Interference Testing

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).

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This page was last updated on August 26, 1999.  Send questions and comments to: