Astrometric Pipeline

 

A separate great circle coordinate system is defined for each stripe (§ 3.2.2). In these systems, the stripe center is the equivalent of the equator in the equatorial system. Pixel coordinates are corrected for empirically derived optical distortion terms, and the resulting mapping from corrected CCD row and column pixel positions to great circle longitude and latitude is linear to very good approximation. Astrometric solutions are carried out in this coordinate system. One of two reduction strategies is employed depending upon the coverage of astrometric catalogs:

• Whenever possible, stars detected on the r photometric CCDs are matched directly with stars in the USNO CCD Astrograph Catalog (UCAC) zacharias00. UCAC extends down to R = 16, giving approximately 2 magnitudes of overlap with unsaturated stars on the photometric r CCDs. For the EDR, this technique was used for the south equatorial stripe (runs 94 and 125) and for the north equatorial stripe (runs 752 and 756).
• If UCAC coverage is not available to reduce an imaging run, bright stars detected with the astrometric CCDs are matched with the Tycho-2 catalog hoeg00. The fainter stars on the astrometric CCDs are then matched with the brighter detections on the r CCDs, enabling us to transfer the pixel coordinates of the catalog stars onto the r CCD coordinate system. In the EDR, this technique was used for the SIRTF area, runs 1336, 1339, 1356, and 1359.

For each r frame, these mappings result in an affine transformation relating corrected pixel positions to celestial coordinates. A secondary catalog is produced from the detections on the r CCDs. This secondary catalog is then matched to centroid positions on the i, u, z, and g CCDs to derive affine transformations in those filters. The transformation also includes terms to correct for differential chromatic refraction and those terms are applied when the colors of objects are known (Table 16). Positions of detected objects given in this EDR have had this correction applied.

 
• Astrometric Quality
• WCS