SDSS 20-inch Photometric Telescope

General

The 20-inch SDSS photometric telescope is a classical Cassegrain with a parabolic primary and hyperbolic secondary.  Such telescopes have small fields so Shu-i Wang at Yerkes Observatory designed a corrector system of two concave-skyward meniscus lenses made of fused silica to expand the field to almost a degree in  diameter, the diagonal of the CCD detector.  The image size, about 1 arc second FWHM, would not be considered good for most professional applications but the detector pixels are large (about 1.15 arc seconds across) and are a good match for the modified telescope.

The primary mirror is made of Pyrex and has a bare aluminum coating maintained and applied at the National Solar Observatory coating facilities in Sunspot, NM. The secondary, also of Pyrex, has a protected aluminum coating applied by Denton Vacuum. The two corrector lenses are made of fused silica and were manufactured by Coastal Optical. They have broad band antireflection coatings from QSP similar to that on the SDSS 2.5 m telescope first corrector. The second corrector also serves as the vacuum window on the detector cryostat. The CCD is a SITe 2048x2048 device with 24 micron pixels and a UV antireflection coating.

The new wide field design changed the telescope f-ratio from f/8.0 to f/8.8 and added a significant amount of field distortion.  The optical prescription (in inches) is shown in the tables below. Details of the field distortion and the Zemax file are also here.

Prescription

Field Distortion

Stray Light

Zemax file

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