[Aavso-photometry] Flat Stuff

[Chuck Pullen]

Michael - I used to use frames with very different median counts on them, 
and let them be scaled.  However, Arne suggested that could introduce bias 
to a process that is supposed to introduce uniformity.  As I understand it, 
the pixel to pixel variation with low S/N flats is the intrinsic 
variability **plus** the statistical light noise that comes from any low 
S/N frame.  While you can scale that up to a median of 1/2 well depth, you 
are simply multiplying those individual low pixel values **plus** the high 
noise by some scaler factor.  So, you actually get more noise, not less, 
then using frames that are all well exposed to begin with.

Practically, especially in Blue - I have a hard time getting enough 
frames.  So I do let IRAF scale them.  But I shoot for a range of 30 to 40 
K counts for the frames I then median combine, no less or more.

There is a difference, as I understand it, between twilight flats and "sky 
flats".  Sky flats are doing a median combine on your entire nights run of 
images, (which has to include many different fields) to generate your flat 
field.  I've never shot enough different fields to make it worth 
trying.  But for a SNe patrol type set up where you take several hundered 
images, each of a different field, it can supposably produce very good results.

Chuck


At 12:38 1/18/05 -0600, Michael Koppelman wrote:

>I have had very good luck with twilight/dawn flats (sky flats?) too. Mira 
>lets you combine with an intensity scaling. It basically does a weighted 
>average so you don't have to worry tons about the low S/N frames. I 
>usually change the exposure time a few times during the run to try to keep 
>the counts reasonable. It's a great way to go but I'm not always around 
>during twilight/dawn (in fact rarely) so I need an alternative.
>
>I wonder about these temperature compensating focusers. How can you get a 
>good flat when your focus is changing continually? For pretty pictures, I 
>can see why the temperature compensating focuser would be nice but for 
>photometry I can't imagine it is really necessary, is it?
>
>Cheers,
>Michael Koppelman
>http://www.lolife.com/astronomy/