The Hobby-Eberly Telescope Low Resolution Spectrograph: optical design

The Hobby Eberly Telescope (HET) is a revolutionary large telescope of 9.2 meter aperture, which is currently undergoing commissioning at McDonald Observatory. First light was obtained on December 11, 1996. Scientific operations are expected in 1998. The Low Resolution Spectrograph (LRS, a collaboration between the University of Texas at Austin, the Instituto de Astronomía de la Universidad Nacional Autónoma de México, Stanford University, Ludwig-MaximilliansUniversität, Munich and Georg-August-Universität, Göttingen) is a high throughput, imaging spectrograph which rides on the HET tracker at prime focus. The LRS will be the first HET facility instrument. The unique nature of the HET has led to interesting optical design solutions for the LRS, aimed at high performance and simplicity. The LRS is a grism spectrograph with a refractive collimator and a catadioptric f/1.4 camera. The beam size is 140 mm, resulting in resolving powers between λ /∆λ ~ 600 and 3000 with a 1 arcsec wide slit. The LRS optics were designed and partially fabricated at the IAUNAM. We present a description of the LRS specifications and optical design, and describe the manufacturing process.