COMBINE takes a reference file and a data file, both in DAOPHOT style, and merges them, matching stars by position and magnitude. There are two main modes. The first produces a .MER file which has the magnitudes from both frames, the difference, and lots of other informations. The second mode, used when the CMB= keyword is used, produces a file which has the positions and magnitudes of both frames. This file can be rerun through COMBINE to add more frames. This is used to combine multiple observations of a given set of stars. The first mode is useful only for comparing 2 frames. It is largely historical.
It is strongly suggested that both files be sorted from brightest to faintest before running. Files can be specified by REF=file and DAT=file, or will be prompted for. They must have star number, column position, row position, and brightness in the first 4 columns. The brightness will be assumed to be in magnitudes if the file has extension .NST, .TST., or .COM, or if the keyword REFMAG is specified.
Stars will be based on a matching criteria of magnitudes and/or distance, with the maximum magnitude difference specified by MAG=maxm and the maximum position difference by DIST=maxd. Either criterion can be shut off by supplying a very large value. The defaults are infinity magnitudes and 1. pixel for maxm and maxd. Several passes may be made through the reference with successively less stringent matching criteria in distance by specifying several distance criteria with the DIST=d1,d2,... keyword.
If the REF keyword is specified, the coordinates of objects in each frame will be transformed according to transformations loaded in the header using the REGISTER command. Alternatively, use the OFFSET command to transform the coordinates before running COMBINE.
The output in the first mode is a .MER file with a variety of information, as follows, by column:
|1||True mag||9||2 sigma|
|2||Obs mag||10||true frac dist|
|3||diff||11||obs. magnitude error|
|4||obs. row||12||position error|
|8||2 sigma||16||DAOPHOT star number|
To do the noise estimates properly, one must specify the sky value with SKY=, gain with GAIN=, readout noise(electrons) with RN=, NEFF=. To accommodate tests with the same reference file at various levels, we have the keywords FACT=f, which specifies a multiplier to be applied to the reference counts, and NORM=n, which specifies a 0 point. These default to 1 and 0 (for DAOPHOT magnitudes), respectively.
With the COMB or COMB= option, we will produce a new version of the reference file, with additional columns for matched information from the data file. This can be rerun through COMBINE as the reference file to get yet another set of matched observations, etc., up to six times.
With the PLOT option, will produce a plot of REF MAG vs. DELTA MAG and a histogram of matched and unmatched stars as a function of magnitude. With the MER option, this plot is made directly from a previously existing .MER file, if the matching has already been done previously (e.g., in batch).