2.7m Telescope

Cassegrain f/17.7

Large Cass Spectrometer (LCS)
Scientist: Dr. Anita Cochran, (512) 471-1471, anita@astro.as.utexas.edu

A low-to-moderate-resolution spectrometer (R = 600—2,500) for faint object work available with TI1 and CC1 CCDs. Various gratings provide wavelength coverage from 0.3 to 1.1 microns. The LCS has long slit (150 arcsec) capability and CCD-based guiding and acquisition. Limiting magnitude mv = 20. The TI1 CCD has better QE at all wavelengths but suffers from severe fringing for wavelengths redward of 6,000 Å; thus for red work, CC1 is the better choice. Both chips can be used all the way to the atmospheric cutoff, but TI1 is better than CC1 in the blue.

Available Gratings:
Grating No. Blaze Å Effective Blaze Å Grooves/mm
40 4200 3900 300
41 7500 6000 300
42 10000 9200 300
43 4000 3700 600
44 7500 6900 600
45 10000 9200 600
46 4000 3700 1200
47 6000 5500 1200
48 7500 6900 1200

Nominal Resolving power for TI 1:
300 grooves/mm R (= Delta lambda / lambda) = 650
600 grooves/mm R = 1100
1200 grooves/mm R = 2200

For CC1, the resolving power is 15/12 that of TI1.

Three gratings may be mounted simultaneously and their grating tilt can be controlled from the control room. However, flexure in the instrument makes it prudent to clamp the gratings in place, making switching between gratings a process which requires accessing the instrument on the dome floor. It is not recommended to switch often between gratings in one night.

As for speed, it depends on the nature of the object (emission/absorption) and the color and where you want to be spectrally. For wavelengths below 6500Å, TI1 is the chip of choice. Beyond that, fringing sets in so it is prudent to change to CC1. However, CC1 has a lower quantum efficiency than TI1 at all wavelengths.

For grating 40, set to cover from 3000-5600Å, the following S/N is based upon the star Feige 25 observed under photometric conditions:
V=12.1, B-V=-0.04, B6V
At max, S/N=100 per pixel takes ~135 sec. Therefore, an object of similar color of V=17.5 would require 3.7 hours for S/N=100 per pixel or 2.6 hours for two pixel S/N=100 (7Å resolution).

The current guider, the PXL, is reasonably sensitive. Tests show that one could easily see V=17 stars in 1 second, could see V=19 stars in a few seconds and could see V=19 stars in 10 seconds on the slit. However, if autoscaling is enabled, bright objects will limit the ability to see faint so appropriate sub-frame readouts or user controlled scaling is necessary to see faint. The exposure estimates were obtained in good transparency in seeing of 1.5-2 arcsec. The TV can be used to view the slit, a field position or can be used for offset guiding.

Integral part of the instrument is the ATOG (Automated Telescope Offset Guider). The ATOG includes comparison arc lamps of argon and neon, along with an incandescent lamp (though external flat field lamps are much more accurate and should be used instead). There are also two filter wheels with a variety of filters for order sorting. In addition, slit size is controlled via the ATOG. The slit is continuously adjustable and is a bilateral slit. The ATOG is controlled by its own software and runs in a window on the workstation.

The ATOG is attached to the telescope on a rotatable bearing. Thus, the instrument may be rotated on the back of the telescope to orient the slit at any arbitrary angle. This is controlled by an external box hooked up on the dome floor.

A manual for the ATOG/LCS GUI is available in Adobe Acrobat PDF and PostScript formats. The corresponding ICE commands are documented here.

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31 May 2002
UT Astronomy Program • The University of Texas at Austin • Austin, Texas 78712
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