The Sloan Digital Sky Survey data acquisition system, and early results Page: 3 of 6
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The Sloan Digital Sky Survey Data Acquisition System, and Early Results
J. Annis, J. Bakken, D. Holmgren, D. Petravick, R. Rechenmacher
Fermilab, P.O. Box 500, Batavia, Illinois 60510
AbstractThe Sloan Digital Sky Survey will systematically map one-
quarter of the sky, producing detailed images in five color
bands and determining the positions and absolute bright-
nesses of more than 100 million celestial objects. It will also
measure the redshifts of a million selected galaxies and of
100,000 quasars, yielding a three-dimensional map of the
universe through a volume one hundred times larger than
that explored to date. The SDSS collaboration is currently
in the process of commissioning the 2.5-meter survey tele-
scope. We describe the data acquisition system used to
record the survey data. This system consists of twelve sin-
gle board computers and their associated interfaces to the
camera and spectrograph CCD electronics, to tape drives,
and to online video displays, distributed among several
VME crates. A central UNIX computer connected to the
VME crates via a vertical bus adapter coordinates the sys-
tem and provides the interface to telescope operations. We
briefly discuss results from the observing runs to date and
plans for the archiving and distribution of data.
I. INTRODUCTION
The Sloan Digital Sky Survey (SDSS) will image 7r stera-
dians about the north galactic cap in five filters and ac-
quire one million spectra using dedicated telescopes at the
Apache Point Observatory in New Mexico. We describe
the data acquisition system for the survey's three main in-
struments: the 54-ccd imaging camera [1], the 660-fiber
twin spectrographs which use 4 ccds, and the single-ccd
photometric calibration camera.
II. INSTRUMENTS
The SDSS uses facilities and two dedicated telescopes
at the Apache Point Observatory near Alamogordo, New
Mexico. Imaging and spectroscopic data are obtained on
the SDSS 2.5m telescope, which images an area on the
sky equivalent to about 30 full moons (3 degrees) onto a
1-m focal plane. The SDSS 0.5m photometric calibration
telescope continuously measures standard stars during ob-
servations in order to determine photometric calibrations.
The spectrographs and photometric calibration camera
do not impose any extraordinary system requirements. On
the other hand, the SDSS imaging camera is an extraordi-
nary instrument, employing 54 ccds and producing data at
an aggregate rate of 9 Mbyte/second. The imaging camera
drives the data acquisition requirements and is shown in
figure 1. The camera contains a photometric array consist-
ing of 30 2048 x 2048 ccds arranged in 6 columns of 5 de-
tectors each. Photometric filters of five different passbands
Work supported by the U.S. Department of Energy under contract
No. DE-AC02-76CH03000.SOSS CAMERA
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Figure 1. SDSS Imaging Camera
are attached above each ccd to a corrector element; thus,
the camera simultaneously images the sky in five colors.
The remaining 24 ccds make up the camera's astrometric
array. Each is 400 x 2048 pixels in size and is covered with
a neutral density filter. These chips will not saturate when
imaging bright astrometric standard stars, whose positions
are well known. Two of the astrometric arrays ccds serve as
focus chips; these are mounted about 200 microns behind
the focal plane, and are half-covered with a window of 400
microns optical path thickness. A comparison of images be-
tween the two halves will yield a differential measurement
of the focus.
The imaging camera is controlled over a serial line and
generates its own internal timing. The ccds are operated
in Time Delayed Integration mode (TDI). The tracking of
the 2.5m telescope is controlled such that, combined with
the effect of the rotation of the Earth, the image of the
sky moves vertically down the rows of the ccd detectors
at precisely the rate at which ccd scan lines are readout.
After each scan line is converted by the camera electronics,
the pixel data are are transmitted to the data acquisition
system over 10 fibers: one fiber per column of photometric
ccds, and one fiber per row of astrometric ccds.
Imaging data are recorded in their entirety and are
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J. Annis, J. Bakken, D. Holmgren, D. Petravick, R. Rechenmacher. The Sloan Digital Sky Survey data acquisition system, and early results, article, June 29, 1999; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc786439/m1/3/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.