### 8 Introduction

This part of the cookbook provides a set of simple recipes for reducing CCD images and performing various related auxiliary tasks. The recipes are:

• importing data (Section 9),
• displaying images (Section 10),
• calculating image statistics (Section 11),
• simple removal of instrumental effects (Section 12),
• advanced removal of instrumental effects (Section 13),
• combining target images (Section 14),
• reading FITS files from tape (Section 15),
• handling large images (Section 16).

The first three recipes (Sections 9 to 11) demonstrate preliminary, auxiliary tasks. Two alternative recipes are provided for removing the instrumental effects from CCD images. Section 12 is a very simple recipe using software operated from an easy-to-use GUI (Graphical User Interface). Section 13 is a more advanced recipe using software operated from the Unix command line. Though more complicated than the preceding recipe it offers more flexibility and perhaps greater insight into the operations being performed on the data. Section 14 is an example of combining reduced images.

Recipes 9 to 14 form a natural sequence and are intended to be worked through in the order in which they are given. Each of these recipes consists of a set of numbered steps which you can follow and example data are provided with the cookbook so that you can work through them yourself. On Starlink systems these example data are kept in directory:

/star/examples/sc5/data

These data are R band images of the spiral galaxy NGC 2336 obtained with the Jacobus Kapteyn Telescope (JKT) on La Palma. They were obtained from the CD-ROM Astronomical Images by Jaffe[14]. However, the same data are publicly available from the ING (Isaac Newton Group) data archive maintained at the Institute of Astronomy, University of Cambridge. See URL:

For technical reasons there are some differences in the arrangement of the FITS headers in the archive and CD-ROM versions of the files but both contain identical astronomical information.

The final two recipes, Reading FITS Files from Tape (Section 15) and Handling Large Images (Section 16) are rather different. They contain material which could not be presented conveniently as a set of numbered steps which you can follow, but rather are given as a set of hints and tips.

The packages used in the recipes in this cookbook are GAIA (see SUN/214[11]), CCDPACK (see SUN/139[10]), KAPPA (see SUN/95[6]) and ESP (see SUN/190[12]). These items should all be available at all Starlink sites. If you have any difficulty in accessing them then see your site manager in the first instance. In all cases on-line hypertext versions of the manuals are available via the command showme. For example, to display the CCDPACK manual, SUN/139, you would simply type:

% showme sun139

IRAF examples equivalent to the recipes for displaying images (Section 10) and calculating statistics (Section 11) are included in SG/12[22], the introduction to IRAF on Starlink systems. IRAF includes the powerful and flexible package CCDRED for reducing CCD data; see Section 6.1 for a list of the documentation for it.

#### 8.1 Getting started

In order to work through the recipes you should use a colour display capable of receiving X-output (typically an X-terminal or a workstation console). Before starting you should ensure that your display is configured to receive X-output.

To work through the recipes as they are presented you should take a copy of the example data provided with the cookbook. Proceed as follows.

(1)
Create a new subdirectory, perhaps called sc5, in some convenient location and make it your current directory:
% mkdir sc5
% cd sc5
(2)
Copy the example data files into this subdirectory:
% cp -r /star/examples/sc5/data .

Note that the data files are kept in various subdirectories of /star/examples/sc5/data and a recursive copy (the ‘-r’ option) is used to preserve this structure. Each subdirectory contains a different type of file, as follows:

bias
– bias frames,
flats
– flat fields,
targets
– images of target astronomical objects.

Keeping the different types of files in separate subdirectories makes them easier to manage.

As an alternative to using the data supplied with the cookbook you could use data of your own. Suitable data are available on the CD-ROM Astronomical Images[14] or from the ING data archive. However, if you substitute your own data you will need to ensure that you know the extent of any bias strips on the CCD frames and similar auxiliary information. For data from Astronomical Images these details are given in file descript.ccd included on the CD-ROM.