Selecting the Best Direct Image Predecessor for a Grism Tile

General comments:

This document describes finding the best predecessor for existing data. Here, we attempt, to find the 'best' predecessor even though that predecessor may not have all the qualities as required by the Guest Investigator Program. (TAB, July 2008)


Program usage:

To see the best MCAT catalog for a grism visit, try the program:

    /home/galex/tab/bin/pipecheck -getmcat (tile name)_(visit number)  [ -verbose ]
So, for example, the grism tile 'GI2_048001_ABELL0426' visit number 14 command line would be:
    /home/galex/tab/bin/pipecheck -getmcat GI2_048001_ABELL0426_0014
in which case the best predecessor catalog is 'GI1_098001_A0426-xd-mcat.fits'.
Description of selection process:

For each grism tile, there must be only one best direct image predecessor. Every grism visit for a given tile must be reduced with the same direct image source catalog.

To decide on the best predecessor catalog, first we look at all the direct visit exposures. We look first at those direct imaging visits whose tile name matches the predecessor tile. The predecessor (direct imaging) tile for a given grism tile is found using the 'getpred' program which looks in the MPS database (see '/home/galex/tim/src/perl/misc/getpred').

To find the best predecessor tile direct imaging visit, find the visit with the largest exposure time which has:

  1. an exposure larger than the minimum predecessor exposure time (at least 5% of total expected grism exposure).
  2. separation from the grism visit of less than 5 arcminutes.
  3. a passing QA grade.
If a suitable visit is not found, increase separation limit in 5 arcminute intervals up to 20 arcminutes. If a visit is still not found, try again without the minimum predecessor time requirement.

Repeat this procedure for every direct imaging visit that has the same tile name as the grism visit. Repeat this procedure again for all direct imaging visits regardless of tile name. Now we have 3 best direct visit catalogs:

  1. Best direct visit which matches the MPS predecessor tile name.
  2. Best direct visit which matches the grism tile name.
  3. Best direct visit with any tile name.
From these 3 choices, choose (1) if it is 'adequate'. Where 'adequate' is defined as being within 10 arcminutes and exceeding the minimum (5%) exposure time. If (1) is not adequate, choose (2) if it is adequate, otherwise choose (3). In some cases, we choose (3) over (2) if it is much better choice, i.e. if (3) is within 5 arcminutes of the grism visit and the exposure time is at least 3 times longer than (2).

Then we repeat all of the above procedures this time for coadds, not visits. The final choice of best predecessor catalog will be a coadded catalog, as long as it meets the basic separation (< 20 arcminute) and exposure time (> 5%) limits. However, sometimes a visit can be selected over a coadd if an adequate visit is more than 5 arcminutes closer than the best coadd. In other words, a small separation is a more important than a large exposure time.

However, normally, all direct visits will be coadded, so we should always have a coadded predecessor catalog. The program described above 'pipecheck' will also look for 'pending' coadds, and will give a warning if a pending coadd may be the best predecessor.