The 2.5 m Telescope of the Sloan Digital Sky Survey
James E. Gunn, Walter A. Siegmund, Edward J. Mannery et al
TL;DR
The SDSS 2.5 m telescope tackles the challenge of a wide-area, multi-band optical survey by marrying a fast, wide-field Ritchey–Chrétien optical design with a large, low-distortion focal plane, enabling efficient time-delay-and-integrate imaging and concurrent spectroscopy. A dual-corrector system optimizes imaging and spectroscopy separately while preserving a common optical path, supported by a precision mechanical design, wind/light baffling, and a roll-off enclosure that minimize thermal and wind effects. The project delivers high-quality imaging (sub-arcsecond PSF across a 3° field) and robust spectroscopic performance, supported by sophisticated software, control systems, and data pipelines, culminating in a transformative data set for cosmology and extragalactic astronomy. The telescope's innovations—mass-efficient, thermally stable structure; advanced active mirror control; and integrated instrument switching—pave the way for scalable, automated survey operations and long-term legacy datasets.
Abstract
We describe the design, construction, and performance of the Sloan Digital Sky Survey Telescope located at Apache Point Observatory. The telescope is a modified two-corrector Ritchey-Chretien design which has a 2.5-m, f/2.25 primary, a 1.08-m secondary, a Gascoigne astigmatism corrector, and one of a pair of interchangeable highly aspheric correctors near the focal focal plane, one for imaging and the other for spectroscopy. The final focal ratio is f/5. The telescope is instrumented by a wide-area, multiband CCD camera and a pair of fiber-fed double spectrographs. Novel features of the telescope include: (1) A 3 degree diameter (0.65 m) focal plane that has excellent image quality and small geometrical distortions over a wide wavelength range (3000 to 10,600 Angstroms) in the imaging mode, and good image quality combined with very small lateral and longitudinal color errors in the spectroscopic mode. The unusual requirement of very low distortion is set by the demands of time-delay-and-integrate (TDI) imaging; (2) Very high precision motion to support open loop TDI observations; and (3) A unique wind baffle/enclosure construction to maximize image quality and minimize construction costs. The telescope had first light in May 1998 and began regular survey operations in 2000.