Re: GSC 00279 00321 paper (plus an item for Tom about sky)

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

  Michael K. asked some questions about the TASS data.

    1. Astrometry is done relative to a network of Tycho-2 stars
       in each field.  The positions are taken directly from 
       the Tycho-2 catalog, and do not include proper motion
       to the time of observation.  A simple third-order polynomial
       in position in the frame is used to fit the observed positions
       to the catalog positions, projected onto a tangent plane at
       the center of the frame.  No terms for color or magnitude
       are included.  

       I plan to write a Tech Note on the astrometry of Tom's DS23
       in mid-to-late June.

    2. The photometry is based on the Tycho Bt, Vt magnitudes for
       stars in each frame, which I convert to Johnson V and Cousins I
       using equations suppled by Arne Henden.  You can find these
       equations at the top of the big reference catalog file used
       by the pipeline, I think, and certainly in the documentation
       for that file; see the pipeline document for the link.

       I would like to investigate the photometry for systematic
       errors sometime soon.  The reductions DO include first-order
       color terms to bring the instrumental magnitudes to the
       (converted) V, I magnitudes, so I don't expect the errors
       to be _too_ big.


  Now, Tom asked:

> If you have energy to do a (not so) big software project, how about 
> thinking about how to pre-process the tass raw data to filter out problems 
> caused by clouds.  That project is next on my personal list.  That is why I 
> want iraf.  My hope is that a simple call to Iraf will give me the mean and 
> sigma for a portion of an image.  I could then try my scheme of cloud 
> rejection.

  Well, Tom, if you've installed the pipeline software, then you can 
try these XVista commands, right from the command line:

      box 1 cr=1000 cc=1000 nr=50 nc=50
      mn image.fit box=1

The first command defines a box near the center of a raw frame
(so that the nasty edge pixels don't affect the frame),
and the second command calculates the mean and stdev of the
pixels in that region for the raw image file called "image.fit".
You might try 

       sky image.fit box=1

which tries to fit a gaussian to the histogram of pixel values
in order to find the sky value and stdev. 

  Voila!  No IRAF needed :-)

                                        Michael Richmond