The McDonald Large Cassegrain Spectrograph is a long slit low-moderate (R~600-2500) resolution CCD spectrograph optimized for the spectral range 0.3-1 micron. The instrument is used on the Harlan Smith 2.7m telescope at the f/18 Cassegrain focus.


The instrument is attached to the Automated Telescope Offset Guider (ATOG) module at that focus, which provides acquisition and guiding capabilities, as well as calibration lamp illumination. The LCS offers on-axis guiding off the slit jaws, as well as off axis guiding options. This page provides basic information for proposal development.


New observers will want to consult the latest versions of the LCS/ATOG User Manual (pdf) for detailed instructions, and the 2.7m Telescope Control System Users Manual before starting an observing run. The LCS/ATOG ICE commands are documented here. See also the McDonald Observatory Observer Information page. On the Request for Services form, available at this page, LCS observers must specify needed gratings, filters, and CCD.

 Contacts: (e-mail
 Scientist:  Anita Cochran (anita)  512-471-1471
 Observatory Support:  David Doss (doss)  915-426-4111


LCS Observing Options:

 Slit: Width remotely continuously adjustable (bilaterally) from 0.8" to 15".
 Decker: Remotely selectable between two options : 80" long and 250" long.
The location of the slit and decker on the science array is remotely adjustable.
The orientation of the science CCD array with respect to the slit can be adjusted manually at the telescope, if necessary.
The position angle of the slit on the sky can be adjusted manually at the telescope by rotation of the entire ATOG/LCS. Nominal default orientation of the slit is EW (PA=90).


 Filters: Located below the slit are two filter wheels. These can be used in any combination to isolate grating orders. Default filter set is given below.

Filter Wheels
 position  wheel 1  wheel 2
 1  clear  clear
 2  (CuSO4)  GG375
 3  BG18  GG420
 4  UG5  GG475
 5  ND2.0  OG515
 6    OG550
 7    RG610
 8    RG645

The CuSO4 filter may not be installed. Be sure to specify if you need it.
Wheel #1 has five locations. Wheel #2 has eight locations.


 CCDs: Two science CCDs are available for LCS. TI1 chip has higher overall QE.

McDonald CCD Arrays available for LCS
device name device type format pixels  plate scale
(no binning)
TI1  thinned, back-illuminated
Texas Instruments

CC1  unthinned, front-illuminated
 1024x1024 12µm  0.51"/pix

The TI1 chip has serious fringing at >6500Å. Flat fields should be done carefully.
 CCD dewar LN2 hold time is >24 hours.


 Gratings: The LCS offers remote selectability among three gratings chosen from those in the list below.

Diffraction Gratings available for LCS
lines/mm blaze
blaze (Å)
l/ one-pixel Dl
40 300  4200 3900  550
 41  300  7500  6000  550
 42  300 10000   9200  550
 43  600  4000  3700  1100
 44  600  7500  6900  1100
 45  600  10000  9200  1100
 46  1200  4000  3700  2200
 47  1200  6000  5500  2200
 48  1200  7500  6900  2200

 Gratings are blazed for use in first order.
 For CC1 array, the dispersion is 15/12 that of TI1 (ratio of pixel pitch).
Note that there is considerable flexure in the grating holder. Once a grating is set up to the desired tilt, the assembly should be manually clamped to ensure wavelength stability.
Grating tilt drive cannot reach >7500Å for gratings 47 and 48. These gratings must be mounted in the rightmost grating slot, and therefore cannot be used together.


Calibration Lamps: Three calibration lamps are mounted within the ATOG and above the LCS and are usable remotely for calibration. Argon and neon can be used in situ without moving the telescope.

Calibration Lamps available for LCS
lamp name lamp type and location
argon  hollow cathode argon lamp
in the ATOG above LCS
neon  hollow cathode neon lamp
in the ATOG above LCS
 incandescent  incandescent lamp
projected on dome
flat-field plaque

The hollow cathode lamps have fairly unique line ratios. It is suggested that observers use the linked spectra tofind their way around.
A flat field plaque with UV-relfective paint, illuminated by quartz lamp, is available on the inside of the dome for flat fielding.
The calibration lamps offer a filter wheel for attenuation. ND0.3, 0.5, 1.0, and 2.0 filters for the lamps can be selected remotely.


 Sensitivity: TI1 chip, grating 40, 1-pixel resolution
S/N=100 on V=17.5 B-V=0 in 3.7 hours of integration



 Useful Links

 McDonald Observatory Home Page

 McDonald Observatory Observer Information