Re: [APML] Sky fog

[Aplanatic@aol.com]

Hi Rick,

> I have a nagging
>  feeling that the difference between a great site and a good site is only a
>  few tenths of a mag, which is about the limit of repeatability using these
>  instruments.

Yes, all else being equal (and linear), an increase in sky background of one 
magnitude would require 2.5X the total exposure to produce the same SNR.
 
>  The STV has a green filter which the manual says approximates a V filter.
>  For the most part I have not used this, as it also attenuates about 20x.

That much, eh?  

>  Last night, I took some measurements with the STV, both with and without
>  filter, and also with the ST-4.  The results are below.

I'd say that the correlation between the ST-4 and STV is quite good.  Your 
average ST-4 measurement of the sky background was about 17.15 and using the 
STV it was about 17.4 in V-band.  That's actually better than I had hoped 
for.  I realize it was only a single case, but it is encouraging.  For your 
cal star, did you use the accepted visual magnitude when analyzing the 
green-filtered data? 

>  By the way, I made STV measurements on Saturday also, and they were
>  different than the readings on Sunday by 0.25 mag, yet the measurements 
were
>  repeatable on each day to better than 0.1 mag.

You attribute the difference to a change in sky background between the two 
nights?

Thanks for the info,

Dave Rowe

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