[APML] Use ST-4 to Measure Sky Background

[Aplanatic@aol.com]

Hi List,

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.

Thanks,
Dave Rowe

PS  It's cloudy in LA and I measured 14.4 mag/arcsec^2 skies.  Yuk!

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