### 4 Introduction

This section briefly gives some background information about astronomical photometry. It starts with the basic definitions and equations of radiation theory. Subsequent topics include: the definition of the magnitude scale, photometric systems, atmospheric extinction, selecting and observing standard stars and finally techniques for measuring and calibrating the magnitudes of objects recorded in CCD frames.

You do not have to read the sections sequentially. You can skip some of the sections if you are already familiar with the material that they cover. Indeed, if you are already familiar with the background to astronomical photometry you can skip this part of the cookbook entirely and go straight to the recipes in Part II.