The SDSS Data Release 7 (DR7)
The Sloan Digital Sky Survey (SDSS) consists of a series of three
interlocking imaging and spectroscopic surveys, carried out over an eight-year
period with a dedicated 2.5m telescope located at Apache Point Observatory in
Southern New Mexico. This seventh data release (DR7) from the SDSS
represents the completion of this project.
The three surveys that make up the SDSS are:
- Legacy, an imaging survey in five bands over a contiguous
7646 deg2 high-latitude elliptical region in the Northern Galactic
Cap, plus an additional 750 deg2 in the Southern
Galactic Cap, together with spectroscopy of complete samples of
galaxies and quasars covering about 8200 square degrees.
The total imaging area in the Legacy survey is 8423 square degrees.
- SEGUE (Sloan Extension for Galactic Understanding and Exploration),
additional imaging of 3240 deg2 of sky at lower Galactic latitudes,
together with spectroscopy of 240,000 stars towards 200 sightlines covering
1400 square degrees (spread throughout the Legacy and SEGUE
imaging footprints), to study the structure of the Milky Way.
- Supernova, The equivalent of about 80 repeated imaging scans of the Southern
Equatorial Stripe (ra > 310 or ra < 59; -1.25 > dec < 1.25) obtained
in variable weather conditions (some clouds) to search for
supernovae in the redshift range 0.1 < z < 0.4.
The catalog derived from the images includes more than 350 million
celestial objects, and spectra of 930,000 galaxies, 120,000
quasars, and 460,000 stars. The data are fully calibrated and reduced,
carefully checked for quality, and publically accessible through
efficient databases. The data have been publicly released in a series of
annual data releases, culminating in the final data release, DR7.
Contents
New for DR7
- The Data Archive Server (DAS) for DR7 has a new interface, offering users the ability to browse all the imaging
runs and spectra by run or plate number. Color (g-r-i) pictures of 1.3M
imaging fields and wavelength-flux plots of each of 1.6M spectra are available.
Bulk download access to over 70TB of raw and processed data are
available via wget or rsync interfaces.
- DR7 contains 577 new plates of spectra beyond DR6 (277 new Legacy, 285 SEGUE, 15 other) for a total of 2564 plates and 1,640,960 spectra (including
sky and unidentified objects). All SDSS spectroscopic plates are available from the
Spectroscopic Plates page of the DAS.
- DR7 contains 3240 square degrees of SEGUE lower-latitude and Southern imaging. SEGUE imaging is available as FITS files
from the Data Archive Server; you can upload a list of coordiantes and download the appropriate files.
- DR7 contains the complete Legacy and SEGUE imaging catalogs in the same CAS SQL database as all the Legacy and SEGUE spectra,
simplifying cross-id searches. All these catalog data are available from the Catalog Archive Server (CAS).
- DR7 CAS contains 'propermotions' table with information for over 205M
Legacy and SEGUE objects, derived from the UCAC-2 and UCAC-3 USNO astrometric catalogs,
with the alignment re-calibrated against the SDSS imaging positions. The parameters stored in this table are available in the
CAS Schema Browser entry for the propermotions table.
- DR7 CAS contains updated (from DR6) astrometry solutions for most objects, using the UCAC-3
catalog when available, and replacing most Tycho-2-based solutions with UCAC-based solutions, which have more matches and include fainter
objects. The rms difference in coordinate positions is less than 20 mas for nearly all imaging fields.
- DR7 CAS contains an all new sppParams table with updated radial velocities of over 460,000 stars and estimates of [Fe/H], log g
(surface gravity), effective temperature and stellar type classification information for over 300,000 stellar spectra
(sppParams table entry in the CAS
Schema Browser).
- DR7 CAS and DAS contains "ubercalibrated" magnitudes for all
objects as the default magnitude selection (psfmag_[ugriz], petromag_[ugriz], etc). The target version of the calibrated magnitudes are
still available in a separate table.
- DR7 CAS 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 of imaging fields from the beginnings and ends of DR7 strip scans, which did not
resolve into a complete rectangular stripe on the sky. All photometry from the RUNS database is available in the
RUNS Catalog Archive Server (RUNS CAS).
- DR7 CAS 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 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.
- For DR7, as part of the SEGUE effort, the spectroscopic pipeline was enhanced to obtain more
accurate radial velocities of stellar objects. The plate-to-plate systematic velocity variations are approximately 2 km/s. Including random
scatter and systematics, the overall velocity accuracy of stars has a sigma of 5.5 km/s at g=18.5 mag, degrading to about 10 km/s at g=19.5
and 20 km/s at SEGUE's limit of g=20.3. Over-zealous cosmic ray rejected removed some real emission line features in spectra of narrow emission
line galaxies in DR6. This effect has largely been corrected in DR7. Spectrophotometry has been
recalculated for all objects in DR7, and is good to levels of 10% or better over the full spectral coverage range of 3900-9100 Ångstroms.
The accurate spectrophotometric calibration is a unique feature of the SDSS and SEGUE spectroscopic surveys, present in no other large spectroscopic
dataset.
- With SEGUE's addition of several thousand square degrees of low-latitude (|b| < 30 deg) imaging, the standard high-latitude object detection
and measurement code was tested in very crowded fields. In these regions, the standard PHOTO pipeline was augmented by a version of the
PanSTARRS object detection pipeline. PanSTARRS-assisted PHOTO also was optimized for stellar PSF measurement and object deblending in
low-latitude SEGUE fields. The accuracy of the crowded field photometry approached that of the Legacy survey area, except in regions of
extreme crowding (such as the cores of globular clusters). Catalogs of objects processed with this special code are marked in the database
with a rerun number of 648 and are available in the DR7 CAS and the DR7 DAS.
- Updated photo-zs are available in the
PhotoZ tables of the CAS. The photoz table is current through DR7,
using ubercalibrated magnitudes, and it contains information on nearest neighbor galaxies in a volume to allow analysis of photometric redshift
statistics. The PhotoZ2 table, an independent method of deriving
photometric redshifts, is up to date through DR6.
- DR7 CAS contains a number of plates of spectra targeted on known globular and open clusters. The spectra on these plates identified with
programnames in the plateX table of the form 'segcluster%' and are
used to calibrate the SSPP (SEGUE stellar parameter pipeline) code that derives [Fe/H], log g, and Teff for each star.
Except for the addition of new data and the changes in the
spectroscopic, imaging, and sspp pipelines described above, the pipelines
and databases are essentially identical in
DR7, DR6, DR5, DR4, DR3 and DR2. Thus, DR7 is a proper
superset of DR6, which is a
superset of DR5, etc. The DR2 included reprocessing of all data
included in DR1, and those data in EDR that pass our data-quality
criteria for the official survey.
What DR7 contains
The DR7 imaging data cover about 8423 square degrees of
"legacy" sky, with information on roughly 230 million
distinct photometric objects, and about 3240 square degrees of SEGUE
sky, with about 127 million distinct objects (including many stars
at low latitude). The DR7 spectroscopic data
include data from 1802 main survey plates of 640 spectra each, and
cover 8200 square degrees. In addition, DR7 contains 762
"extra" and "special" plates:
- 86 "extra" plate/MJD combinations which are repeat
observations of 77 distinct main survey plates
- 676 distinct "special" plates, which includes 410
SEGUE plates, and about 270 plates with
observations of spectroscopic targets, mostly in the southern galactic cap, which
were selected by the collaboration for a series of specialized science
programs. A few of these plates are outside of the
regular DR7 imaging area.
- 15 "extraspecial" repeat observations of
"special" plates (7 of SEGUE plates, and 8 of other
special plates)
There is a separate page describing the special plates in
DR7
The DR7 footprint is defined by all non-repeating survey-quality
imaging runs within the a priori defined elliptical survey area in the
Nothern Galactic Cap, and three stripes in the Southern Galactic Cap
obtained prior to 14 July 2008, and the spectroscopy associated with
that area as well as the extra and special plates
obtained before that date.
In fact, about 34 square degrees of imaging data
in the Nothern Galactic Cap lie outside this ellipse.
While the DR7 scans do not repeat a given area of sky, they do
overlap to some extent, and the data in the overlaps are included in
earlier releases as well. The sky coverage of the imaging and
spectroscopic data that
make up DR7 are given on the coverage
page. The natural unit of imaging data is a run; the DR7 contains
data from 362 runs in the best database, 530 in the RunsDB,
and 305 runs in the Stripe82DB, including 2 coadd runs 106 and 206
(the coadd is the co-addition of about 50 repeated North and 50 South scans of stripe 82).
A total of 183 square degrees of sky are different runs between target
and best, the majority along the Equatorial Stripe in the Fall sky.
We also make available images and associated catalogs from
three categories of special runs in the DAS only. A subset of runs in
DAS ALL imaging contains the DAS-only data release, and catalogs are not present in the
DR7 CAS, though some (not all) are available in the RUNS CAS database.
These runs are indicated by a non-standard stripe
number, such as 100, 205, 61, 62, 301, or 293, 90. In DR6, this
data was referred to as DR supplemental.
They are:
- Scans through or near M31 (run 3366,3367, stripe 293) and the Perseus Cluster (stripes 61,62);
- Sagittarius tidal stream scans (stripe 205) and scans near the South Galactic Pole (stripe 90);
- Scans taken at low Galactic latitude as part of the SEGUE project (plate 797, stripe 301, stripe 100); these runs were used to target stars on
special plates.
Imaging caveats
The following caveat is new to DR7
Systematic errors in the photometry of bright galaxies
There is a subtle systematic effect present in the photometry of
faint galaxies in the wings of very bright extended galaxies.
The effect is due to the way the DR7 version of the PHOTO treats
sky subtraction; in particular, the binning of sky over large regions
affected by extended wings of low-surface-brightness galaxies.
This effect has been carefully documented, and it can be removed
in a statistical sense for weak-lensing and other galaxy clustering
studies. See the Imaging Caveat on
Overestimation of sky
levels near bright galaxies for more detail.
The following caveats apply unchanged to DR7
Red leak to the u filter and very red objects
Bias in sky determination
Zeropoint of the photometric system
The SDSS photometry is intended to be on the AB system (Oke
& Gunn 1983), by which a magnitude 0 object should have the
same counts as a source of Fnu =
3631 Jy. However, this is known not to be exactly true, such that the
photometric zeropoints are slightly off the AB standard. We continue
to work to pin down these shifts. Our present estimate, based on
comparison to the STIS standards of Bohlin,
Dickinson, & Calzetti~(2001) and confirmed by SDSS photometry and
spectroscopy of fainter hot white dwarfs, is that the u
band zeropoint is in error by 0.04 mag, uAB =
uSDSS - 0.04 mag, and that g, r, and
i are close to AB. These statements are certainly not
precise to better than 0.01 mag; in addition, they depend critically
on the system response of the SDSS 2.5-meter, which was measured by
Doi et al. (2004, in preparation). The z band zeropoint is
not as certain at this time, but there is mild evidence that it may be
shifted by about 0.02 mag in the sense zAB =
zSDSS + 0.02 mag. The large shift in the
u band was expected because the adopted magnitude of the
SDSS standard BD+17 in Fukugita
et al.(1996) was computed at zero airmass, thereby making the
assumed u response bluer than that of the USNO system
response.
Holes in the imaging data
About 0.3% of the DR7 imaging footprint area
(about 25 square degrees) for DR7 are marked as
holes. These are indicated in the CAS by setting
quality=5 (HOLE) in the tsField file and
field table and given in the list of quality holes, which
contains further details about the holes and quality flags, including
a information about a table in the CAS which allows
one to query for quality information about each field of data.
Problems with one u chip
The u chip in the third column of the camera is read out
on two amplifiers. On occasion, electronic problems on this chip
caused one of the two amplifiers to fail, meaning that half the chip
has no detected objects on it. This was a problem for only two of the
105 imaging runs included in DR5: run 2190, which includes a total of
360 frames in two separate contiguous pieces on strip 12N (centered
roughly at delta = +5 degrees in the North Galactic Cap; NGC), and run
2189, which includes 76 frames on stripe 36N near the northern
boundary of the contiguous area in the NGC. The relevant frames are
flagged as bad in the quality flag; in addition,
individual objects in this region have the u band flagged
as NOTCHECKED_CENTER (or, for objects which straddle the
boundary between the two amplifiers, LOCAL_EDGE). Richards
et al (2002) describe how the quasar selection algorithm handles
such data; the net effect is that no quasars are selected by the
ugri branch of the algorithm for these data.
Spectroscopy caveats
The following caveats from DR6 have been corrected in DR7
- Over-zealous cosmic ray rejected removed some real emission
line features in spectra of narrow emission line galaxies in DR6.
This effect has largely been corrected in DR7; see the caveat on
Clipped Spectral
Lines on the Spectra page for more information.
- In DR6, 28 plates, had a flux scale offset due to replacement of
fiber magnitudes by psf magntidues. This problem has been corrected in DR7.
- In DR6 CAS, the mag_0, mag_1, and mag_2 values in the specobjall
table were incorrectly pointing to the u, g, and r respective band
synthetic magnitudes for each spectrum. u synthetic magnitude are meaningless
since the spectral coverage does not include the u band.
In DR7 CAS, these values have been updated to point to the (correct) g, r and i
respective synthetic magnitudes.
- An instability in the spectroscopic flatfields was found in some spectra prior to DR7;
this effect has been corrected in DR7. Details on the correction are available in the
Spectrophotometry Algorithms page.
The following caveats and notes are new to DR7
- DAS Only Plates: A few Legacy and SEGUE plates did not have sufficient exposure time to reach
their target S/N in order to be declared 'done'. Also, in the case of SEGUE, 3 plates
have bright but not faint equivalents (or vice-versa). These plates do not appear in
the CAS or in the list of all plates; however, they are available in the DAS. The
plate-mjd numbers for these plates are:
plate-mjd Why not in CAS platex table:
-----------------------------------------------------------------------
0356-51779 S/N too low -- plate not done (not enough exposure)
1112-53180 S/N too low -- plate not done
1858-53271 SEGUE test plate, high DEC, never made it to CAS
2309-54441 S/N too low -- SEGUE plate
2333-53676 M71 very bright plate, lots of saturation, only 320/640 done
2535-54632 SEGUE bright, faint matching plate not done
2640-54474 BAOTest plate, test plate for SDSS-III
2716-54629 SEGUE bright, faint matching plate not done
2851-54485 SEGUE dup pointing of plate 2045.
2962-54578 don't have correct tsObj file, Low S/N, plate not done
- Some very strong emission lines appear clipped due to saturated pixels, but were not
flagged as such. This occurs very rarely, but the user should be aware of the possibility of
objects with extremely strong emission lines and unusual line ratios. Details on this caveat
are available under Clipped Spectral Lines
on the Spectra page.
The following caveats are unchanged from DR6
A small number of plates suffered from a variety of minor problems
affecting the quality of the spectrophotometry (but not of
redshifts). See the list under Plates with
problematic spectrophotometry on the data products page for
spectra.
Zero equivalent width of emission lines, especially H alpha
There is a bug in the line-measurement code that has been in use
since DR3 which gives some emission lines an equivalent width of zero,
even though there is a significant line detection. The aim of the
change introducing the bug had been to determine the equivalent width
by integrating the spectrum, instead of using the parameters of a
fitted Gaussian. The Gauss-fit equivalent width can be recovered from the
fit parameters using the usual expression EW = 2.5066 * sigma *
height / continuum.
Note about galactic extinction correction
In the EDR and DR1, the spectroscopic data were nominally corrected
for galactic extinction. The spectrophotometry since DR2 is vastly
improved compared to DR1, but the final calibrated spectra in DR2 and
beyond are not corrected for foreground Galactic reddening (a
relatively small effect; the median E(B-V) over the survey
is 0.034). Users of spectra should note that the fractional
improvement in spectrophotometry from DR1 to DR2 and beyond was much
greater than the extinction correction itself. As the SDSS includes a
substantial number of spectra of galactic stars, a decision has been
taken not to apply any extinction correction to
spectra, since it would only be appropriate for extragalactic objects,
but to report the observational result of the SDSS, namely, the
spectrum including galactic extinction.
Mismatches between the spectroscopic and imaging data
For various reasons, a small fraction of the spectroscopic objects
do not have a counterpart in the best object catalogs. In
addition, the DR7 does not contain photometric information for some of
the special plates, and
the retrieval of photometric data from the CAS database requires
special care for objects from the special plates, and even more care
for SEGUE spectra. See the caveat
about mismatches
between spectra and images on the data products page for
spectra.
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