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

[Aplanatic@aol.com]

Fellow APMLers:

I very much need your help.  The next couple of weekends should 
see quite a few of you out taking images.  If you can find the time 
to perform the simple measurement detailed below, I would really 
appreciate it.  Chuck Vaughn and Robert Bickel have both 
contributed to this, but I need a lot more data.  

Again, I am trying to correlate limiting visual magnitude (LVM) to 
the sky background measured by the ST4.  With it we can all learn 
many interesting things.

Thanks so much for your participation,

Dave Rowe

--------------- Original Message ----------------------

> I've done some calculations and believe that we can use our ST-4's for
>  estimating the sky background brightness.  Here's how to perform the
>  measurement:
>  
>  Set the boost to 4, i.e., set b=4.
>  Set the mode to faint, i.e., bA=F
>  Set the exposure to 1 sec, i.e., EA=1.0
>  
>  Take a dark frame.
>  
>  Now, put the ST-4 in find and focus mode, take the cover off 
>  the ST-4 and point it straight up without optics.  Just let the 
>  sky background fall directly on the chip.  If the reading is 
>  below 20, increase the exposure time to make it at least 20 
>  (if possible).  If the reading is saturated (99), then decrease 
>  the exposure time until the unit is out of saturation.  Using a 
>  little geometry and the calibration given by SBIG, here's 
>  what I get:
>  
>  Background (magnitudes per arcsec^2) = 23.2 - 2.5*log10(Count/T)
>  
>  where Count = the reading on the ST-4, and T = exposure time 
>  in seconds.
>  
>  If your skies are so bright that you saturate the ST-4 with 
>  b=4 and bA=F, then set b=1 and bA=A and use the formula:
>  
>  Background (magnitudes per arcsec^2) = 19.4 - 2.5*log10(Count/T)
>  
>  Remember, if you change any of the exposure settings (b,EA or bA), 
>  you must take a dark frame before making a measurement.
>  
>  OK, here are the caveats.  First, I'm sure ST-4's will have 
>  variable calibration factors.  Perhaps Michael Barber can 
>  comment on this.  Secondly, stars that are within the field 
>  of view of the open ST-4 will have a small effect on the reading 
>  if the background is very low.  How much of an effect?  Well, 
>  if the background is 21.5 mags/arcsec^2 (very dark) then a
>  0 magnitude star in the field of view will have a 1% effect 
>  on the reading.  Not much.  
>  
>  So, let's try it.  Next time you are out, do the following for 
>  me.  Estimate the limiting visual magnitude of the sky 
>  directly overhead and then perform the ST-4 experiment.
>  Report the conditions (b,EA,bA), the reading, the limiting
>  visual magnitude, and the location.  If you do so, I will keep
>  a log on a web page.
>  
>  If we all do this from time to time, we can build up a self 
>  consistent picture of the sky background and use it to find 
>  better sites, report our local shooting conditions, and finally
>  have an objective measure of our changing skies.
>  

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