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Overview

The layout of the A&G unit is shown in Figs. 8i and 8ii. This unit is arguably the most complex part of the observing system to understand. This is because a number of different light-paths exist, in order to support different A&G functions, and these have to fit into a very small space envelope. This situation can be contrasted with the spectrograph itself, where there is just one lightpath, and far more relaxed space constraints. The following light-paths exist within the A&G:

• The main beam from the telescope to the spectrograph slit

• The return beam from the spectrograph slit to the acquisition TV

• The return beam from the spectrograph slit to the CCD autoguider, for guiding on the slit image.

• The beam from the telescope to the CCD autoguider, for offset guiding.

• The beam from the calibration unit to the spectrograph slit.

In the next few subsections, a description of each of the lightpaths is given, along with instructions on how to select them. Finally, how the system is expected to be used in practice is described.

An upgrade of the UES A&G unit is in progress at the time of writing and should come into use in mid--late 1994. The upgraded A&G unit will have higher throughput enabling guiding on objects down to 17 mag. In section 16.8 details of the enhancements are given.