In message , Tony
Flanders writes

Somewhat more baffling, I detected a "floor" of 9 - 11 for
readings in New Lebanon, regardless of sky brightness.

What was the value for the inside of your lens cap taken at the same
time?

The CCD dark current is both temperature (and so time dependent) and
position sensitive on the array. If you take a long enough exposure on
most consumer digicams you will be able to see which is the "warm"
corner nearest to the readout electronics and stray IR photons.

I believe that the floor of 9-11 is due to some nonlinear
effect from faint stars. One reason that I believe this is
that the floor seems to rise when I shoot at full resolution
rather than the 640x480 which I normally use. Another reason
is that the 11 reading always lies in or near the Milky Way,
while the 9 reading lies far from the Milky Way. It is also
extremely intriguing that the Milky Way is no more visible
in the shots from New Lebanon than in the shots from Westford,
or even from Lincoln, although it is certainly immemsely
much more prominent to the naked eye. This hints to me at
some kind of non-linearity, possibly due to the fact that
stars are point sources.

Sometimes it is the eye's own non-linearity that means the camera shows
you more accurately what is really there, but the eye fails to see
enough contrast to latch on.

Try a few shots with M33 in the frame - that ought to show if it is
diffuse faint starlight. I'd also be inclined to shoot at full
resolution - you have no idea what dirty tricks the camera firmware may
use to downsample from the CCD to 640x480.

Very interesting results and with simple easily available equipment too!
- I hope more people try measuring their skies this way.

Regards,
--
Martin Brown