Re: [TASS] Flat fielding

[Tom Droege <droege@WWA.COM>]

Andrew,

Do you want some raw data?  I have lots, but it will take me some time as I
have to move it around on floppy disks to get it to where I can write you a
CD ROM.  Perhaps someone else (Nick?) can send you a CD ROM full of data.
I would hate to have you stay up nights thinking ways the data can go wrong
in theory when you can have real data to munch on.

Tom Droege

At 02:39 PM 9/16/99 GMT, you wrote:
>On Tue, 14 Sep 1999 10:46:45 -0700, "Arne A. Henden"
><aah@NOFS.NAVY.MIL> wrote:
>> ...
>>  There are lots of these sources of error.  You should
>>understand the process before you start calling on
>>the statistician gods.
>
>I was struck speechless. The effect is now wearing off.
>
>The whole reduction process is an exercise in statistics -
>the conversion to I magnitudes a particularly nasty one
>involving extrapolation and all kinds of possibilities
>for selection effects. It is just not possible to
>understand the process without becoming at least a bit
>of a statistician.
>
>But, to return to my original point (and the subject of
>the thread) and rephrase it as offensively as I can as a
>series of assertions:
>
>1) Flatfielding by dividing by a vector derived from a
>single image is a waste of time and introduces errors.
>The large scale errors (gradients) can be partially
>removed by FlatComp. The small scale errors arising from
>incomplete removal of stars cannot be so removed and
>unnecessarily increase the final photometry errors.
>
>2) One should instead use a flat vector derived from
>a large number of images. Because the drift scan method
>averages over a complete column, the pixel to pixel
>scatter is very small and should be ignored. The flat
>vector should be a low order polynomial fit to the
>large scale (vignetting) response.
>
>None of this applies to the MK IV which is a whole
>new can of worms.
>
>Disclaimer: These conclusions are based on the TN's, etc.
>plus a purely mental examination of the raw data to
>which I do not have access. Any connection with fact
>is accidental.
>
>Andrew Bennett, Avondale Vineyard, Nova Scotia, Canada.
>
>