Re: Analyzing Data

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

  Tom and Arne are discussing the ways one might mark images
as "bad".

  Tom is currently using

        a) overall sky level
        b) "clumpiness" in the sky level from one area of the image to another

as criteria.  Arne suggests using

        c) standard deviation from the mean sky level
        d) number of stars detected in a frame

instead.

  Perhaps Tom and Arne will be happy to hear that I have _just_ modified
the Mark IV pipeline so that one can use methods a, c, or d to flag
a frame as "bad", thus discarding it from further processing.  I hope
to release the new version before Monday.  I still need to modify 
the "photom" program to include an option to account for differential
extinction within each frame using fixed coefficient values (since 
ignoring differential extinction is significant in V-band, at least).

  Arne writes:

>    Regarding use of data below the threshold when performing photometry:
>    you need to use all data within your aperture for the star measure,
>    and all data surrounding your aperture for sky measure.  If you use
>    "zero" then your results will get skewed dramatically.  I may be
>    misunderstanding, as usual.

  The pipeline acts as Arne suggests: all pixels within the aperture
and all pixels in the sky annulus around it are included in the 
calculations.  

                                   Michael