The photograph below shows the secondary mirror tested against its 5" diameter plate glass tool. The tool was figured spherical by null testing with a Foucault tester. In performing this interference test the mirror rested atop the testing plate and was illuminated by an alcohol flame soaked with table salt. A convenient way to provide this illumination is to use a ~1" wide pure cotton wick (an old T-shirt) inserted in a slot cut into a mason jar lid. The salt is piled on the mason jar lid around the cotton wick. Methyl alcohol is a good fuel because it is cheap and provides a colourless burn. The mirror should not be directly illuminated by this lamp, instead a glass plate tilted at 45o is used to direct the light from the lamp straight down onto the mirror. The fringes are observed by looking straight down at mirror; this combination of observing position and source of illumination ensures that both the light source and direction of observation are perpendicular to the mirror surface. The photograph was taken using a 45mm lens at f1, ASA 200 film and an an exposure time of approximately 5 seconds. The departure from "circularity" of the interference fringes was estimated by using a clear plastic template on which circles of incremental size had been printed (the template was drawn up using CAD and then printed on overhead display material).