As John Dobson says, test your mirrors at the focus. Why struggle with
indirect tests, when the bottom line is what you see in the eyepiece. When
the star test shows its perfect, your finished figuring. QED.
Mike Linnolt
On Tue, 5 Apr 2005, Richard Ozer wrote:
> Interferometry is tricky business. I've been peripherally involved in a Bath
> interferometer project, using fringeXP for the final analysis. You can download that
> software on the Ceravolo web site: http://www.ceravolo.com/fringe/FringeXP/FringeXP.htm
>
> It's important to realize that each testing methodology has inherent errors.
> Interferometry is theoretically more precise than a foucoult test, but in practice is so
> sensitive to abberations in the tester, temperature differentials, and positioning of
> the mirror, that your results may not be any more accurate than using a lower tech
> methodology. Depending on the size and application of your project, you may end up
> gaining very little by building an interferometer, except the fun of doing so.
>
> Each type of test is valuable in its own right. Use a ronchi test to get a rough idea
> of your mirror's figure, use a foucault test to get a good reproducable benchmark, and
> use interferometry as a third test. If all three agree; you're right on the money.
> But, you can't necessarily believe one over the other unless you've calibrated your
> testers against a "known" piece of glass... and then you have to ask how it is known!
>
> I recall one TAC member (I think it was Jeff) who purchased a sizable mirror from a well
> known manufacturer. That mirror came with the results of an interferometry test. It
> turned out that it had a terrible turned down edge.. the placement of their
> interferometer was off, just enough to discount edge abberations. A simple ronchi test
> demonstrated the problem.
>
> If you come up to the Chabot Telescope Makers' Workshop, you should hunt down Gert
> Gottchalk (also a Tri-Valley Stargazers Member). He's the builder of the interferometer
> I mentioned above... you should also talk to Dave Barosso, who has a great deal of
> experience with other types of tests.. caustic and wire.
Received on Tue Apr 5 19:44:28 2005