Re: More from ASAS

[Andrew Bennett <andrew.bennett@ns.sympatico.ca>]

On Wed, 16 Oct 2002 21:52:23 -0500, Tom wrote:

>Andrew Bennett writes:
>
>>So - bite the bullet: the MK V has to put the same star on the
>>same pixels night after night!
>
>Well, this brings up one of the points that I am trying to make.  If you 
>measure the same star on the same pixel night after night, the result looks 
>nice.  It even has a small rms error.  But the real error in absolute
>photometry is hidden by the process.  The error is still there, it is just 
>harder to see.  If we move the star around on the ccd, then the error 
>becomes obvious.  We still may not have all of it, but at least we are 
>fooling ourselves less.

Yes. You are right. I should have said:
So - if the aim is to find medium-long period variables, rather
than spend more time tracking down mysterious systematic errors
then bite the bullet etc etc.

And it will be nice to be able to move that star around the ccd
in a planned manner. My code that didn't work supposedly coped 
with observations at random locations but I never managed to 
pull a consistent pattern out of the residuals. Observations on 
a regular grid should be a lot easier to handle - for example,
a lot easier to handle the hypothesis that systematic errors
have a component that depends on more or less constant flat 
fielding errors. With the observations in random positions and
corresponding random choice of reference stars, I was quite
unable fit any pattern to my residuals ... of course, I
was using the Bennett PSF fitting code too ...

Andrew Bennett, Avondale Vineyard