This group is for people interested in making their own high quality telescope mirrors. Even though such mirrors may be purchased from professional manufacturers, there is no greater satisfaction than making one yourself, inexpensively, with basic materials in your own home!
The purpose of this group is the compilation, discussion and dissemination of the essential information needed to produce and test your own telescope mirror to your own satisfaction.
While automated machine mirror making techniques exist, this group is primarily aimed at hand figuring methods, most appropriate for the individual enthusiast.
It is also for discussing the principles and practices of testing astronomical optics, particularly star-testing parabolic mirrors. This test, along with a knife edge test for a balanced edge, is the simplest and most accurate way for measuring the quality of your mirror, especially during final figuring under the actual conditions it will be used in the telescope.
The knife edge images shown here illustrate the difference between a perfect mirror (top) and one with some common faults:
1. A turned down edge shows clearly when a knife edge test is nulled near the outer zone of the mirror. The edge diffraction ring is brighter on one side than the other. A perfect mirror has a balanced edge appearance.
2. Surface roughness is apparent as a mottled appearance.
3. A central depression shows as a sharp point in the shadow at the very center.
A good polishing and figuring technique is essential to avoiding these common defects!
Image courtesy of Carl Zambuto.
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