THE SANDIFORD 2.1-M CASSEGRAIN ECHELLE SPECTROGRAPH FOR MCDONALD OBSERVATORY - OPTICAL AND MECHANICAL DESIGN AND PERFORMANCE

We describe the design, construction, and performance of an efficient new flexure-compensated Cassegrain echelle spectrograph for the 2.1-m (82-in.) Struve reflector at McDonald Observatory. The instrument has a resolving power R = lambda/DELTAlambda of 60,000 for two CCD pixels (for a reciprocal velocity dispersion of 2.5 km s-1 per pixel) and provides continuous wavelength coverage for lambda < 8000 angstrom using a thinned backside-illuminated Reticon 1200 X 400 CCD detector. Total wavelength coverage in a single exposure varies from roughly 500 angstrom at lambda = 4400 angstrom (range 4200-4700 angstrom) to 2500 angstrom at lambda = 7500 angstrom (range 6500-9000 angstrom), and the total system efficiency at lambda = 6000 angstrom from the top of the atmosphere through the telescope, spectrograph, and CCD detector is 10% or more. The mechanical design of the Cassegrain-mounted spectrograph incorporates a unique cantilevered counterweight system designed to drastically reduce the effects of gravitational flexure. In spite of the large physical size of the Cassegrain instrument, worst-case flexure shifts over 60-degrees (4 hr) of telescope motion are less than 2 pixel and are typically on the order of 0.2 pixels or less from all sources. A subsequent paper will describe the CCD and associated electronics in detail.