Observations
SDSS uses a dedicated 2.5-m wide-angle optical telescope, and takes images using photometric system of five filters (named u, g, r, i and z). These images are processed to produce lists of objects observed and various parameters, such as whether they seem pointlike or extended (as a galaxy might) and how the brightness on the CCDs relates to various kinds of astronomical magnitude.
The SDSS telescope uses the drift scanning technique, which keeps the telescope fixed and makes use of the Earth's rotation to record small strips of the sky. The image of the stars in the focal plane drifts along the CCD chip, instead of staying fixed as in tracked telescopes. This method allows consistent astrometry over the widest possible field and precision remains unaffected by telescope tracking errors. The disadvantages are minor distortion effects and the CCD has to be written and read in the same time.
The telescope's camera is made up of thirty CCD chips each with a resolution of 2048×2048 pixels, totaling approximately 120 Megapixels. The chips are arranged in five rows of six chips. Each row has a different optical filter with average wavelengths of 355.1, 468.6, 616.5, 748.1 and 893.1 nm, with 95% completeness in typical seeing to magnitudes of 22.0, 22.2, 22.2, 21.3, and 20.5, for u, g, r, i, z, respectively. The filters are placed on the camera in the order r,i,u,z,g. To reduce noise the camera is cooled to 190 kelvin (about −80° C) by liquid nitrogen.
Using these photometric data, stars, galaxies, and quasars are also selected for spectroscopy. The spectrograph operates by feeding an individual optical fibre for each target through a hole drilled in an aluminum plate. Each hole is positioned specifically for a selected target, so every field in which spectra are to be acquired requires a unique plate. The original spectrograph attached to the telescope was capable of recording 640 spectra simultaneously, while the updated spectrograph for SDSS III can record 1000 spectra at once. Over the course of each night, between six and nine plates are typically used for recording spectra.
Every night the telescope produces about 200 GB of data.
Read more about this topic: Sloan Digital Sky Survey
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