In message , Tony
Flanders writes
Martin Brown wrote in message
...

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?

Yes, I thought of that. Readings from trees in the same frames, or
in frames near by, are consistently 4, the standard value for a dark
frame. Note that the camera does have some kind of correction for
thermal effects at speeds below 1 second. I suspect that it internally
takes a shot with the shutter closed and subtracts it from the actual
shot. In any case, a 15 sec exposure starts 15 sec after you press
the shutter release.

Seems highly likely then.

Another thing to consider is that the in camera JPEG compression will
generate some positive bias in the lowest values near stars due to the
effect of quantising the coefficients. Unresolved point sources on a
flat black background are just about worst case pathological for JPEG.

If the camera has a TIFF mode at full it would be interesting to see if
using that made any noticeable difference. It might or might not
depending on the actual JPEG compression settings used in the camera.

Regards,
--
Martin Brown