The primary sources of error are uncorrected extinction variations that affect both the PT patches and the 2.5m scans, incidents of bad seeing in the 2.5m scans coinciding with the crossing of PT patches, and statistical errors due to the small number of matching stars in certain PT patches in areas of the sky with low star density. Consequently, after final calibration, the data are subjected to a suite of tests to assess the reproducibility of photometric calibrations. These tests are neither exhaustive nor precise, but they do catch egregious errors. They are designed to uncover systematic calibration errors in the scan direction and across the imaging camera.
The principal tests for photometric accuracy include:
These tests consistently indicate that the photometric zeropoints for the EDR data are internally consistent to within % in all bands for 90% of the frames in the EDR data. The worst outliers are all in u, and are up to 10% peak-to-peak in the worst of our data; as we saw above, these are due to ghosting in the u chip.
Because errors in different bands are often correlated, the colors of objects have smaller errors than might otherwise be indicated; stellar locii from different columns are typically aligned to better than 1% (measured in the g r i z bands). The median stellar locii from different runs are also aligned to better than 1%.
We discussed in § 4.4.5 that the estimated PSF magnitude errors are accurate to 10-20%. The errors themselves are impressively small, as is manifest by the width of the stellar locus; the rms scatter in bright star PSF photometry (where photon statistics are negligible), after correcting for zero-point offsets, is 0.02 mag in , , and , and 0.03 mag in and . The separation between the stellar locus and quasars in Figure 13 is due to this excellent photometry.
In addition to errors on the internal consistency of the photometry, the tie of the 2.5m photometry to an AB system has additional errors.
Thus the system zeropoints could differ from the AB zeropoints by as much as .