MMT Spectrograph Overview and Optical Layout
Contents
- Overview
- Observer's Responsibilities
- Shared Assemblies
- Blue Channel Optical Parameters
- Red Channel Optical Parameters
- Contacts & More Information
Overview
The MMT Spectrograph is actually composed of two spectrographs sharing a common focal plane assembly and two filter wheels. The spectrographs are denoted the Blue Channel and Red Channel, after their approximate wavelength bands. The spectrograph functions are all articulated by stepper motors under microprocessor control. Click here to see the MMT Spectrograph Control System User Manual.The Blue Channel is a low-to-intermediate resolution instrument optimized for spectroscopy in the range from 3200-8000 A. A number of gratings are available giving resolution as high as 30 km/sec. Three gratings can be resident in the spectrograph at any one time, thereby facilitating rapid reconfiguration of the spectrograph. The current detector is a highly-optimized ITL/STA 2688x512 CCD.
The Red Channel is a 3.75" beam spectrograph optimized for the region 5000 A to 1.0 micron but usable to about 3700 A. A highly-optimized ITL/STA 1200x800 CCD is used as the detector. The spectrograph has several operating modes including high-throughput long-slit modes at a variety of spectral resolutions from 20 A to about 2 A and prism cross-dispersed modes yielding complete coverage from 4500 A to 1.0 micron at moderate spectral resolution. As in the case of the Blue Channel, three gratings can reside in the spectrograph at any one time.
The MMT Spectrograph mounts to the standard MMT Top Box which houses the TV acquisition system as well as the comparison lamp system.
Observer's Responsibilities
As most observers know, CCD detectors are very fragile and expensive items. Fortunately, once the CCD is installed in a cryogenic dewar it is relatively sturdy. The following precautions are to be observed by all observers:- Never disassemble the dewar or the electronics box that is mounted on it (the camera head electronics). Never separate the camera head electronics box from the dewar.
- Never let air into the dewar. Never touch the valve at the rear of the dewar.
- Never disconnect or jiggle any cables.
- When thunderstorms are imminent and MMTO personnel are unavailable, turn off the CCD power supply, CCD controller, and VME computers and peripherals. It is extremely rare that an observer is on the mountain alone. Please try to locate a staff member to do this.
Shared Assemblies
Slit AssemblyA vector normal to the slit assembly is tilted 12.5 degrees with respect to the optical axis. Slit plates are to be changed only by trained MMTO personnel.
Click here to see a drawing of the MMT beam configuration.
Shutter
A Uniblitz shutter is located below the slit assembly and above the filter wheels. Control of the shutter is by the data acquistion computer and the CCD controller.
Upper and Lower Filter Wheels
Each wheel contains seven positions and a clear position. Each position can accommodate a 2x2 inch square filter with thickness up to about 6 mm. Filters cannot be changed without removing the spectrograph from the telescope. Contact the MMTO staff for special filtering requirements.
Blue Channel Optical Parameters
Click for a drawing of the Blue Channel layout. The view shown is as seen with one's back to the observing chamber doors (i.e. facing north when the building is stowed normally).
- Camera
- Type: folded Schmidt
- Focal length: 9.86 inches
- Monochromatic beam size: 6.0 inches
- Unvignetted field: 9.0 degrees
- Collimator
- Type: off-axis paraboloid (10 degrees)
- Focal length: 54.25 inches
- Diameter: 8.0 inches
- Folding Prism (TIRP)
- Type: Total Internal Reflection Prism (TIRP), 110 degree beam deviation
- Material: fused silica
- Beam steering range: +/-7 arcmin (+/-0.5 mm at camera focus)
Red Channel Optical Parameters
Click for a drawing of the Red Channel layout. The view shown is as seen with one's back to the observing chamber doors (i.e. facing north when the building is stowed normally).
- Collimator: Two element lens of 6.0 inch clear aperture (3.75 inches
used by axial beam).
- Focal Length: 33.75 in.
- Element 1: BaK5 meniscus. Outer surface is asphere.
- Element 2: Calcium Fluoride. Spherical both sides.
- Range of use: Achromatic from 3700 A to 1 micron.
- Folding Flat -- Approximately 6 inches in diameter. Fold angle = 32 degrees.
- Gratings -- Located at the pupil formed by the collimator.
- Beam diameter at pupil: 3.75 in.
- Ruled area: 102 mm x 128 mm.
- Camera/collimator angle: 32 degrees.
- Details of available gratings tabulated below.
- Cross-dispersing Prism. 45 degree wedge of SF-6. Can be used as the primary disperser for low, nonlinear (3 A /pixel at 5000 A to 18 A /pixel at 8700 A) dispersion with high throughput.
- Camera
- Folded Schmidt: focal length = 6.0 inches. f/1.6 monochromatic.
- Corrector element 1: LF5 meniscus. Both sides spherical.
- Corrector element 2: FK5 meniscus. Inner surface asphere.
- Corrector clear aperture: 5.75 in.
- Schmidt folding flat diameter: 6.75 in. Fold angle = 62 degrees.
- Sphere: Clear aperture = 6.90 inches, diameter = 7.00 inches. Radius of curvature = 12.46 in.
Contacts and More Information
MMT Personnel: Dennis Smith (Instrument Specialist)
Multiple Mirror Telescope Observatory
University of Arizona
Tucson, AZ 85721
Office: (520) 670-6747
Home: (520) 398-9609
Mark Wagner (Instrument Scientist)
Steward Observatory
University of Arizona
Tucson, AZ 85721
Office: (520) 626-3006
Home: (520) 219-1363
For CCD problems, contact a representative of the Steward Observatory CCD Group:
Dave Baxter 628-2078 x107 (O)
Mike Lesser 628-2078 x119 (O), 742-9302 (H)
Dave Ouellette 628-2078 x 110 (O), 219-9185 (H)
To report minor problems or quirks, send e-mail to `ccdlog'.
Further (but somewhat outdated) descriptions:
Beckers, et al., 1981, Ann. Rev. Monograph
"Telescopes for the 1980's," ed. Burbidge and
Hewitt, p. 63.
"A Moderate-Resolution, High Throughput CCD
Channel For the MMT Spectrograph," G. Schmidt,
R. Weymann, and C. Foltz 1989,
Pub. Astr. Soc. Pacific, 101, 713.