[Aavso-photometry] linearity test for SBIG ST2000XM

[BailyHill at aol.com]

Hello Gianluca;

Your should measure this on your camera and scope combintion.  Just take a 
star--say 12 mag or so, and expose it for 1,2, 3,4, .....seconds--until the star 
blooms.   Since this is an antiblooming camera, this may not occur as ususal. 
 Then plot the adu's of the star vs exposure time.  It should be  clear where 
it deviates from linear by more than 1%.  Since this is an antiblooming 
camera I believe, the camera may not be linear above 1/2 full well.  This is the 
only save way to do this.  It takes into account the version of your camera, the 
software, etc.

I observe from suburban Boston--probably not too different from Rome.  I 
rarely get below 2.5 arc secs FWHM.  Quite often the stars are more like 3-4 arc 
sec FWHM.  Under bad conditions, it goes to 5-6.  To properly sample for 
photometry, you need 2-3 pixels per FWHM.  This works out to an image scale of 1 arc 
second per pixel.  I do not know the size of the pixels in this camera, but 
with 800mm focal length, I do not think you are close.  Let me know the pixel 
size and I can calculate it for you.  

Clear Skies
Gary Walker