FRAMES uses the astrometric calibrations to match up detections of the same object observed in the other four filters. The r CCDs are calibrated directly against the primary astrometric reference catalog. There are approximately 2 - 3 magnitudes of overlap between UCAC and unsaturated stars on the r CCDs. Together UCAC2 and r14 cover the whole sky. Outside the UCAC2 area we use an "internal" UCAC data release known as "r14".
UCAC2 extends up to around a declination of 41 degrees north. 2000), which has a precision of 70 mas at its catalog limit of r= 16, and systematic errors of less than 30 mas. Stars detected on the r CCDs are matched directly with stars in the United States Naval Observatory CCD Astrograph Catalog (UCAC2, Zacharias et al. The r CCDs are calibrated by matching up bright stars detected by SDSS with the UCAC astrometric reference catalogs. That is, the positions for SDSS objects are based on the r centroids and calibrations. The r photometric CCDs serve as the astrometric reference CCDs for the SDSS. Portions of that discussion are summarized here. A detailed description of the astrometric calibration is given in Pier et al.
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