Photom -- programs to perform photometric calibration of stellar photometry

Version 0.8: March 27, 2005

Version 0.8 has one big enhancement and one small fix:
  1. The user can now request that stars detected in only a single passband be retained and included in output. They will have no color corrections made, only zero-point and extinction shifts.
  2. A minor big in initializing memory has been fixed; it caused the selftest script to fail on some machines.

Added March 25, 2006: Version 0.8 (and all earlier versions) have one important bug.

  • The code requires the names of the filters for which data will be reduced to be "V" and "I". It turns out these are the filters used in the TASS Mark IV project, for which this software was written. I was just too lazy to create the proper routines to handle arbitrary filter names.

    So, whatever filters you are using, you'll have to supply "V=" and "I=" as the arguments on the command line, even if the data of interest are, e.g., "B" and "V".

Thanks very much to Andisheh Mahdavi for pointing this out. I ought to fix it in a future release ....
Added Sep 7, 2007: If the code compiles and links, and passes some of the self-tests -- but fails on others -- try editing the Makefile to change
        CFLAGS = -g -O2
        CFLAGS = -g 

These programs are designed to take raw, instrumental measurements of stellar brightness and convert them to calibrated magnitudes on some standard system. They were written as part of the TASS Mark IV pipeline , but may work on other datasets.

This package creates two executables:

The photom program tries to fit the raw measurements from N frames to the catalog magnitudes with in one of three modes:

  1. if given command-line option mode=noextinct, an equation of N + 1 free parameters and no extinction
       calibrated mag  =   raw mag  +  a   +   b * (raw color)

  2. if given command-line option mode=noextinct and fixk=, an equation of N + 1 free parameters and differential extinction within each frame with user-supplied coefficient
       calibrated mag  =   raw mag  +  a   +   b * (raw color)  -  k*X
  3. if given command-line option mode=extinct, an equation with just 3 free parameters
       calibrated mag  =   raw mag  +  a   +   b * (raw color)  -  k * airmass

There is no default; the user must specify the mode in the command line. If no values for fixk are given, then no differential extinction is included in the solution.


The source code is vanilla ANSI C. It links against the standard math library, and also against the GNU Scientific Library (GSL). It works off the command line, with no user-friendly GUI. The package includes a self-test is written in Perl, but it's not necessary for the operation of the programs. The GSL will take about 75 MB of disk space to download, compile and install, but much of that is needed only temporarily. The photom code itself requires only a few tens of MB.

Last modified March 27, 2005 by Michael Richmond