The purpose of this book is two-fold. Firstly to provide a handy quick source of ref­ erence of the terms, techniques, instruments, formulas, processes, etc. , for practising observers, whether it is their first look through someone else's small telescope, or whether they have spent decades building their own instruments, observing with them and are regularly producing results to rival those of the professionals. It is not principally aimed at professional observers, but in order to be sufficiently compre­ hensive for its intended audience, many entries have been included which will be relevant at that level. In particular though, the more esoteric parts of spectroscopy have not been included. References to specific observatories are included if their instrumentation includes optical telescopes over 1 m in diameter or radio dishes over 20 m. Individual entries for telescopes of 4 m or over are included, and for the larger radio instruments, plus other telescopes that may be of interest for historical or other reasons (for example the 1m Yerkes refractor). Spacecraft have generally not been included (apart from the Hubble space telescope) because their short working lives mean that most current spacecraft will no longer be operating by the time that this book is published. Also the names of spacecraft are frequently changed after launch making it difficult to identify which mission is which. References to commercial organisations, and to some widely available commercial products have been includ­ ed, but an intending purchaser should always obtain up-to-date information.