Photometric calibration

The purpose of the photometric calibration functions in CURSA is to convert a list of instrumental magnitudes, typically measured for a set of objects in a series of CCD frames, into calibrated magnitudes in some standard photometric system. To fix ideas, think of a group of programme objects for which instrumental magnitudes have been determined from a set of CCD frames using an aperture photometry package such as PHOTOM (see SUN/45[14]). These instrumental magnitudes are to be calibrated into standard $R$ magnitudes in the Johnson-Morgan UBVRI system.

Astronomical photometry is a diverse subject. There are many different standard photometric systems, many ways of making photometric observations and many ways of reducing them. CURSA provides only some simple and basic facilities. Though they will be useful and give reasonably accurate results in many circumstances they are certainly not appropriate in all circumstances. In particular, they are not suitable for high precision photometry. Whether they are suitable for you will depend on the details of your programme.

This section is not an introduction to how to calibrate photometric observations. Rather, it describes the principles behind the CURSA photometric calibration functions so that you can decide whether they are suitable for your purposes and describes how to use them. For a more general introduction to calibrating photometric observations see SC/6: The CCD Photometric Calibration Cookbook[22]. SC/6 also includes a tutorial example (a `recipe' in the jargon of cookbooks) of using the CURSA photometric calibration functions.