-square (2 adaptors). At the 0.9-m (f/7.5), these 4-inch filters illuminate approximately 6K X 6K pixels (56% of the total sky area); At the 4-m, 4-inch filters illuminate approximately 5.5K X 5.5K pixels (46% of the total sky area).

The positioning of the filter track is highly repeatable. However, the acceleration of the track can occasionally dislodge dust particles between filter moves, particularly if intervening movements have turned the filter upside down. In all cases, the filter track software moves the track in the direction that minimizes the distance moved to reach the requested filter position.

In addition to the 14-position filter track, there is a manual slide that can hold a single filter of the same size (5.75 inches square).This may be used, for example, to hold a bandpass filter when polarization filters are used in the track. Use of an additional 'hand-insert filter' changes the focus. At the 4-m, changing, inserting, or removing this filter can only be done at the maintenance (Southeast Annex) position.

3.6. Filters For Mosaic

To fully utilize the field of view of the 8Kx8K, filters must be 5.75 inches (146 mm) square, and have 5.43 inches (138 mm) clear aperture. The optimum thickness that preserves image quality over the entire field of view is 0.47 inches (12.0 mm). All NOAO mosaic filters adhere to these specifications to maintain a parfocal condition. Thus, neither the telescope nor the guide TVs should require a focus change when switching between filters. There is one exception, the CuSO₄ U filter (k1001), for which there is a focus offset.

A list of all available filters and their properties can be found at http://www.noao.edu/kpno/mosaic/filters/. Note that in order for post-processing command to display the correct on-the-fly flat the official filter name needs to be specified in the correct parameter set (i.e. wheel1). The official names can be found at http://www.noao.edu/kpno/mosaic/filters/filter_names.

Some of the currently available filters and approximate count rates (e -/sec) for a 20^th mag star are:

			4-m			0.9-m
Filter	TV	Peak T%	Central Wave	FWHM	e^-/s	Central Wave	FWHM	e^-/s
U	S8612	79.5	3577	646	35	3577	647	2
B	BG-38	69.3	4360	990	330	4360	990	14
V	BK-7	88.4	5370	940	340	5370	940	15
R	RG-610	86.2	6440	1510	410	6440	1510	16
I	BK-7	93.9	8220	1930	225	8220	1930	9
H^α	BK-7	94.3	6569	80		~6575	~80
H^α+4	BK-7	91.2	6611	81		~6615	~81
H^α+8	BK-7	89.5	6650	81		~6656	~81
H^α+12	BK-7	86.1	6692	81		~6695	~81
H^α+16/[SII]	BK-7	90.7	6730	80		~6736	~80
SDSS g'	BK-7	90.2	4813	1537		4813	1537
SDSS r'	BK-7	91.8	6287	1468		6287	1468
SDSS i'	BK-7	94.6	7732	1548		7732	1548
SDSS z'	BK-7	94.8	9400	2000		9400	2000
[OIII] #2	BK-7	75.2	5021	55		~5027	~53
[OIII]+29#2	BK-7	90.5	5305	241		~5305	241
White	BK-7	97.2	5600	6800		5600	6800
Wash M	BG-38	87.1	5100	1140		5100	1140
Wash C	S8612	75.4	3860	1034		3860	1034
DDO 51	BK-7	85.1	5132	161		~5132	161
WR CIII	BK-7	68.4	4653	52		~4660	~50
WR HeII	BK-7	73.3	4690	51		~4695	~49
WR 475	BK-7	78.8	4750	51		~4755	~49
WR CIV	BK-7	72.7	~5816	46		5823	42

See Figures 3.6.1 through 3.6.5 for plots of the current filter transmission curves. ASCII Tables that describe the transmissions are available on the Mosaic Web Pages.

The U-band filter is based on the same formulation as our 4" filter set (liquid CuSO₄ + UG-1). Because containment of the liquid requires a thickness around the edge that exceeds the nominal Mosaic dead zone, some vignetting is present. At the 4-m, the vignetting introduces a 20% loss of light at the edge, but recovers to zero-loss at 200 pixels from the edge.

Figure 3.6.1: The broad band filter set, including the 'White' filter.

Figure 3.6.2: The SDSS g', r', I', and z' filters, along with Washington C and M (M is the smooth curve slightly redder than g').

Figure 3.6.3: The current set of H^α (plus redshifted) filters. Note that H^α+16 serves as a [SII] filter.

Figure 3.6.4: The blue Wolf-Rayet filters for [CIII], He II, and a continuum at 4750.

Figure 3.6.5: The [OIII] on-band and off-band filters, plus DDO 51.

Figure 3.6.6: The V-band filter installed in the filter track. The 2 TV guider filters are visible to the lower left and upper right of the science filter. An Arcon controller can be seen in the background to the right.

3.7. Operation of the Guider TVs

Guiding with the Mosaic is accomplished using one of two TV cameras on the north and south sides of the science field. These are intensified fiber-optically coupled CCD cameras ("ICCDs"), and so, they can be damaged if exposed to bright light. The video signal from the selected TV camera is fed to the guider system. The field of view of each camera is about 2.2 arcmin on a side at the 4-m and about 5 arcmin on a side at the 0.9-m.

The field of view of the TVs is fixed with respect to the science field. At the 4-m, the fields are approximately 1440 arcsec north and south of the center of the science field. At the 0.9-m, the fields are approximately 2400 arcsec north and south of the center of the science field. TV focus can be moved remotely; offsets are -0.9 and -1.7 at the 4-m, and -0.1 and -1.9 at the 0.9-m, for the north and south TVs, respectively.

At a given location, suitable guide stars are almost always available without moving the telescope from the desired position. We find that we can guide at the 4-m on stars as faint as V=20 in full moon, and at the 0.9-m to V~17 near full moon.

The TVs and guider are controlled by the telescope operator at the 4-m, but by the observer at the 0.9-meter.The observer needs to first select the N or S TV on the distribution panel (see Figure 3.7.1). For the selected TV, on the ICCD Control Panel:

Turn the high-voltage potentiometer completely counterclockwise (10 turn pot)
Toggle the power switch on (on TV screen, pixel defects will appear)
Neutral density switch should be down (off)
Push the momentary button to enable high voltage
Slowly turn the high voltage potentiometer clockwise, monitoring TVs until guide stars appear
Acquire a guidestar on the computer moss and initiate guiding (consult the 0.9-m telescope user's guide (http://www.noao.edu/0.9m/Tel_Op.html) for details on step 6)

When switching between the two TVs, be sure to turn the high voltage potentiometer counterclockwise and turn off high voltage on the TV no longer in use.

Figure 3.7.1: A schematic drawing of the layout of the TV control panels at the 0.9-m. Only the two leftmost panels in the lower rack are used with the Mosaic TVs. The upper rack is used to select which TV/video signal is seen on the monitor.

3.8. Correctors

The 4-m corrector is a 4-element fused silica (for maximum U-band efficiency) design with additional internal prisms that serve as an atmospheric dispersion corrector (ADC). See Figure 3.8.1 below for the optical layout and refer to Jacoby et al. (1998, SPIE 3355, 721) for a detailed de