[Aavso-photometry] Short-Exposure Photometry

[Michael Koppelman]

So I'm taking a look at V972 Her. It's bright, at V=6.7 or so. I did 
BRVI photometry with the following exposures times:

I  10 sec.
R   5 sec.
V   9 sec.
B  20 sec.

The signal-to-noise ratio is really good 'cause I got lots of counts, 
so the random error is like 0.002. What's weird is the comp star 
standard deviation is really high, upwards of 0.03 in V. (This is a 
variable that, as far as I can tell, varies less than 0.1 magnitude.) 
This is usually more like 0.008 or something for me.

Is there some reason why short exposures would cause higher errors?

I do not believe I am saturated. Most images have a max pixel value 
between 30,000 and 55,000 or so (on an SBIG ST-7XE). I don't think I 
should have shutter effects at exposures at 5 seconds or greater (see: 
http://www.lolife.com/astronomy/lintest/lin5.jpg ), at least not more 
than 1%.

I have a little page about this star here:

http://www.lolife.com/astronomy/V972Her/

Cheers,
Michael

PS - Resending this because of the discussion area downtime...