SAO Reticon Echelle Spectrographs
The SAO echelle reticons are crossdispersed echelle spectrographs using Reticon detectors. Each photon-counting Reticon is a 2x936 pixel self-scanned photodiode array, coupled to a 3-stage image tube intensifier package, and then fed through the video amplifiers and electronics to the photon event discriminator, FIFO, and computer. The Reticon is cooled to about -40C from a Neslab chiller and re-circulating methanol bath.
Each spectrograph has an internal Th-Ar lamp used for wavelength calibrations, and an internal incandescant lamp used for flat field exposures.
Only one echelle order is viewed by the reticon detector. The standard setting for our CfA programs and reduction routines is the order centered near 5187A, and covers a 50A bandpass. It is possible to set the spectrograph to view other orders, however the system efficiency is poor in the blue.
Spectral resolution (R) is about 20,000 as measured from spectra at 5177A. Echelle spectrographs on 60" telescopes yield radial velocity precisions of ~1km/sec. These spectrograph can be used to observe stars down to a magnitude of 13.0
The echelle grating is ruled at 79 lines per millimeter. We have 2 cross-dispersion gratings, a 300-line/mm and 1200-line/mm. The 300-line grating is used for the standard 5187 setup. The spectrograph slit is 80x500 microns, corresponding to ~1.5 arcsec by 9 arcsec on the sky.
The photon event discriminator and FIFO are located in the computer room, on the 60" equipment rack. The image intensifier and MCP controls are located on the north side of the telescope tube assembly.
The instrument exhibits significant flexure as the telescope is slewed to different parts of the sky. Therefore the slit plate is on a motorized stage that is controlled from the observing room. The slit postion is centered on the spectrograph order before beginning each observational sequence by balancing both halves of the reticon counts as displayed on the realtime system tcs. All stellar observations are preceded AND followed by a Th-Ar exposure (Th-Ar, star, Th-Ar). The power to the Th-Ar lamp and the sky/comp flip mirror are controlled by mechanical switches located on the observing desk.