Up: Data Products Sections: Images - Object lists - Spectra - Tiling
Getting and using images
The Data Archive Server provides the survey images, called
"corrected frames", as
fpC*.fits files. See the fpC data model.
The Catalog Archive
Server serves 3-color jpeg images generated from the g, r,
i images as finding charts, cutouts for object lists and for
point-and-click navigation of the sky.
The data access page contains various
query forms to get images by coordinates, or to search for
objects from the imaging and spectroscopic catalogs by redshift,
object magnitude, color etc., and to retrieve the corresponding
data from the archive.
There is a separate set of
files, containing the "postage-stamp" images for individual objects
from the photometric object
lists. See how to read an atlas
New for DR7: Runs and Stripe 82 Databases
The DR7 Catalog Archive Server contains the imaging
RUNS database, separate from the DR7 (Legacy + SEGUE) database. This database contains 530
imaging scans, some of which are not present in the main DR7 database, and thousands imaging
fields from the beginnings and ends of DR7 strip scans which did not resolve into a complete
rectangular stripe on the sky.
The DR7 Catalog Archive Server contains the Stripe82
database, distinct from the DR7 and RUNS databases, containing all imaging from SDSS stripe 82
along the Celestial Equator in the Southern Galactic Cap (Stripe 82). Subsets of the 303
individual stripe 82 scans were made public in previous data releases, but with DR7, we now make
a coaddition (the Stripe 82 coadd) of about 47 South strip and 55 North strip scans into
a scan in which objects are detected and cataloged about 2 magnitudes deeper than in any
individual stripe82 scan. It includes a total of 303 runs (plus 2 coadd runs), covering any
given piece of the approx. 270 deg2 area approximately 80 times.
All 303 runs covering Stripe 82 are part of the Stripe 82 database, which can
be queried to search for variability (and propermotion, with a bit more effort). Only about
one-quarter of the Stripe 82 scans were obtained in photometric conditions, the rest were taken under
variable clouds and often poorer than normal seeing. For those runs which are non-photometric, an
approximate calibration, using the photometric frames as reference, is derived and available in the CAS
stripe82 database, for objects with RA > 310 or RA < 59 degrees.
Overestimation of sky levels near
Because of scattered light (see the Early Data Release paper [Stoughton
et al. 2002]), the background sky in the SDSS images is
non-uniform on arc-minute scales. The photometric pipeline determines
the median sky value within each 100" square on a grid with
50" spacing, and bilinearly interpolates this sky value to each
pixel. This biases the sky bright near large extended galaxies. As was already reported
in the Data
Release 4 paper and in
Mandelbaum et al. 2005, this effect causes a systematic decrease in the number density
of faint objects near bright galaxies. In addition, it also strongly
affects the photometry of the bright galaxies themselves, as has been
reported by Lauer et
al. (2007), Bernardi et
al. (2007), and Lisker et
al. (2007). This effect was overestimated in Data Release 6 but has been corrected for
Data Release 7.
We have quantified this effect by adding simulated galaxies with
exponential or de Vaucouleurs profiles to SDSS images. The simulated
galaxies ranged from apparent magnitude mr=12 to
mr=19 in half-magnitude steps, with a one-to-one
mapping from mr to Sérsic half-light radius
determined using the mean observed relation between these quantities
for Main sample galaxies with exponential and deVaucouleurs profiles.
Axis ratios of 0.5 and 1 were used, with random
position angle for the non-circular simulated galaxies.
Results in the r band are shown in the figure below, which
shows the difference between the input magnitude and the model magnitude
returned by the SDSS photometric pipeline as a function of r magnitude for
galaxies with both exponential and deVaucouleurs profiles. (Figure and text from
Difference between measured model and true r-band magnitudes of a
series of simulated galaxies with Sérsic index of 1 (disk galaxies; upper
panel) and 4 (elliptical galaxies; lower panel).
Bad CCD columns Some chips have bad CCD columns
which get interpolated over by the photometric pipeline, leading
to noticeably correlated noise. The bad columns for each run are
currently available in fpM*.fits. These files can be
found on the Data Archive Server in the
subdirectory of each
run/rerun directory (e.g., http://das.sdss.org/imaging/1740/40/objcs/1/. See
how to read fpM*.fits
Very red objects
The u filter has a natural red leak around 7100 Å
which is supposed to be blocked by an interference coating. However,
under the vacuum in the camera, the wavelength cutoff of the
interference coating has shifted redward (see the discussion in the
EDR paper), allowing some of this red leak through. The extent of
this contamination is different for each camera column. It is not
completely clear if the effect is deterministic; there is some
evidence that it is variable from one run to another with very similar
conditions in a given camera column. Roughly speaking, however, this
is a 0.02 magnitude effect in the u magnitudes for mid-K
stars (and galaxies of similar color), increasing to 0.06 magnitude
for M0 stars (r-i ~ 0.5), 0.2 magnitude at r-i ~
1.2, and 0.3 magnitude at r-i = 1.5. There is a large
dispersion in the red leak for the redder stars, caused by three
- The differences in the detailed red
leak response from column to column, beating with the complex red
spectra of these objects.
- The almost certain time variability of the red leak.
- The red-leak images on the u chips are out of focus and are
not centered at the same place as the u image because of
lateral color in the optics and differential refraction - this means
that the fraction of the red-leak flux recovered by the PSF fitting
depends on the amount of centroid displacement.
To make matters even more complicated, this is a detector
effect. This means that it is not the real i and
z which drive the excess, but the instrumental colors
(i.e., including the effects of atmospheric extinction), so the leak
is worse at high airmass, when the true ultraviolet flux is heavily
absorbed but the infrared flux is relatively unaffected. Given these
complications, we cannot recommend a specific correction to the
u-band magnitudes of red stars, and warn the user of these
data about over-interpreting results on colors involving the
u band for stars later than K.
There is a slight and only recently recognized downward bias in the
determination of the sky level in the photometry, at the level of
roughly 0.1 DN per pixel. This is apparent if one compares
large-aperture and PSF photometry of faint stars; the bias is of order
29 mag arcsec-2 in r. This, together with
scattered light problems in the u band, can cause of order
10% errors in the u band Petrosian fluxes of large
u3 half-bad for runs 2188,2189,2190
In early 2001, the u band CCD of CamCol 3 suffered
from an electronics problem in its 'left' amplifier,
resulting in missing u band data in half the u-band
CCD in CamCol 3 (only) for runs 2188, 2189, 2190 (only).
The problem was then corrected. This data remains
missing. The problem can be seen by downloading
a u3 fpC or drC corrected frame from
the DAS from one of
these runs. There is no work around other than
not using the u-band data in this region.
*Text and figures on this page come from an author-created, un-copyedited
version of the SDSS Data Release 7 paper, an article submitted
to Astrophysical Journal Supplements. IOP Publishing Ltd is not responsible for any errors
or omissions in this version of the manuscript or any version derived from it. A preprint of the
DR7 paper is available from the arXiv preprint server.
Last modified: Thu Oct 18 09:21:04 CEST 2007