14 - Optical design

Summary

First steps

Le mieux est l'ennemi du bien. (The best is the enemy of the good.)

Voltaire

Once the task for the spectrograph has been defined, a suitable type may be chosen and a catalogue search made for a possible manufactured model. Many factors affect the decision. The first of course is fitness for purpose. Time and cost of manufacture may be factors and the facilities available for local or in-house design and construction are primary considerations. A decision on whether to buy or to design and build a dedicated instrument must rest on such an appreciation.

It is a mistake when doing fundamental or academic research to construct a more elaborate, high-performance or expensive instrument than the immediate task demands, possibly in the hope or expectation that it may prove useful for some other investigation at some later date. In the author's experience this is almost never the case and the chief result is delay and unnecessary expense. In the long term the instrument is a sad relic, cannibalised of its optical components and left to decay in the attic with its ingenious mechanisms and precision micrometers.

Initial layout

Once the spectrograph type has been chosen and its main parameters such as wavelength range, resolution, type of detector, number of resolved elements, étendue etc. have been decided, a sketch can be made, and the traditional back-of-an-envelope is as good a place as any for this.