The SDSS Data Release 6 (DR6)
Contents
New for DR6
DR6 is the first release since DR2 to have significant changes to
the processing software. As it includes the first data from SDSS-II,
it also adds substantially different data products - see About SEGUE and the
public data release from the SDSS Supernova Survey. The
qualitatively new items are:
- SEGUE imaging catalogs and quality assurance (QA) are available in the SEGUE
CAS database. SEGUE targeting flags will eventually be available for
all DR6 imaging, including legacy. See the target-selection
algorithms page for information on SEGUE targeting.
For SEGUE imaging runs, note the caveat on low-latitude photometry and
reddening corrections!
- Stellar spectroscopy Stellar
Spectral Parameters Pipeline line
measurements (sppParams, [Fe/H], log g, Teff)
and line
index measurements (sppLinesCa, Mg, etc.) are
available both in the CAS and the DAS for SEGUE and many SDSS
spectra.
Important! The SEGUE spectra count as
special-plate spectra
in the CAS and are not considered to be
"scienceprimary". Therefore, they are not linked to the photometry in
the usual way, e.g., they do not appear in the object
browser's default view like galaxy, quasar and stellar spectra from
the legacy survey. Moreover, all SEGUE spectra are in the
BESTDR6 database, while most of the SEGUE imaging is in
the SEGUEDR6 database of the CAS, so that linking
imaging and spectroscopy data requires special care. See the SEGUE sample
SQL query for instructions on joining SEGUE imaging and
spectroscopy.
- Ubercal improved calibrations using cross-scans
to tie the photometry of the entire survey to each other. See the algorithms entry on ubercal
- Improved spectroscopy reductions The
spec2d code that reduces the spectrograph's
2-dimensional CCD images into 1-dimensional spectra has been
modified substantially. The most important changes are:
- Improved spectrophotometry, now calibrated relative to PSF,
not fiber magnitudes. The
spectrophotometric flux scale is now brighter by 0.35 mag,
or 25%.
- Availability of additional data products, including
individual 15-minute exposures (DAS only) and
sky spectra
- In the
spectro1d pipeline analyzing the reduced
1-dimensional spectra, the algorithm for velocity dispersions
has been changed, see Bernardi
2007, The sigma-L correlation in nearby early-type
galaxies, AJ, 133, 1954 and there are improved
radial-velocity estimates for stars
- In the CAS
database, the following changes have occured:
- All the columns from the
photoAuxAll
table (galactic coordinates and astrometric errors in RA
and DEC) are now part of photoObjAll and derived views
(photoObj, star, photoPrimary
etc.). For backwards compatibility, there are still
photoAuxAll and photoAux views,
but new queries requiring these parameters do not need
these auxiliary tables any more.
- There have been changes in the sector/region code in the CAS
- A "clean photometry" flag has been added to
photoObjAll table to facilitate
photometric flag
checking.
The quantitatively new items are:
- Increased sky
coverage of the Legacy survey in both imaging and
spectroscopy (as with any data release). The contiguous imaging in
the north galactic cap is essentially complete now.
- Additional value-added
catalogs (e.g., the DR5
quasar catalog).
Except for the changes in the spectroscopic pipeline
spec2d and the CAS changes described above, the pipelines
and databases are essentially identical in DR6, DR5, DR4, DR3 and DR2.
Thus, DR6 is (very nearly) a proper superset of DR5, which is a
superset of DR4, 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. For details about what changed in
subsequent releases, please refer to About DR4 and
About
DR5.
What DR6 contains
The DR6 imaging data cover about 8420 square degrees of
"legacy" sky, with information on roughly 230 million
distinct photometric objects, and about 1200 square degrees of SEGUE
sky, with about 57 million objects. The DR6 spectroscopic data
include data from 1520 main survey plates of 640 spectra each, and
cover 6860 square degrees. In addition, DR6 contains 467
"extra" and "special" plates:
- 64 "extra" plate/MJD combinations which are repeat
observations of 55 distinct main survey plates
- 383 distinct "special" plates, which includes 162
SEGUE plates (see above), and 226 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. Some of these plates are outside of the
regular DR6 imaging area; DR6sup provides
that missing imaging, among other things.
- 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
DR6
The DR6 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 7 July 2006, and the spectroscopy associated with
that area as well as the extra and special plates
obtained before that date. In fact, 34 square degrees of imaging data
in the Nothern Galactic Cap lie outside this ellipse. While the DR6
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 DR6 are given on the coverage
page. The natural unit of imaging data is a run; the DR6 contains
data from (about) 244 runs in the best database, and (about) 246 runs
in the target database.
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 as a
DRsup
(supplemental) DAS-only data release. They are:
- A series of repeat scans of the Equatorial Stripe in the Southern
Galactic Cap (Stripe 82);
- Scans through M31 and the Perseus Cluster;
- Scans taken at low Galactic latitude as part of the SEGUE project;
these runs were used to target stars on
special plates.
Imaging caveats
The following caveat is new to DR6.
Systematic relative photometry errors in extreme-coloured
stars
Stars with extreme colours can have inconsistent photometry due to
slight differences in the photometric response for different
camcols. There is a brief description in Ivezic et
al. 2007, AJ in press.
The following caveat has been characterised quantitatively now:
Overestimation of sky levels in the vicinity of bright
objects
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