Dark Frame Check

[Tom Droege <tdroege2@earthlink.net>]

I just did a quick check with some dark frames that were taken about two 
months apart.  The difference of the frame means was about 50 +/- 10 
counts.  The difference of the dark columns was also 50 +/- 10 counts.  Not 
a careful scientific experiment, but I confirmed what I expected to find.

1) The dark frames do not drift around very much.
2) The drift is most likely caused by electronics and such so that the dark 
columns will make the proper corrections.

Because we do a fair job of holding the detectors to constant temperature, 
I think that dark frames will not be a problem.

Any dark variation is apt to be well down in the sky noise.

This to point out that we are operating in a different regime from most CCD 
systems.  It would appear that darks are less important, and flats are more 
important for us than for most.  With the temperature control, the darks 
are well down in the noise.  Because of the wide angle, the flats make a 
large contribution.  They will have to be done right.

At the moment, I see no alternative but to use light box flats.  Possibly 
cloud flats will work.  This will be a pain for TOM2 and TOM3 since someone 
has to climb up inside the dome to attach the flat box to the lenses.  This 
is more than I can do.  Sigh!