Re: aperture photometry and sky variations

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

  Tom wrote:

> Once one does aperture photometry, then if there is a bright star sitting 
> in a bright cloud, then one gets the measurement wrong, as the background 
> aperture gets the bright cloud, and that is wrong.

  Well, it depends on the size of the bright cloud compared to the
size of the star.  If the bright cloud is much larger than the star,
so that the area "under" the star is about the same brightness
as the area immediately surrounding the star, then aperture photometry
can still do a decent job; one must simply define an annulus surrounding
the star, measure the brightness there, and subtract it from the 
light inside the aperture.  The current Mark IV pipeline performs
this sort of local sky subtraction.

  However, if there are small bumps and holes in the background
level, of roughly the same size as (or smaller than) the PSF,
then one will indeed make big errors in the photometry.

  The routines in the version of the pipeline Tom is using try to
remove large-scale, gentle gradients with a simple linear or
quadratic model fit to the entire image, _and_ also use a local
sky annulus around each star during the aperture photometry.
It's not foolproof by any means, but it can do a decent job in
some situations.

                                   Michael