1 Introduction

For the photographical magnitudes much work has been done at Harvard College. In the Cape photographic Dm data about such magnitudes are contained for all the stars south of declination 19, down to about 9.5, together with a great mass of fainter ones.

The difficulty of reducing all these magnitudes to one and the same homogeneous and rational scale has however, not been altogether overcome.

Plan of Selected Areas,
J.C. Kapteyn, 1906.

Nearly a century has passed since Prof. Kapteyn made this remark and during the intervening period much effort has been devoted to developing techniques for the proper calibration of astronomical photographs and latterly CCD images. The techniques for calibrating CCD images are now well established. However, they must be applied carefully if accurate results are to be obtained.

In essence, astronomical photometry is concerned with measuring the brightness of celestial objects. CCD (Charge-Coupled Device) photometric calibration is concerned with converting the arbitrary units in which CCD images are recorded into standard, reproducible units. In principle the observed brightness could be calibrated into genuine physical units, such as W m2, and indeed this is occasionally done. However, in optical astronomy it is much more common to calibrate the observed brightness into an arbitrary scale in which the brightness is expressed relative to the brightness of well-studied ‘standard’ stars. That is, the stars chosen as standards are being used as ‘standard candles’ to calibrate an arbitrary brightness scale.

The reasons for wanting to calibrate CCD frames are virtually self-evident. Once an image has been calibrated it can be compared directly with other photometrically calibrated images and with theoretical predictions. The calibration of CCD frames is simplified because CCD detectors have a response which is usually for all practical purposes linear; that is, the size of the recorded signal is simply proportional to the observed brightness of the object.

This cookbook provides a set of simple recipes for the photometric calibration of CCD frames and sufficient background information about astronomical photometry to allow you to use these recipes effectively. The structure of the cookbook is:

Part I
– background material,
Part II
– the recipes.

You should not require any prior knowledge of astronomical photometry to use the cookbook. Part I gives the necessary background information. You should realise, however, that it only covers the basics of the subject and in sufficient detail to allow you to use the recipes effectively. Many of the finer points and details are deliberately omitted for clarity and brevity. However, references to additional details are given throughout and the next section gives some useful general references. In summary, the cookbook will not turn you into an expert in astronomical photometry, but it does give recipes which you can use for the photometric calibration of CCD frames.

It is not necessary to read the cookbook sequentially from beginning to end. You can skip some or all of the background material if you are already familiar with it. Similarly, the individual recipes are independent of each other and you can use just the ones appropriate for your purposes.

It is also worth noting that astronomical photometry is a diverse subject. There are many different ways of making and reducing photometric observations. Observations can be made for very different types of programmes and require very different standards of accuracy. The cookbook describes the most common method of calibrating observations using standard stars. This method is not the only one, nor always appropriate. You might, for example, only be interested in the relative brightness of two objects, or the brightness of an object relative to the night sky. Some of the recipes in the cookbook are not appropriate in these cases.

It is usually necessary to correct for various instrumental effects which are present in CCD images, such as bias signal, flat field sensitivity variations and cosmic-ray events, before they can be photometrically calibrated. These corrections are usually referred to as ‘CCD data reduction’ and should be carried out before you attempt the photometric calibration. CCD data reduction is not described in this cookbook, but is covered in the companion document SC/5: The 2-D CCD Data Reduction Cookbook[18], which is a good introduction. It is not considered further here.