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Survey instruments

The Sloan Digital Sky Survey Technical Summary (York et al 2000), available in html both at this site and that of the AJ, is the essential reference for survey instrumentation; we present only a brief overview with important parameters here. Further documentation can be found in the SDSS Project Book.


The SDSS uses a dedicated 2.5 m f/5 modified Ritchey-Chretien altitude-azimuth telescope located at Apache Point Observatory, in south east New Mexico (Latitude 32° 46' 49.30" N, Longitude 105° 49' 13.50" W, Elevation 2788m). A 1.08 m secondary mirror and two corrector lenses result in a 3° distortion-free field of view.

Imaging Camera

The imaging camera collects photometric imaging data using an array of 30 SITe/Tektronix 2048 by 2048 pixel CCDs arranged in six columns of five CCDs each, aligned with the pixel columns of the CCDs themselves. SDSS r, i, u, z, and g filters cover the respective rows of the array, in that order. The survey operates the instrument in a drift scan mode: the camera slowly reads the CCDs as the data is being collected, while the telescope moves along great circles on the sky such that images of objects move along the columns of the CCDs at the same rate the CCDs are being read. As an image of an object moves along the column of the CCDs, a CCD in each row collects data on that object. Therefore, the camera produces five images of a given object, all from the same column of CCDs, one from each CCD in that column. It takes an object 54 seconds to move from the beginning of a CCD to the end, so the effective exposure time in each filter is 54 seconds. Because there is some space between the rows of CCDs it takes an image 71.7 seconds to move from the beginning of one row to the next. Each row corresponds to a different filter, so each object has one image in each filter, taken at 71.7 second intervals.

An additional 24 CCDs placed before and after the photometric CCDs collect astrometric data. Neutral density filters cover these CCDs, and allow collection of data on bright reference stars without saturation.

Because there is space between the columns of CCDs, two passes along a great circle are required to obtain a solid area. The second pass is offset from the first such that the area that fell between columns in the first pass is images in the center of the columns in the second pass.


The SDSS uses two spectrographs, each of which has a blue channel and a red channel separated by a dichroic filter. 640 fibers (320 fibers per spectrograph) feed light from the focal plane into the spectrographs. A total of four 2048 × 2048, SITe/Tektronix CCDs (one for each channel of each spectrograph) collect the spectra.

Photometric Telescope (PT)

A single SITe/Tektronix 2048 by 2048 pixel CCD mounted on a separate telescope with a 20" aperture provides the data necessary for photometric calibration. Two corrector lenses expand the field of view of this classical Cassegrain telescope to almost a full degree. The PT has an effective focal ratio of f/8.8. The attached camera has a pixel scale of 1.15 arcseconds/pixel, and a field of view 39.2 arcminutes on a side. The photometric telescope periodically observes fields containing primary standards throughout a night of observing, providing a photometric solution for the night. The remaining time is used to observe secondary patches.

Last modified: 2003-03-19