[Aavso-photometry] Dark Skies / Thin Air

[ben at davies.net]

Arne, thank  you,

I'm very relieved to hear that the negative values are not a problem.

The pixel fov was oversampled at .46 arcseconds.  I didn't get around 
to using any binning this time, but was defocused.

The weather was in rare fine form.  Winds that had been off the desert 
died at sunset to a dead calm.  The temp didn't get below 45 degrees at 
night.  Can you believe it? no moon, no wind at the peak and warm, and 
all this in November yet.  The skies were just magical.  Probably never 
happen again.

Ben


> 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?
>
Readnoise is a random process, and involves the conversion process
from electrons to volts, so is a little complex to describe.  
Basically,
all manufacturers apply a bias to the output signal so that the ADC
always sees a positive signal.  Once you subtract bias and dark, that
subtracts *mean* bias and dark, and the definition of a mean says that
there should be as many negative as there are positive excursions 
about
that mean.  Therefore, if there is no sky background, you should see
small positive and negative values that average to zero.  I never
worry about positives and negatives much except when converting back
to an integer fits image, as floating-point fits images don't care
and software *shouldn't* care.

What is your image scale in arcsec/pixel?  It takes a really, really
dark sky to result in no sky background - I always have sky in wide-
bandpass
filters like Johnson/Cousins.
Arne