[Aavso-photometry] checking flatfields

[arne]

BailyHill at aol.com wrote:
> 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?
>  
What you describe is similar to what I've advocated in the past, and is
the most precise way to check your flatfielding.  By using a single
star and rastering it, you remove the problems of transformation and
dynamic range.  However, it does require a photometric night and takes
a fair amount of time.  If you just want to look for gross effects, do
like I said in my earlier email today - image a field that I've already
done at Sonoita or at USNO, pick stars as similar in color and brightness
as possible in the field, and compare your results against mine.
That way a single image tells the story, at least for gross details.
Since Geir was talking about 0.1mag differences across his field,
the simple method should tell him whether it is a flatfielding problem.
Arne