THE MARS MANUAL (The Multi-Aperture Red Spectometer) Kitt Peak National Observatory National Optical Astronomy Observatory

Staff Contacts:  Daryl Willmarth (willmart@noao.edu) and Arjun Dey (dey@ noao.edu)

Observing Programs That May Benefit From MARS

• Programs that need low-to-moderate dispersion spectra in the red
• Programs requiring high throughput
• Programs requiring a high density of multislits
• Programs that need nod-n-shuffle observing mode

System Overview

• a new high resisitivity  p-channel CCD
• a newly  overcoated-Ag  Schmidt primary mirror
• a new chip mount for the CCD with smaller cross-section
• a new field flattener lens
• a new volume-phase holographic grism (450 g/mm)

Click here for the MARS optical diagram (Figure 1).

    Multi-slit masks are individual masks fabricated for each target field. They require substantial effort to prepare, since good astrometric prositions are required for each object. To maximize the number of objects that can be observed without overlapping their spectra, an ancient UNIX-Fortran77 program, MSLIT, designs the mask pattern*. In this observing mode, one can typically observe up to 30 objects simultaneously. Refer to the manual "Multi-Slits at Kitt Peak" which is available from the KPNO documents website  for details on design and construction. Please note that the multi-slit design software is being significantly updated with an anticipated release date of summer 2002

* Soon to be upgraded.

Exposure Time Calculator

• Click here to link to Exposure Time Calculator

Total System Throughput - The Optimum Case

Data measured through a XX -arcsec slit,  with YY seeing,  etc......  Spline fit.
 
 

DQE Curve for the MARS/LBNL CCD

• Click here for measured DQE values

   Click here to go back to the Table of Contents

Instrument Observing Configurations

• A long-slit configuration using metal aperture plate slits.
• A multi-object mode employing observer designed multislit masks (see "Multislits at Kitt Peak" for mask design parameters).
• An imaging mode can be used by removing the grism and slit-mask. See below for more information.

Any of these modes can optionally use the "nod-and-shuffle" technique for accurate sky-subtraction.  See the section below for more details on using this technique.

Standard Long Slit Masks

Offset Slit Aperture Plates

• Click here to see the List of Offset Slits

Other Setup Aperture Plate Masks

    --- SETUP MASKS ---
    #19 Lynds Test line of holes, 32 holes, 380 micron diam. 
    #20 Knife-edge Hole 1.95mm diameter
    #21 Knife-edge Hole 2.05mm diameter
    --- TEST MASKS ---
    #22 Test Centroid 6 holes
    #23 Garth No.I alternating double hole pattern
    #24 Garth No.II another double hole pattern 
    #25 Fowler Test X-pattern centered at 45 deg., 200 micron holes
    --- MISCELLANEOUS MASKS --- 
    #26 1 hole, 2.5" diam. centered
    #27 2 holes, 2.5" diam. (star/sky)
    #28 2.5" slit with 2" center occultation
    #29 2.5" slit with 4" center occultati

Imaging Mode

   Click here to go back to the Table of Contents

The Optical System

Atmospheric Dispersion Compensation -  The ADC  or Risley Prisms

A set of Risley Prisms is available for atmospheric dispersion correction at the RC focus of the 4-meter telescope. This ADC covers a field slightly larger than MARS field (~5 arcminutes) and is automatically controlled by the telescope control system. One inconvenience  caused by the Risleys is difficulty of locating guide stars.  The Telescope Operator must be aware that suitable guide stars must fall inside the annular area just outside the instrument FOV but inside the field of the prisms. Choosing a guide star in or too close to the MARS FOV will result in vignetting the instrument field.  The ADC is used at the option of the observer and may not be suitable for all types of observations.

Click here for a plot of the ADC throughput in the 5500-10,000A spectral region.

The Collimator Lens

The collimator optics consist of a re-imaging field lens, located just beneath the  aperture plate near the focal plane of the telescope, and a collimating doublet. The field lens is a simple plano-convex lens that images the pupil of the telescope onto the collimating doublet and also reduces vignetting  when the full 50mm spectrograph aperture is used.The plano-side of the lens goes upward (skyward).

The doublet collimating lens is a classical crown-flint cemented lens with the achromatic range centered at a wavelength of 0.63 microns. The clear aperture of the lens is 4.2 inches, and the effective focal length is 31.5inches.  The flint element faces the incoming beam. Both the field lens and collimator are anti-reflection coated. The flint element is not a good  UV transmitter and contributes to the poor throughput below 4000A.
 

The Grism Dispersive Element

The dispersive element consists of a transmission grating replicated on the hypotenuse exit face of a right angle prism. The grism follows the collimator and was chosen for this application because of its high efficiency. The purpose of the grism is to obtain dispersion without deviation at the center of the spectral range. The instrument performs as a direct-vision spectrograph with the ``zero" order of the grism falling outside the field of view of the detector. The grism is fixed and there is no provision for adjusting the central wavelength by tilting the assembly. Adjustments of a few hundred Angstroms are possible by using offset slits as described in an earlier section of this manual.
This instrument uses two types of grisms, a "conventional" grism, shown below, and a new volume-phase holographic grism (see NOAO Newsletter, 67, Sept. 2001, p 32). The new "VPH" grism employs a sinusodial varying index of refraction medium produced by holographic interference as the disperser.

The undeviated central wavelength (UDCW) depends upon the wedge-angle and refractive index of the prism, and the particular ruling used. When the vertex of the prism  equals the groove angle of the rulings,  maximum energy throughput is obtained.
 
 

Available Grisms

 

Grism Number	4950-400	5970-300	6560-150	8050-450**	9700-300	9700-300	7300-300
Old Grism Number	650	770	810	none	780-1	780-1	780-2
g/mm	400	300	150	450	300	300	300
Order	1	1	1	1	1	2	1
UDCW* - Angstroms	4950	5970	6360	8050	9700	4850	7300
Resolution - Angstroms@	12	15	30	8	15	8	15
Dispersion - A/Pixel	3.2	4.3	9.1	2.0	4.3	2.2	4.3
Spectral Range - Angstroms	~4000- 8100	~4000- 10,000	~4000- >11,000	~5500- 10,800	~4000- >11,000	~4000- 7100	~4000- >11,000

* Undeviated central wavelength
@ With a 2.5" aperture
**   New Volume-Phase Holographic Grism, see NOAO Newsletter  #67, September 2001, p. 32. Has a built-in OG-550 filter.

---

   Click here to go back to the Table of Contents

---

The Upper Filter Bolt

The upper filter bolt is normally reserved for up to four order separation filters. These filters are 3-1/2 inches square and are NOT interchangeable with the lower filter bolt.

Order Separation Filter List

•  Click here for individual filter curves.

Filter Transmission Curves

•     Click here for UV-Blue sharp-cut filters.
•     Click here for bandpass filters.
•     Click here for Red sharp-cut filters
•     Click here for individual filter curves.

The Copper Sulfate Crystal Filter - Click Here for Precautions

The Lower Filter Bolt

    The lower filter bolt (four slots plus an open position) normally contains neutral density attenutors or direct imaging filters.  The normal filter size is 3-7/8 inches square but two slots have been enlarged to hold 4x4-inch filters. Consult with your Instrument Assistant for proper filter installation, if you want to use this bolt . Be aware that even though a filter list appears in the Spectrograph Setup GUI the bolt  itself may NOT  be installed in the instrument.
 

The Camera Optics

For best matching between an image at the focal plane of the 4-meter telescope (1 arcsec = 150 microns) and the detector (1 pixel = 15 microns), a demagnification of  ~10 is required to give an image scale of 1 arcsec per pixel. This requires that the camera optics have an effective focal  length of 4 inches, resulting in an f/1.0 optical system.  A Schmidt-type design was selected because it introduced a minimum amount of glass to the optical path. In addition, a fused silica corrector is used for aspheric correction and as an entrance window for the cryogenically cooled dewar that houses the detector. The Cervit primary mirror has a spherical figure with a radius of  curvature of 8 inches. It is held in position by a specially designed kinematic mount to ensure that the optical alignment is independent of temperature. The field  flattener is mounted almost in contact with the detector to compensate for the field curvature produced by the Schmidt design. The images produced by the original design contain ~75 percent of the energy within a 25 micron diameter circle. A new corrector was installed in December 1983 and improved the imaging beyond the above figures.  A new LBNL 1980x800 CCD was installed in December 2001. As with most fast camera systems, there is significant distortion that must be calibrated out in the reduction process.

Some important parameters of the system are:
 

Telescope Scale	6.6245 arcsec/mm
Camera Demagnification	~8.4x
Spatial Scale on CCD	~0.86 arcsec/pixel

The CCD Detector

Click here for a system DQE curve.

Click here for measured DQE values

Click here for a link to the LBNL CCD website.
 
 

Nod-and-Shuffle Observing Mode

ICE Observing Commands

References