Re: [APML] Help Needed -- ST-4 and Sky Background

[Aplanatic@aol.com]

Rick,

> They state (pg 3) a value for C of about 12000 for the ST-4 and about 1000
>  for other SBIG cameras. Units are counts/sec/sq-in-aperature
>  for 0th mag star.
>  
>  But other SBIG cameras are move sensitive than the ST-4 (in counts/electron
>  and correspondingly (including QE),  counts/photon).
>  
>  So for other cameras, shouldn't the factor C be greater (not less) than 
that
>  for the ST-4?

Hmmm...  I really don't know the answer, but it does seem a 
little odd, especially since the newer SBIG cameras have more 
bits in their A to D converters, which would presumably mean 
that the constant should be larger, not smaller.  I guess it's where 
they decided to set the pre-amp gain and ADC quantization 
levels.  Or perhaps the app note assumes different settings?

>  BTW, I'm going on so much because I want to use the same technique
>  to measure background sky with the STV (with no lens), as it is a lot 
easier
>  (once calibrated) than using a known reference star.

Yes, that's what's fascinating to me about this technique.  I really 
do suspect that we have a wonderful, calibrated (good enough, 
anyway) method of measuring sky background anywhere on the 
planet.  We need to do some side-by-side comparisons of ST-4s 
and STVs to make sure that they are reasonably close to each 
other (within product type).  I will be encouraging people with 
ST-4s to do these simple tests.

Do you have the C-factor for the STV?  Is it 
1000 counts/0 mag/sq in/sec?

Dave Rowe
 


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