[Aavso-photometry] Dark Skies / Thin Air

[ben at davies.net]

I had never imaged in dark skies, so last weekend I lugged a 12" SCT up 
to the top of a 7,000 ft mountain.  I brought back some BVR  images, 
have been looking them over, and am a little confused.

The trouble is, in 60 second images there is no sky noise at all.  
Zip.  The average sky background runs something like 104-105 adu 
(before dark subtraction) and the dark frames run 105 adu as well.  
Read noise is ~7 adu.

This is an SBIG ST-10 camera at -25 degrees and processing is with 
AIP.

Both single images and track and stack, with bias, darks, flats and 
flat darks, leave typical mean PV of the sky background at around minus 
2 adu.  I am thinking that it might be read noise fluctuations are 
causing the negative values.  (I'm pretty sure) there are not negative 
photons out there.  The only other possibility I can think of is 
differential noise in the bias frames.  I didn't do them up there 
because the batteries ran out. Drat.

1. What can I do about the negative pixel values?    The signal is 
already as saturated as I want it to be.  And, the snows have come to 
the Sierra - so there will be no more images from there until at least 
next summer.

2.  What is the nature of Read Noise?  I had always assumed that it 
was addative - that excess electrons were generated by the alu.  But 
maybe it is actually just uncertainty expressed in units of electrons?

Thanks for any guidance here,

Ben Davies