Re: Bits and pieces

[Stupendous Man <richmond@a188-l009.rit.edu>]

  I wrote:

>       b) we have a separate tool to perform basic image reduction
>                on a single frame, and then analyze the cleaned frame
>
>   The pipeline I wrote is not designed for step b).  One could
> use IRAF or XVista or any commercial package to do this ... but one
> would then _not_ get exactly the same results as the pipeline,
> because the exact steps and parameters used won't be the same.

  Tom asked:

> Ahhhh!  I just realized why this is true.  (I think)  I recall that Michael 
> generates photometric corrections that depend on all the data taken.  If 
> one does one image, then the photometry may be different???

  Correct.  If one uses the measurements from a single frame, one
will not get the same photometry as if one uses measurements from
many frames.  The color terms will certainly be different, and the
zero point will also differ by a small amount.

  (One may argue reasonably that, for most purposes, one can measure
differential magnitudes on the re-cleaned frame.)

  Even before one gets to the photometry, there could be differences
if one has not saved the master dark and master flat frames used
in the original processing.   One can recreate the master dark
and flat from the raw images, but that will take several minutes.

                                        Michael