Bath Interferometer page:

How to Build a Bath Interferometer
How to Build a stable 3-Axis Translation Stage
Lessons Learned - Aligning the Bath Interferometer Optics
Interferrogram Analysis Software

(This is a random collection of links, pics and email exchanges until I find time to organise it - apologies!)

My Bath interferometer:

This image shows the interferometer sitting atop my DIY 3-axis stage for fine adjusting the beams. The whole assembly sits on top of a camera tripod.
The top two layers of the stage slide in X and Z axies (perpendicular to and along the optical axis of the mirror under test) each on two polished steel rods
sitting in V-shaped grooves that I routed in to the 1/2" MDF. This works very well. I got the polished steel rods from old 5.25" floppy drives... I *never* throw anything out!
The Y-axis (up-down) is controlled by a screw-jack arrangement (bottom two layers) and doesn't work very well... it was a nice idea at the time :-) but needs re-thinking.

Plan view of the interferometer... which looks like it got attacked by an army of woodworm during construction!

First Light - using a commercial 8" f/6 paraboloid as a test subject.
I haven't eliminated the two internal reflections (bright spots) from the setup yet... and I need to work on enhancing the image contrast big time!

Email exchange on the Interferometry Group mailing list - Re: [interferometry] Re: Foucault testing with a Bath Interferometer? 25 Jan 2008 and later...

Thanks Dale,

I do use Open Fringe, but I have been tracing the fringes.  How do go about not tracing them.  I tried the auto trace but my images are low in contrast and the auto feature doesn't seem to work.  Am I missing something?

Cheers,
Matt.

Dale Eason wrote:

It is good to have a second test method. Many people including me
built interferometers because we did not trust our Foucault
technique, it was tedius to preform, and it can not see non-symetric
errors like astig. So I find it ironic that you want to use Foucault
because you don't trust your interferometery. That makes me think
that you may never have done a Foucault test with data reduction.
Here are some points I would like to make sure you understand.

There is no need to trace fringes anymore if you use OpenFringe. I
almost never do.

I still use foucault to make sure I know the sign of the conic since
that is ambigious with interferometery. There are other methods for
determining that but I have not mastered them. However I cheat
because I have a computer controlled Foucault test system that I just
turn on a let it do the work. I use visual Foucault to check for
smoothness.

If you want to use the same stage to control the Foucault tester then
you will need a way to determine its position accuractly to .001
inch. That is easily done but I just want to remind you of it.

There are very bright LEDs these days. So bright that it hurts your
eye to look at them.

I tried the laser as a Foucault light source some time ago. I found
the laser does not make a good light source for Foucault because of
the diffraction effects of a monochromic light source make it much
harder to read the mask openings. I prefer the smoothing of the
broader band light source. Be carefull about looking at the return
of the laser directly. I took the lens off of mine to make it a
point light source.

One of the advancements in Foucault testing was the slit. Point
light sources cause diffraction artifacts around objects in your
eye. The slit source helps reduce that to make it easier to read.
The bath/foucault will behave like a point light source. You may be
able to put a slit after the expanding lens.

Dale Eason

Matt wrote:

Re: [interferometry] Re: Foucault testing with a Bath Interferometer?

I agree that Interferometers are better, I just happened to notice
what appeared to be a Foucault image and I wanted to investigate.
Plus it will give information about the smoothness of the surface.
Additionally, it's nice to know that two test agree with whatever
result you get therefore you would know that you are doing the tests
correctly. I am still new at tracing interferograms and I like to
know if I am on the right track. If one wishes to use both tests
this would be more efficient in terms of time as you would only have
to setup one test. Also the only Foucault tester I have seen used an
extremely dim LED and the image was not readily visible. The Bath
modification image is plainly visible with the unaided eye.

Use the FFT/vortex method. Use tilt to get many fringes. Something
like 30 or more.
Dale Eason

It says to make an Igram with as much tilt as possible.  I am not familiar with that process, can you give me a quick lesson?
Cheers,
Matt.

Adjust the interferometer so that you see fringes. If you move it
toward the mirror's COC you will get less fringes. Once you have the
smallest number of fringes now move it left/right or up/down. You
will get more fringes. I also like to move it a little behind the
COC. Moving it left/right/up/ down is called adjusting the tilt of the
interferomgram.

You can try simulations using Openfringe to see what I mean. Start
OpenFringe. Click the Simulations and Graph tab. Click the
Simulated Igram button on the right side of the screen.

You now will have a green and black igram that probably has
concentric rings. Next click the "Zernike" menu at the top of the
screen and then select "View Terms". A list of zernike values will
be displayed. There are check boxes on the left that enable each
term. The X and Y tilt terms will not be checked. To the right of
the check box the values field is probably 30. Check the X or Y and
see what happens to the simulated igram. We just added tilt.

It is also possible to change the value fields of any of the terms to
see what happens to the igram. You can also go back to the 3D
display to see what happens to the surface.

Dale Eason

26 Jan 2008 - [interferometry] On Bath interferometer translation stages