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Re: more flatfields



aah@nofs.navy.mil wrote:
> If you want to do 0.01mag photometry, then many
> experiments need to be performed to find systematic errors and improved
> extraction/reduction techniques.  Tom spent a fair amount of time building
> these cameras; why not take an equivalent amount of time to learn the
> best way to use them?

Sounds good.

Here is a simple Idea that may help with both the gradient and
non-constant
PSF problem:  Take each frame and cut it up into a set of sub-frames.
Run each subframe thru the existing pipeline.  Now you will compute a
median background and latter on a zero point for each much smaller
region

This should not require any software changes except a program that can
chop up a FITS file.  Seems like FTOOLS or the like could handle that.

Or, you could keep the raw frame in tact and simply sort the extracted
star list by Dec. band making four lists then find a zero point for
each sublist.

If this helps then
I think the true soluution is to take the above "sub-region method"
to the limit of where the "number of sub-regions approaches infinity"
In other words just accept that our images have continously varying
PSFs and gradients and process accordingly.  I have been arguing for
using 2D curve fitting with a  varying PSF model for some
time now.  I appears to me from recent posts that the assumption of a
constant PSF made by the current software cost us about mag. 0.02+
error.

-- 

--Chris Albertson

  chris@topdog.logicon.com                Voice:  818-351-0089  X127
  Logicon RDA, Pasadena California          Fax:  818-351-0699