### 1 Introduction

The ECH package consists of a set of routines which model the behaviour of the UCL coudé echelle spectrograph (UCLES) and of echelle spectrographs of similar design. Such spectrographs can differ in

• collimator focal length (which is in fact irrelevant to the model package)
• cross dispersing prism apex angle
• echelle ruling frequency
• echelle blaze angle
• camera focal length

It is not capable of dealing with spectrographs which differ in any more major ways from UCLES, although it could be extended to do so should the need arise.