Good news. When I look at the r-band sky levels without having swarp first do the background subtracting, I get some really crazy stuff happening. Exposure 181 drops nearly to 0 on chips 4, 5, and 6, and exposures 179 and 180 are much higher than the others.Yesterday I had only looked at the sky levels for the background subtracted frames. Now looking at the stack of 7 exposures (without the 3 weird ones) I don't see the bad patch that was there before in ds9 OR in atv, so I'll proceed using the 7 exposures instead of 10. I still have yet to decide which stack to use for the g-band--that decision probably won't be made until I see the CMDs.
I calculated the median of medians from the un-subtracted sky levels to figure out what value I should add back in. I did this for the 10-exposure and 7-exp stacks in both r- and g-bands--4 values in all:
It seems reasonable to me that the median levels differ slightly between the two sets of stacks, but not by too much within each band. This makes me think there's not anything really crazy going on. These are the constant values I'll add to the sky-subtracted stacks to put the sky back in.
I added the sky to both the 10-exp and 7-exp g-band stacks as well as the 7-exp r-band stack.