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-inch) Struve reflector at McDonald Observatory. The instrument has a resolving power R = lambda/Delta-lambda 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 A using a thinned backside illuminated Reticon 1200 X 400 CCD detector. Total wavelength coverage in a single exposure varies from roughly 500 A at lambda = 4400 A (range 4200 - 4700 A) to 2500 A at lambda = 7500 A (range 6500 - 9000 A), and the total system efficiency at lambda = 6000 A from the top of the atmosphere through the telescope, spectrograph, and CCD detector is 10 percent 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 hours) of telescope motion are less than 1/2-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.

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