A detailed description of the astrometric calibration is given in
Pier et al. (2003)
(AJ, or astro-ph/0211375).
Portions of that discussion are summarized here, and
on the astrometry quality overview
The r photometric CCDs serve as the astrometric reference CCDs for the SDSS.
That is, the positions for SDSS objects are based on the r centroids and
calibrations. The r CCDs are calibrated by matching up bright stars detected
by SDSS with existing astrometric reference catalogs. One of two reduction
strategies is employed, depending on the coverage of the astrometric catalogs:
- Whenever possible, stars detected on the r CCDs are matched
directly with stars in the
United States Naval Observatory CCD
(UCAC, Zacharias et al. 2000), an (eventually)
all-sky astrometric catalog with a precision of 70 mas at its catalog
limit of r= 16, and systematic errors of less than 30 mas.
There are approximately 2 - 3 magnituds of overlay between UCAC and
unsaturated stars on the r CCDs. The astrometric CCDs
are not used. For DR2/DR3, stripes 9-13, 76, 82, and 86 used UCAC.
- If a scan is not covered by the current version of UCAC, then it is
reduced against Tycho-2
(Hog et al. 2000), an all-sky astrometric catalog
with a median precision of 70 mas at its catalog limit of
VT = 11.5, and
systematic errors of less than 1 mas. All Tycho-2 stars are saturated
on the r CCDs; however there are about 3.5 magnitudes of overlap between
bright unsaturated stars on the astrometric CCDs and the faint end of
Tycho-2 ( 8 < r < 11.5), and about 3 magnitudes of overlap between bright
unsaturated stars on the r CCDs and faint stars on the astrometric CCDs
(14 < r < 17). The overlap stars in common to
the astrometric and r CCDs
are used to map detections of Tycho-2 stars on the astrometric CCDs onto
the r CCDs. For DR2/DR3, stripes 30-43 used Tycho-2.
The r CCDs are therefore calibrated directly against the
primary astrometric reference catalog. FRAMES uses the astrometric
calibrations to match up detections of the same object observed in the
other four filters. The accuracy of the relative astrometry between
filters can thus significantly impact FRAMES, in particular the
deblending of overlapping objects, photometry based on the same
aperture in different filters, and detection of moving objects. To
minimize the errors in the relative astrometry between filters, the
u, g, i, and z CCDs are
calibrated against the r CCDs.
Each drift scan is
processed separately. All six camera columns are processed in a
single reduction. In brief, stars detected on the r CCDs
if calibrating against UCAC, or stars detected on the astrometric CCDs
transformed to r coordinates if calibrating against
Tycho-2, are matched to catalog stars. Transformations from
r pixel coordinates to catalog mean place (CMP) celestial
coordinates are derived using a running-means least-squares fit to a
focal plane model, using all six r CCDs together to solve
for both the telescope tracking and the r CCDs' focal plane
offsets, rotations, and scales, combined with smoothing spline fits to
the intermediate residuals. These transformations, comprising the
calibrations for the r CCDs, are then applied to the stars
detected on the r CCDs, converting them to CMP coordinates
and creating a catalog of secondary astrometric standards. Stars
detected on the u, g, i, and
z CCDs are then matched to this secondary catalog, and a
similar fitting procedure (each CCD is fitted separately) is used to
derive transformations from the pixel coordinates for the other
photometric CCDs to CMP celestial coordinates, comprising the
calibrations for the u, g, i, and
Notes: At the edges of pixels, the quantities objc_rowc
and objc_colc take integer values.
Early SDSS imaging runs were astrometrically calibrated against
Tycho-2 (Hogg et al. 2000), which yielded statistical errors per
coordinate for bright stars (r < 20) of approximately 75 mas and
systematic errors of 20 — 30 mas. Later runs were calibrated against
preliminary versions of the USNO CCD Astrograph Catalog (UCAC,
Zacharias et al. 2000), which yielded improved statistical errors per
coordinate of approximately 45 mas, with systematic errors of 20 — 30
mas (Pier et al. 2003). Proper motions were not available for the
preliminary versions of UCAC. Since the typical epoch difference
between the SDSS and UCAC observations is a few years and the typical
proper motion of UCAC stars is a few mas
year-1, this introduces an additional
roughly 10 mas of systematic error in the positions due to the
uncorrected proper motions of the calibrating stars.
All of DR7 has been recalibrated astrometrically against the second
data release of UCAC (UCAC2; Zacharias et al. 2004). While the
systematic errors for UCAC2 are not yet well characterized, they are
thought to be less than 20 mas (N. Zacharias, private communication).
UCAC2 also includes proper motions for stars with δ < +41º.
For stars at higher declination, proper motions from the SDSS+USNO-B
catalog (Munn et al. 2004) have been merged with the UCAC2 positions.
With these improvements, all DR7 astrometry has statistical errors per
coordinate for bright stars of approximately 45 mas, with systematic
errors of less than 20 mas. The mean differences per run between the
old and new calibrations is a function of position on the sky, with
typical absolute mean differences of 0 to 40 mas. The rms differences
are of order 10 to 40 mas for runs previously reduced against UCAC,
and 40 to 80 mas for runs previously reduced against Tycho-2,
consistent with what we would expect given the errors in the
Note that the formal SDSS names of objects in the CAS are of the form
SDSSJhhmmss.ss±mddmmss.s. Because of the subtle changes in the
astrometry, these names will be slightly different for many objects
between DR6 and DR7. The user should be aware of this in comparing
objects between DR6 and DR7.
The CAS includes proper motions for objects derived by combining SDSS
astrometry with USNO-B positions, recalibrated against SDSS (Munn
et al. 2004). These are given in the ProperMotions table in the
CAS. (This table was called USNOB in the DR3 and DR4 versions of the
CAS.) An error was discovered in the proper motion code in Data Releases
3 through 6, which causes smoothly varying systematic errors, in the
proper motion in right ascension only, of typically 1—2 mas
year-1 (see Munn et al. 2007 for a full description of the problem
and its effects). This error has been corrected in DR7, thus any use of
proper motions should use the DR7 CAS.
Errors on RA, DEC available in photoObj tables in CAS from DR6 on
photoObjAll and derived views
etc.) in the CAS now contain
the errors and covariances for the right ascension and declination as
well as galactic coordinates for all objects (see the CAS
Schema Browser entry for photoAuxAll). These errors are calculated
by adding the centroiding errors in quadrature with the estimated
astrometric calibration errors. The centroiding and calibration
errors are calculated in great circle coordinates and are assumed to
be uncorrelated in the great circle longitude and latitude directions;
the covariance between the right ascension and declination errors is
then introduced when transforming the uncorrelated errors in great
circle coordinates to the celestial coordinate system.
In DR4 and DR5, these errors were available only in the
photoAuxAll table (as were galactic coordinates). For
backwards compatibility, there are still
photoAux views, but new queries requiring these
parameters do not need these auxiliary tables any more.
*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 on $Date: 2009/03/25 16:49:12 $ (UT).