Sky coverage
The DR1 contains that part of the SDSS imaging and spectroscopy
taken through July 2001. Here we give coverage plots and the detailed
coverage tables from which they were
constructed. The survey terms are explained in the sections on imaging coverage and spectroscopic coverage below.
The quickest way to check whether SDSS data are available for a
list of objects is the DR1 footprint
server. You can also use the SDSS finding
chart to check the survey coverage. For other interfaces, see the
DR1 data access page.
Coverage tables
For reference, the coverage tables describe the sky coverage of DR1 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.
Note also the list of list of quality
"holes" and missing data, the list of runs differing for target and
best, and the list of fields which are meant
to be missing.
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.
Stripes
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 along a stripe. 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.
The calibrated object lists are accessible in the archive by stripe
number and startMu on that stripe.
For historical reasons, DR1 contains both photometric and
spectroscopic data which lie outside the official survey limits, i.e.,
the startMu/endMu for some runs extend beyond the
lambda limits for the stripes they cover. Please
refer to this note about DR1 data
outside the SDSS survey area for details on how to retrieve
these.
Fields
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
Reruns
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
contained in DR1:
- A target rerun, which is the reduction used to select
targets for the spectroscopic survey. This is referred to as
skyVersion = 0. All target reruns in
DR1 have rerun < 20.
- 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 DR1 best reruns
have rerun >=20.
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.
Notes
Spectroscopy
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.
Last modified: Tue Apr 15 19:02:13 CDT 2003
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