You need a high numerical aperture measurement beam to cover the whole
mirror. This means either a very short focus lens in the Bath or widen
the beam from the laser.
I did the latter for an F/3 and F/2.4 mirror respectively, although not
as big as yours. For beam expansion I took off the laser lens, and
replaced with a longer focal length F/1 projector objective. See here:
Steve Follett wrote:
> I'm a member of a group who are constructing a 40 inch
> telescope for Robert Ferguson Observatory in Sonoma County, CA.
> The mirror we are working on is
> 40 inch F:3.66 paraboloid
> 2.25 inch thick (at the edge)
> Can I use a Bath interferometer to reliably test a mirror
> this big and this fast?
> I posted this question a couple years ago, and it wasn't clear
> then that it would work well. Some posts suggested that a PSI
> Bath might work. In the meantime we went ahead and made the
> mirror and tested it as best we could.
> We are now at the point that I would like to switch to
> interferometry if we can.
> We have tested up to this point using the King wire test and
> Foucault testing. We were able to tell we had astigmatism by
> testing on several axes and comparing the results.
> We used a test that Richard Schwartz described in a post to
> the Amateur Telescope Making listserv to measure the
> astigmatism. The test used a mask with holes in a 'X' pattern,
> similar to a Hartmann mask. The mask was set up so that the
> pattern fell on the 45 degree axes of the mirror.
> We then measured the ROC of each axis. Any difference between
> the two axes would indicate astigmatism and allow us calculate
> the size of the error (on those two axes). We then rotate the
> mirror 22.5 degrees and tested again until we had measurements
> on 16 axes (i.e. every 22.5 degrees).
> We had measurable astigmatism initially of about 5 waves. We
> corrected this using local polishing to the point that we
> couldn't reliably detect it anymore.
> We have as a result a mirror with some remaining astigmatism
> (detectable a star test) and probably a very bumpy figure from
> the local polishing. The problem is that we are seeing things
> in a star test that I don't understand. The image does come to
> a focus but outside of focus two lobes appear to collapse back
> toward the center of the pattern.
> Here's a link to video of the star test:
> I would really like to have a surface map to understand what's
> going on and improve the mirror.
> BTW, I want to try making a PSI Bath and I need a source for a
> polarizing beam splitter and a suitable ¼ wave plate at minimal
> Steve Follett