The majority of today's amateur astronomers do not make their own telescope optics. Nonetheless most of us are aware of the basic procedures involved and are familiar with the basic principles of the Foucault and Ronchi grating optical tests. I would, however, wager that very few telescope users realise that simple versions of these tests can be used to evaluate the overall accuracy of both figuring and alignment of the assembled telescope's optics with much greater ease than the toiling mirror maker can assess the individual components in his/her workshop.

The easiest method involves using a Ronchi grating. Many optical firms, especially those dealing in telescope optics and telescope-making materials, sell these gratings. They are very cheap to buy. If a choice is given select a grating of at least four lines per millimetre (100 lines per inch), though more lines per millimetre will provide a more sensitive test.

Simply mount a small piece of the grating over the central hole in a ‘dummy eyepiece’ (a collimation tool described in Appendix 1) and you have all you need to make the evaluation.

Actually I mounted my own grating in place of the eyelens of a cheap discarded eyepiece – all the lenses first being removed from it. I store the unit in a container, to keep it clean, and so it is always ready for use at a moment's notice.

Using this device could not be easier. Simply set the telescope on a fairly bright star in the normal way.