Collimation is the process that is used to align the optics of a telescope in order to provide images of the highest possible quality, whilst star testing is a way of testing the optics of a telescope. These two topics are combined in this chapter because star testing can show that a telescope needs collimation and also provides the way of making the final collimation adjustments. Star testing can also highlight other optical problems and in some cases allow them to be corrected but, be warned, it is exceedingly sensitive and very few telescopes, though providing excellent images, will give perfect star tests. In more complex optical designs, star testing might well give somewhat odd results and will not be a good indicator of the telescope’s optical performance.

Refracting telescopes rarely, if ever, need any maintenance but, after a year or two, the primary mirror of a Newtonian or the corrector plate of a Schmidt-Cassegrain may need cleaning. This is not too frightening a prospect, provided that it is done carefully, so let me first describe how best to do it. If the mirror surface is in really bad condition, with the reflecting surface beginning to erode, this might be the time to have it re-coated. If so, it is worth having one of the new multi-coated, high-reflectivity coatings applied, such as the Hi-Lux coatings provided by Orion Optics in the UK. Not only should such coatings last for many years, the overall contrast of the telescope will be significantly improved.