1.) First of all Rough grinding is used to 'cut out' the glass and produce a concave curve in to the mirror's surface. During this period one uses the 'chordal' stroke and applies pressure to ensure that the grit cuts away glass. One also offsets the mirror about one-third to half of its diameter (taking care not to allow it to 'tip' over the edge of the tool.) This is done around successive 'chords' of the mirror and tool by 'walking round' the grinding table (or by turning the tool and mirror if working on a bench,) so that the grinding is spread evenly all the way around both mirror and tool. When the depth of curve is very close to that required - the stroke is changed to a 'W' stroke. This improves the accuracy of the curvature during the last stages of grinding to the required depth.
2.) During the next process, which is smoothing the mirror - the pressure is simply the weight of the mirror and the hands resting gently on top, to guide the mirror through its 'W' strokes. In this process which consists of using gradually finer grades of grit, the purpose is to 'let the grit' do the work. The aim is to get the surface of the mirror smooth enough to finally polish it, so that even with no coating applied it will reflect light - like any highly polished surface. When really smoothed the surface should show reflections of bright objects at a shallow angle. Also at the start of smoothing it is best to adjust (if necessary) the depth of curve as accurately as possible this can be done by reversing the discs and working with either the mirror on top (to increase the depth) or using the tool on top (to reduce the depth.)
3.) Polishing - this is a different process again from the previous grinding and smoothing. A polishing 'lap' is made by applying pitch squares to the tool and 'wet-pressing' it against the mirror. This makes the pitchcovered surface adopt the exact shape of the finely smoothed mirror's surface. Then an optical grade polish mixed with water is used (usually Cerium Oxide or Zirconium Oxide.) Polishing a telescope mirror calls for patience and many hours of work. To properly polish even a 6 inch diameter disc can take from 8 to 10 hours, and a 10 or 12 inch disc may need 15 hours or more.Back to Mirror Making Page