[AZ-Observing] Gegenshein and a question

Hi All-

Steve Coe mentioned the other day how visible the Gegenshein was on the
weekend - I was having some backlash issues with my mount, so went to wide
field imaging.  Unfortunately, I forgot my adaptor to use my Nikon lenses,
so I had my Canon zooms to use with the 20Da.

Anyway, one of my targets was the Gegenshein, and for it used the 10-22
Canon zoom at 10mm wide open at F/3.5.  Cropped slightly, this is 4X3
minutes at an ISO of 1600:

http://alice.as.arizona.edu/~ketelsen/Gegenshein%204X3m.jpg

I'm really impressed with the sharpness of this lens - it is fun to cruise
around the frame at full resolution.  There is a little vignetting and
that brings me to my question - how do you shoot flats with a lens with a
107 degree diagonal FOV?  I usually shoot sky flats during twilight, but I
don't think the sky is uniform enough that wide.  Any ideas?

BTW, here are a couple other shots - the winter Milky Way from south of
Canis Major to Comet Holmes and a little Zodiacal Light, here with 18
minutes total exposure:

http://alice.as.arizona.edu/~ketelsen/WinterMilkyWay%206X3m.jpg

Here a quick shot of the western sky after moonset about 2200 local time
with 9 minutes total exposure:

http://alice.as.arizona.edu/~ketelsen/West%203X3m.jpg

Thanks in advance for thoughts on flat fielding...

-Dean


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