Convex surfaces may be tested by the knife edge from behind if we test through a known good concave surface of similar radius (S & T Jan. 1970). Since such a concave surface already exists on the tertiary mirror, all that is needed is to `fuse' the two spherical mirrors together with household glycerine whose refractive index is identical to Pyrex. The glycerine will fill in any scratches or pits remaining on the backsides.
A custom stand is required. Mine took one evening to throw together from 1/2" plywood and pine strips. The photographs should speak for themselves on this matter. Notice that I purposely angled the test stand slightly forward. I no longer feel this is necessary; supporting the rear disk behind its center of gravity should insure that the two stay in contact.
To test the convex backside, the Foucault tester must be closer to the assemblage than it would be to test the concave front. Under test, the shadows will have a beautiful indigo color while striae remain uniformly gray. Test for a null.
Practice will show you just how much glycerine to pour on the back of the secondary before putting it on the rack. Squeeze the sides of the disks to spread it out. Slide the convex off when done testing. The concave mirror isn't removed from the stand until the figuring is finished. Plain water will clean up the secondary for the next round of figuring.
Curator: Hartmut Frommert
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