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Sky coverage

The DR4 contains that part of the SDSS imaging and spectroscopy taken through July 2004. Here we give coverage plots and the detailed coverage tables from which they were constructed. Further below, we outline the future sky coverage of SDSS. The survey terms are explained in the sections on imaging coverage and spectroscopic coverage below.

For data interfaces, see the data access page.

Equatorial imaging spectroscopy
Imaging (ps) Spectroscopy (ps)
Galactic imaging coverage spectroscopy coverage
Imaging (ps) Spectroscopy (ps)

Coverage tables

For reference, the coverage tables describe the sky coverage in terms of survey coordinates. To translate the sky coverage into celestial coordinates, please refer to the coordinates section of the EDR paper and the description below.

Note: The coverage tables, and a number of other files we distribute, are given as ASCII parameter (.par) files.

SDSS entire survey atStripeDef.par (survey coordinates)
Best tsChunk.dr4.best.par (great circle coordinates) - see Target and Best below
Target tsChunk.dr4.target.par (great circle coordinates) - see Target and Best below
Spectroscopy DR4 for the first time contains additional plates beyond the main survey plates: "extra" plates which are reobservations of main survey plates; "special" plates which take spectra beyond the main survey targets, and "extraspecial" reobservations of special plates. There are 5 coverage files: one for all the plates, and 4 for each of the categories.
The coverage files give plate centers by plate number and MJD of observation.
All platesalldr4spectro.par
Main survey plates maindr4spectro.par
Extra plates extradr4spectro.par
Special plates specialdr4spectro.par
Extraspecial plates extraspecialdr4spectro.par

Note also the list of list of quality "holes" (small areas of bad seeing etc.) and missing data, the list of fields which are missing only apparently (e.g., data from different runs were used in target and best) and the list of differences in sky coverage between target and best.

Imaging coverage: stripes, runs, reruns

The survey coverage is defined in survey coordinates, as described in the coordinates section of the EDR paper. For detailed astrometry please see the following note about parts of the survey which have a slightly different survey coordinate system from the rest.


The imaging survey scans the sky along great circles, which are circles of constant survey latitude eta. Scans are obtained along stripes spaced 2.5° in survey latitude. Each stripe has an integer stripe number n such that the inclination of a stripe with respect to the celestial equator is -25° + 2.5° × n. The boundaries of the region eventually to be surveyed by the SDSS are defined in terms of survey longitude lambda in atStripeDef.par.

Runs cover strips, two strips cover a stripe

An imaging run designates a continuous scan of the imaging telescope. The survey camera's focal plane has six columns of CCDs, so that a single run consists of six camera columns (camcols) (see the instrument descriptions). The camera columns have a gap which is filled by an offset second run. The six camcols taken together are called a strip. Thus to completely cover the two strips making up one stripe, at least two runs are required. The imaging data reduction is done on a run-by-run basis, hence the imaging data in our Data Archive are accessible by run number.

Great circle coordinates

Every stripe has its own great circle coordinate system. The coordinate along a great circle is called mu. The range of data actually obtained for a given stripe is reported in terms of a range in great circle mu along that stripe as startMu and endMu in the coverage tables above. Both startMu and endMu are given in integer arcseconds.

The calibrated object lists are accessible in the archive by stripe number and startMu on that stripe.

For historical reasons, DR1 contained both photometric and spectroscopic data which lay outside the official survey limits, i.e., the startMu/endMu for some runs extend beyond the lambda limits for the stripes they cover. Since DR3, only the target data in those areas are still marked non-primary, but the best data are primary. Please refer to this note about DR4 data outside the SDSS survey area for details on how to retrieve these.


A run is divided into images 2048 pix × 1361 pix which are called fields. Each field then has the first 128 rows of the following field attached to it, so that all survey images actually have a size of 2048 pix × 1489 pix.

Object detection, photometry and classification are done field by field. In particular, the calibrated object lists and survey images are distributed separately for each field. To account for the overlap, object detection is not performed on the first 64 rows or the last 64 rows of each corrected frame.

Note that one therefore should use only objects in the "primary" area of each survey field, in order to avoid duplicate area and duplicate object detections. Near the center of a stripe, the primary area will be the full 2048 pix × 1361 pix, but frames overlap near the ends of stripes, reducing the primary area there. Please refer to how "primary" is set in the "status" flag of an object and please also see the details of how overlapping areas are resolved. The primary area of each field is recorded as entry stripeArea in the tsField*.fit file


Every time we (re)process our imaging data, we assign the outputs a new rerun number. Thus, a rerun does not imply re-observation, but is simply a re-reduction of the same run. The re-reduction can use more recent software or calibrations than a previous rerun, or both. Across runs, the rerun is an arbitrary index, so the same rerun of different runs does not have to be produced with the same software or calibrations.

Target and Best

We publish two reruns for each run:
  1. A target rerun, which is the reduction used to select targets for the spectroscopic survey. This is referred to as skyVersion = 0.
  2. A best rerun, which is the best available reduction. This is usually a different rerun from the target version, since our photometric pipelines have been evolving. The best rerun is referred to as skyVersion = 1. All best reruns have rerun >=40.
There is one coverage table each for the target and best version. The tables contain the following information:
  • The survey area on the sky, given as a stripe number and a range for the mu coordinate (startmu <= mu <= endmu). These were used to create the plots above.
  • The runs and reruns and the range of fields which cover the given area. This is the information needed to find data for a given part of the sky in the Data archive.


The spectrograph's 640 fibers are distributed on a plug plate of roughly 3° diameter (7 square degree area) projected on sky. Spectroscopic coverage is given in terms of the plate centers. Plates are identified by a unique number assigned during targetting and the MJD on which they were observed, to distinguish re-observations of the same plate.

Incremental sky coverage of SDSS

A map of the incremental coverage of the last three Data Releases by DR number.

Future sky coverage of SDSS

Future SDSS sky coverage

Current and anticipated sky coverage of SDSS (Aitoff projection of equatorial coordinates as above, but centered at RA=120).

The plot above shows the DR3 and the anticipated sky coverage of the spectroscopic survey (also see the SDSS data release schedule). The solid outline shows the original survey ellipse in the North Galactic Cap as defined above. Dark areas show the coverage of DR3. Light areas show spectroscopic sky coverage expected at the end of the currently funded survey in June 2005 (to be relased in full in 2006). As DR3 includes only data collected up through June 2003, data for part of the light areas are alrady in hand. The imaging survey leads the spectroscopic survey and will therefore not leave a gap in the sky coverage of the North Galactic Cap. Filling the gap in the spectroscopic survey is contingent upon the availability of funding for survey operations beyond June 2005.

Last modified: Tue Jul 26 21:47:32 CDT 2005