or linearity variations, the sky levels as traced across CCD boundaries were not continuous, having discontinuities of 0-3%. These were worst in the [*U*]{} and $B$ bands, especially for CCD 1 (the one in the lower left/southwestern corner). After determining that the bias levels could not have varied enough to cause these jumps, we decided to apply a scale factor correction to each CCD. While the presence of the galaxy makes it impossible to assume that the background is flat across the image, we can still assume that the background is continuous across the chip gaps. We made the assumption that CCD 2 is “truth” — this was the clear choice as most of our standard stars fell on CCD 2. While the halo (NE and S) fields required corrections in all filters, the CTR field only required corrections in the $U$ and $B$ band.

We independently checked the validity of these corrections by plotting the positions of stars in the various color-color diagrams, the “stellar locus”, for each chip assuming that the positon and tilt of the locus should not vary from chip to chip. Only with these corrections is the stellar locus seen to be identical in all 