Standard star catalogue

Figure  shows an example catalogue of standard stars. The observations used in this example were kindly provided by John Lucey. The example is available as file:

/star/share/cursa/photostandards.TXT

The catalogue must contain columns containing the instrumental magnitude, the catalogue magnitude and the air mass (or alternatively the observed zenith distance). It may optionally contain a column containing a name for each of the standard stars and a column of `include in the fit' flags. All five columns are included in the example. If supplied, the star name is listed in the table of residuals produced when the fit is made. Often being able to identify each standard star will be useful to you. The `include in the fit' flag column is of data type LOGICAL and determines whether each star is included in the fit or not. To include or exclude a given star in the fit you simply edit the STL format catalogue and toggle the value of the flag for the star to `T' (or `TRUE') or `F' (or `FALSE') to include or exclude it as appropriate. This procedure is much less troublesome and error-prone than deleting and reinserting stars from the catalogue. Initially set the flags for all the stars to `T' (or `TRUE') so that they are all included in the fit. In the example all the stars are included in the fit except 99Z367 (the penultimate one in the list). This star is excluded as an illustration. When preparing your own catalogues you will usually initially include all the stars.

Figure: Example of a catalogue of photometric standard stars

The zenith distance is an angle and if it is used it must ultimately be presented to the CURSA applications in radians. If you wish you can simply type the values into the STL catalogue in radians. Alternatively, if it is more convenient, you can define the zenith distance column as containing a sexagesimal angle, usually in degrees, and type in the values as sexagesimal degrees. The example catalogue of programme objects in Figure  includes a column of zenith distances in this form.

Though both the columns of star names and `include in the fit' flags are optional I recommend that you use them.

The columns do not have to have the names shown in the example. However, if you use these names you will be able to accept the defaults from the prompts in the CURSA applications.

Obviously the catalogue can contain additional columns, though these are not used. For example, if you are calibrating multi-colour photometry you could prepare a single catalogue containing the instrumental and catalogue magnitudes in all the colours observed. Obviously the columns for magnitudes in different colours would have to have different names. If you did not observe all the stars in all the colours simply use the STL mechanism for indicating null values (see Section ) to represent the missing measurements.