[Aavso-photometry] checking flatfields

[BailyHill at aol.com]

In a message dated 6/16/2007 7:33:23 A.M. Eastern Standard Time,  
arne at aavso.org writes:

I should  mention that the SRO system has been tested and has less
than one percent  flatfielding error across its field. 
 
Hello Arne;
 
Could you please define what you mean by "Flatfielding Error" and how to  
measure it can calculate it?  Let me take a pass at it, and you correct  me.
 
I think you mean that one would take a cluster like NGC7790 and raster it  
5x5, take also a beginning image and a ending image at the center, giving 27  
images.  Then flat field these images.  Now do photometry--an here I  am a 
little fuzzy, It seems like one would do absolute photometry, ie like the  less 
accurate "Information Tool" in Maxim.  Then see what the std deviation  of a star 
is at these 27 positions.  One could repeat this for a half dozen  stars.  
 
If our flatfielding is good, then either the peak to peak values of a  
particular star must vary by less than 1%(.01 mags);  implying a std dev of  .16% 
--ie +/- 3 std devs = 1%, so 1 std dev = 1/6 percent.  This .16  percent is 
equivalent to .0016 mags.
 
Or our 27 values for each particular star could have a std dev of 1%, which  
would be larger.
 
 
 
How would you suggest doing this so that we can compare to the Sonoita  scope?
 
 
 
Clear Skies
Gary
 
 
 
 






************************************** See what's free at http://www.aol.com.