Photom -- calculate and apply a photometric solution to astronomical data

The basic idea is to calculate a photometric solution for a night, given a set of catalog stars with known magnitudes and a set of detected stars with instrumental magnitudes.

Usage:

        photom   catalog_file  racol deccol cat_nfilt  V=4 B=6 
                 detected_file racol deccol jdcol aircol flagcol
                 det_nfilt a,b,c d,e,f 
                 mode=   [fixk= ... ]
                 [det_files ... ]
                 [radius=] [outfile=]
where
       catalog_file   is name of the ASCII data file containing catalog
                            of standard star data

       racol          is the column of the catalog file in which are the
                            RA values (decimal degrees, J2000)

       deccol         is the column of the catalog file in which are the
                            Dec values (decimal degrees, J2000)

       cat_nfilt      is the number of "filter=column" pairs for the
                            reference catalog, immediately following.
                            Must be >= 1

       V=4            is a "filter=column" pair; here, "V" is the name
                            of a filter, and "4" is the index of the
                            column containing magnitudes in that filter
                            in the catalog

                      There must be exactly "cat_nfilt" such arguments
                            denoting passbands in the catalog file.


       detected_file  is the name of the ASCII data file containing
                            stars detected in some image, with instr. mags

       racol          is the column of the detected file in which are the
                            RA values (decimal degrees, J2000)

       deccol         is the column of the detected file in which are the
                            Dec values (decimal degrees, J2000)

       jdcol          is the column of the detected file in which are the
                            Julian Date values

       aircol         is the column of the detected file in which are the
                            airmass values

       flagcol        is the column of the detected file in which are the
                            "quality flag" values

       det_nfilt      is the number of "filter=magcol,magerrcol" pairs 
                            for the detected files, immediately following.
                            Must be >= 2

       a,b,c          denotes some passband in the detected file:
                            The values a,b,c are integers, where
                                     a   =   column with passband name
                                     b   =   column with magnitude val
                                     c   =   column with magerr val;
				               negative value means a
					       saturated star
                            A comma "," separates the integers.

                      There must be "det_nfilt" such arguments denoting
                      the passbands in the detected file.

       mode           describes the form of the photometric solution.
                      Valid options are:
                              extinct       include extinction term,
                                            so equation looks like
                                        V = v + a + b*(color) + kX

                              noextinct     no extinction, but use
                                            one zeropoint per frame j:
                                        V = v + a  + b*(color)
                                                 j

       fixk           Valid only in "noextinct" mode, so that we are
                      NOT trying to determine the extinction coefficients.
                      Use a fixed value of first-order extinction in 
                      each passband, so that
                                        V = v + a  + b*(color) + kX
                                                 j

                      The user must specify the passband name and
                      first-order extinction value, separated by commas:
                                fixk=V,0.20      sets k=0.20 in V band
                                fixk=I,0.08      sets k=0.08 in V band

                      This option can be used to correct for differential
                      extinction across a frame, while using standards
                      within the frame to set the zero point.

       det_files ...  are optional additional names of files with data
                            on stars detected in other images.
                            They are assumed to have the same arrangement
                            of data columns and filter names as the
                            first file

       radius         (optional) radius (in arcseconds) to use when 
                            matching stars in catalog against the
                            detected stars
      
       outfile        (optional) base name of file into which to 
                            write diagnostic output.  

Note that all column numbers should be supplied by the user in 0-indexed form -- this, the first column of data in a file is "0", the second "1", and so forth.


Modes of photometric solution

The photom program can produce three different forms of solution for a set of measurements.

mode=noextinct and no fixk options
Do not include any terms for extinction. In this case, we give each pair of frames its own zeropoint, and use a single color term for all frames. The equation connecting the input, raw magnitudes (lower-case) to the output, calibrated magnitudes (upper-case) is
            V  =  v  +  a    +  b*(v - i)
                         j
    
where V is the output, calibrated magnitude, v is the input, raw magnitude in one passband, a_j is the zero-point offset for frame j, and b is the color coefficient. The term (v - i) is the color of a star in raw magnitudes.

If there are 20 pairs of V,I frames, then the program will solve for 21 unknowns pertaining to the V-band: 20 zeropoint offsets a_j and a single color term b. It will also solve for 21 different unknowns pertaining to the I-band.

mode=noextinct together with fixk options
Do not try to solve for extinction, but use fixed values provided on the command line. In this case, we give each pair of frames its own zeropoint, and use a single color term for all frames. The equation connecting the input, raw magnitudes (lower-case) to the output, calibrated magnitudes (upper-case) is
            V  =  v  +  a    +  b*(v - i)  -  k*X
                         j
    
where V is the output, calibrated magnitude, v is the input, raw magnitude in one passband, a_j is the zero-point offset for frame j, b is the color coefficient, k is the first-order extinction coefficient provided by the user, and X the airmass. The term (v - i) is the color of a star in raw magnitudes.

One might use this mode with very wide-field images. There are plenty of stars of known magnitude, but differential extinction causes systematic errors for a fixed zeropoint.

mode=extinct
Include a term for first-order extinction. In this case, we use a SINGLE zero-point value for all frames in a passband. Once again, we also apply a single color term for all frames. But now we also solve for an extinction coefficient based on the airmass of each measurement. The equation connecting the input, raw magnitudes (lower-case) to the output, calibrated magnitudes (upper-case) is
            V  =  v  +  a    +  b*(v - i)  -  k*X
    
where V is the output, calibrated magnitude, v is the input, raw magnitude in one passband, a is the single zero-point offset, b is the color coefficient, k is the extinction coefficient, and X is the airmass of each measurement. The term (v - i) is the color of a star in raw magnitudes.

If there are 20 pairs of V,I frames, then the program will solve for 3 unknowns pertaining to the V-band: 1 zeropoint offsets a, 1 color term b, and 1 extinction coefficient k. It will also solve for 3 different unknowns pertaining to the I-band.


Input files

Here's an example of the input file formats. Lines which start with a pound character "#" will be ignored. One can use input files which have a different number of columns of data, or columns in different order, as long as the command-line arguments to photom correctly describe the required data columns.

We could run the program with a command line like this:

   photom refcat 0 1 2 V=3 I=4 rawmags 1 2 3 4 11 2 5,6,7 8,9,10 mode=extinct


Output files

We create three types of output file:

Here are examples of the output file formats. Continuing the example used to illustrate the input file formats (using data files with hundreds of stars), we create the following files:


Last modified Aug 2, 2003, by MWR.