MOPEX: Running PRF Estimate

Warning: PRF Estimate will not work for undersampled data or data with intrapixel variability, such as IRAC data. PRF Estimate should only be used for well-sampled data such as Spitzer MIPS or IRS.

Running PRF Estimate on a Mosaic Image

1. Run apex_1frame.pl using first a simulated PRF simply to get a list of bright sources to use for the PRF estimation. To get a list of the brightest sources only, set the "Detection_Threshold" input parameter to a large value (for example, 30). The user might also want to have a threshold on extracted fluxes in the "select_conditions" to further guarantee that only bright sources are included:

   apex_1frame.pl -n apex_1frame_step1.nl

   NOTE: Make sure you are using the same parameters for the MEDFILTER module while running APEX and PRF_estimate (see the example namelists below under "MEDFILTER").

2. Use your favorite image viewing tool to examine the sources in the mosaic_extract.tbl and make a final selection of point sources to use for the PRF estimate. For example:

   skyview
   > pa data/mosaic.fits
   > table output_apex_1frame_step1/mosaic_extract.tbl mark $RA $Dec circle red 17

   Save your final sample of point sources to be used for PRF estimate in a separate file called mosaic_extract_clean.tbl.

3. Run prf_estimate.pl to create your PRF to be used for point source extraction:

   prf_estimate.pl -n prf_estimate.nl

   Copy this file to the cal/ directory:

   cp prf_estimate/PRF.fits cal/PRF_estimate.fits

4. Run apex_1frame.pl again, this time using the estimated PRF, for your final point source extraction:

   apex_1frame.pl -n apex_1frame_step2.nl

5. Run apex_qa.pl to produce a subtracted image:

   apex_qa.pl -n apex_qa.nl

   The subtracted image produced is called: output_apex_qa/Mosaic/residual_mosaic.fits

Known Pitfalls with PRF Estimate

• PROBLEM
  The MOPEX module prf_estimate sometimes fails with a Perl error. It can also fail with an error about incorrect "flux" columns in a PRF file.
  
  WORKAROUND
  The Perl error is most likely a memory limit because you are trying to derive a PRF from too many stars x too many frames x too big a fitting area x too fine sampling. For example, if you have a big fitting area, you could reduce this by lowering PostStamp_Xsize and PostStamp_Ysize.
  
  The second error is due to MOPEX having trouble with the flux column in the input table. This is a bug that remains to be fixed. You can get around it by having a table with just RA and Dec, or RA and Dec and X and Y. It will then determine an approximate flux for each PRF star for its fitting.