Introduction
For astronomical observations, reflecting telescopes have replaced the refracting type of instrument. Large mirrors with great light-gathering power are lighter, more rigid and easier to control than equivalent lenses; lenses are also plagued by chromatic aberration which brings different colours to a focus at different points. One of the largest refractors made, the 28″ at the Old Greenwich Observatory, is a beautiful instrument but suffers badly from chromatic
aberration as you will find if you get an opportunity to use it. Both reflectors and refractors, however, have an intrinsic limit to their resolution, that is to their ability to distinguish two objects which appear very close together in the sky with a very small angular separation. This quality of a telescope, called its angular limit of resolution, or just its resolution, is determined by its aperture — the larger the aperture, the greater the resolution (and hence the smaller the
angular separation that can be resolved). The object of this part of the exercise is to set up a reflecting telescope, to measure its resolution, and to compare this both with that of the unaided eye and also with an estimate from theory. An example of an angular measurement in astronomy completes this part.