1 Introduction

What are the roots that clutch, what branches grow
Out of this stony rubbish? Son of man,
You cannot say, or guess, for you know only
A heap of broken images

The Waste Land,
T.S. Eliot, 1922.

Two-dimensional optical CCDs (Charge-Couple Devices) and the infrared arrays which are their close kin are now the type of detectors usually used to produce direct astronomical images (that is, simple pictures of a region of sky) at optical and infrared wavelengths. These arrays are much more sensitive and have a much larger useful dynamic range than the panoramic detectors used hitherto (principally the photographic plate) and it is hardly an overstatement to say that their widespread adoption in the past two decades has effected a revolution in astronomy. However, the un-processed images, as they are obtained from CCDs, are affected by a number of instrumental effects which must be corrected before useful results can be obtained. This cookbook is concerned with removing these instrumental effects in order to recover an accurate picture of the field of sky observed. This process is normally called ‘CCD data reduction’ though, figuratively at least, it can just as well be thought of as repairing a ‘heap of broken images’.

The cookbook is primarily concerned with reducing direct images observed with optical CCDs. However, it contains additional material covering reducing direct images obtained with infrared arrays. Also, much of the material is relevant for reducing spectra recorded with two-dimensional CCDs or infrared arrays: the preliminary stages of reducing CCD spectra are the same as those for reducing direct images. The techniques for reducing CCD data are now well established and suitable software is readily available. However, the procedures must be applied with care if accurate results are to be obtained.

The cookbook includes a set of recipes for reducing CCD data and a set of scripts which automate some parts of the process. It also presents sufficient background material to allow you to use the recipes and scripts effectively. No prior knowledge of CCD data reduction is assumed. The structure of the cookbook is:

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

It is not necessary to read the cookbook sequentially from beginning to end. If you are already familiar with the principles of CCD data reduction you can simply skip Part I and go straight to the recipes or scripts. Similarly, you do not need to follow all the recipes or use all the scripts: just try the ones appropriate for your purposes.

The final product of CCD data reduction is an image which accurately reproduces the brightness distribution in the field of sky observed (subject to the limits on spatial resolution imposed by atmospheric seeing and the instrumental profile, of course). This image is in entirely arbitrary units. Such images are adequate for some sorts of programme (for example, for comparing the surface brightness of different parts of a nebula or galaxy). However, for other sorts of programme you will need to calibrate the arbitrary brightness of objects in the image into a known photometric system. This calibration (which can be quite involved) is beyond the scope of this cookbook, but is covered in the companion document SC/6: The CCD Photometric Calibration Cookbook[26]. It is not considered further here.