Algorithms: Astrometry
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
page.
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
Astrograph Catalog
(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 DR1, stripes 9-12, 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 DR1, stripes 34-37, 42-44, and 76 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 z CCDs.
Notes:
At the edges of pixels, the quantities objc_rowc and objc_colc take integer values.
Last modified: Tue Mar 18 18:00:22 CST 2003
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