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.
In any case; don't be discouraged by my remarks. You should join the interferometry
list at http://groups.yahoo.com/group/interferometry.
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.
RO
David Wright wrote:
> Has anyone built the Ceravolo spherical interferometer and used the
> recommend interpretation software? I would like any comments on this. I am
> planning to figure about a dozen optical over the coming year and would like
> to assemble a great testing setup.
> Any other information on semi affordable test rigs and other methods for
> quantifying a surface figure would be appreciated.
> I also would like to have someone machine and assemble the necessary parts
> for me as well.
> Thanks
> David
>
>
Received on Tue Apr 5 18:29:44 2005