How do I use color terms when calibrating fluxes?

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#1

How do I use color terms when calibrating fluxes?

#2

Do you mean in standard astronomical way with linear terms, or using
LSST approach? If the latter, see eqs. in Sec. 4.1 from LSE-180.

#3

This post describes how to apply color term corrections using the LSST DM software stack as of 2015-08-28.

Color term corrections are applied by the PhotoCalTask, which is called by ProcessCcdTask. Applying color term corrections requires two things:

  1. You must enable colorterm corrections by setting config parameter calibrate.photocal.applyColorTerms=True. You may also set calibrate.photocal.photoCatName to to the name of the reference catalog, but the default value of None uses the name of the astrometry_net_data product you have setup, and that will usually work.

  2. You must have color term correction data available for the camera and reference catalog in question. Color term correction data is contained in obs_* packages in the directory config/[camera/]colorterms.py. For an example see obs_subaru. Color term data is stored in a ColortermLibrary, a dictionary of reference catalog name: (dictionary of science filter name: Colorterm). Colorterm contains color term correction coefficients for one science filter.

For more information see PhotoCalTask, Colorterm and ColortermLibrary, all presently in pipe_tasks.

Many improvements are planned, including:

  • Improve the way color term data is stored and retrieved. We want to add a temporal component and manage it the same way we manage other calibration data. This awaits the so-called “butler 2”.
  • Move the classes out of pipe_tasks, possibly into meas_photometry.
  • Improve our color term correction model.