### CATPHOTOMFIT

Fit instrumental to standard magnitudes

#### Description:

This application fits instrumental magnitudes (typically measured from images in a CCD frame) to standard magnitudes in some photometric system for a set of photometric standard stars. The instrumental and standard magnitudes, and other quantities, are read from a catalogue.

The transformation coefficients determined by the fit are written to a file and can subsequently be used to calibrate the instrumental magnitudes of a set of programme objects. The equation used to relate the instrumental and standard magnitudes is:

Mstd = Minst - arb $+$ zero $+$ (atmos $\ast$ airmass)

where: Mstd - standard or calibrated magnitude, Minst - instrumental magnitude, arb - arbitrary constant added to the instrumental magnitudes, zero - zero point, atmos - atmospheric extinction, airmass - air mass through which the standard star was observed.

Note that this relation is a particularly simple way of relating standard and instrumental magnitudes. In particular no correction is made for any colour correction between the standard and instrumental systems.

The application has a number of options, including the following.

$\ast$ Either or both of the zero point and atmospheric extinction can be supplied rather than fitted. If both are supplied then no fit is necessary and the file of transformation coefficients is simply written.

$\ast$ A table showing the residuals between the standard magnitudes and the calibrated magnitudes computed from the instrumental magnitudes can be listed.

$\ast$ The table of residuals may optionally include a column showing a name for each of the standard stars.

$\ast$ Optionally a column containing the observed zenith distance may be supplied instead of a column containing the air mass. In this case the air mass is automatically calculated from the observed zenith distance.

$\ast$ By default all the stars in the input catalogue are included in the fit. However, optionally a column of ‘include in fit’ flags may be supplied and a star will only be included if it has the flag set to TRUE. This mechanism provides an easy way to exclude stars which give a poor fit.

catphotomfit

#### Parameters:

##### FIXED = LOGICAL (read)
Flag; are any of the coefficients fixed or are they all determined by the fit. It is coded as follows: TRUE - some or all of the coefficients are fixed, FALSE - all the coefficients are determined from the fit.
##### ZENITHDIST = LOGICAL (read)
Flag; is the air mass read directly from a column or is it computed from the observed zenith distance? It is coded as follows: TRUE - computed from the observed zenith distance, FALSE - read directly from a column.
##### FZEROP = LOGICAL (read)
Flag; is the zero point fixed. It is coded as follows: TRUE - the zero point is fixed, FALSE - the zero point is determined from the fit.
##### ZEROP = DOUBLE PRECISION (read)
Value of the fixed zero point.
##### FATMOS = LOGICAL (read)
Flag; is the atmospheric extinction fixed. It is coded as follows: TRUE - the atmospheric extinction is fixed, FALSE - the atmospheric extinction is determined from the fit.
##### ATMOS = DOUBLE PRECISION (read)
Value of the fixed atmospheric extinction.
##### RESID = LOGICAL (read)
Flag; are the residuals to be listed? It is coded as follows: TRUE - list the residuals, FALSE - do not list the residuals.
##### CATALOGUE = CHARACTER (read)
Name of the catalogue containing the standard and instrumental magnitudes.
##### NAME = CHARACTER (read)
Name of a column containing names of the standard stars. The special value NONE indicates that a column of star names is not required.
##### INCLUDE = CHARACTER (read)
Name of a column of ‘include in fit’ flags for the standard stars. The special value ALL indicates that all the stars are to be included in the fit.
##### CATMAG = CHARACTER (read)
Name of the column or expression holding the standard or catalogue magnitudes.
##### INSMAG = CHARACTER (read)
Name of the column or expression holding the instrumental magnitudes.
##### AIRMASS = CHARACTER (read)
Name of the column or expression holding the air mass.
##### ZENDST = CHARACTER (read)
Name of the column or expression holding the observed zenith distance.
##### INSCON = DOUBLE PRECISION (read)
Arbitrary constant previously added to the instrumental magnitudes.
##### FILNME = CHARACTER (read)
The name of the file which is to contain the transformation coefficients.
##### QUIET = LOGICAL (read)
Operate in quiet mode where warnings are suppressed. The permitted values are: TRUE - quiet mode, FALSE - verbose mode.

#### Examples:

catphotomfit
The input catalogue and various other details will be prompted for. The transformation coefficients and a table of residuals will be displayed. The transformation coefficients will be written to a file.
catphotomfit zenithdist=true
You should supply a column containing the observed zenith distance rather than one containing the air mass. This column will be used to calculate the air mass automatically.
catphotomfit fixed=true
You will supply either the zero point, the atmospheric extinction or both, rather than allowing them to be fitted. You will be prompted for the appropriate details.
catphotomfit resid=false
A table of residuals will not be listed. However, the transformation coefficients will still be displayed and written to a file.