Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1477
Loc: Sin-sin-atti
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Here's an interferometer that I built for a mirror maker friend of mine. He wants to test both diagonal mirrors as well as telescope mirrors. I used a HeNe laser I bought off _bay, a 4 mm half ball lens, PBS and 1/4 wave film (surplus at work), a 4 inch lens an cell (from my collection of odds an ends) and a Cervit flat. It needs a cover and a better way of changing between the flat and null lenses (not shown) but I'll let my buddy worry about that.
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Ed Jones
Edited by Ed Jones (10/17/08 10:06 PM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1477
Loc: Sin-sin-atti
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Just to try it out here's a flat tested with it, actually the back side of a mirror that was polished. There's a reflection fom the front side.
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Ed Jones
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kfrederick
scholastic sledgehammer
Reged: 02/01/08
Posts: 783
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hi ed what type is it? does not look like a bath/ thanks kevin
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1477
Loc: Sin-sin-atti
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Kevin this is a Fizeau much like a Zygo.
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Ed Jones
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 906
Loc: rushville, indiana
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can we delve more into this design and its concepts as i am a not familiar with these tools as i would like to be. and perhaps some pictures and explainations as to your concept shown here. i am trying to build one just have not decided on what kind to build as of yet. if ther is a place that shows more info other than a wiki which i have read and i joined the yahoo group but does not have any design info perhaps you can help and direct me to a place.
doc
doc
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Longitude -85.42786 Latitude 39.59153
"Current Build 127mm refractor "Cosmos I" getting closer
"current build 6 inch F6.5 "Little Toe" waining
"if all else fails use duct tape "works for me"
"Remember there is no spoon"
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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Could be that the fellows need a schematic, showing where the image is formed???
Mark
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So many projects, so little time!
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1477
Loc: Sin-sin-atti
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Here is a diagram of this interferometer. I would probably use a regular beamsplitter cube if I were to do it again. A PBS and 1/4 wave film is more efficient (doesn't lose light) but the image is uncomfortably bright anyway and more complicated. The transmission flat is replaced with a null lens for testing mirrors.
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Ed Jones
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reodds
member
Reged: 05/04/06
Posts: 76
Loc: Merrick, New York
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Ed, not being an optician like you, would you please give us a materials list, and likely sources.
This is a very important testing method, perhaps THE definative mirror test, and should be spelled out in much more detail, for the non optical experts on this forum.
Perhaps if you would be so kind as to make this into a realtively easy to follow step by step project, the moderator could (and should) make it into a sticky.
Thanks for your great contibutions to CN!
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Roger
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 906
Loc: rushville, indiana
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agree with reodds in the fact that many of you are very enlightened and very good with information that the time has come to make a sticky of this sort of a gatherering of the minds,if you will,for those of use not so well equipped. the drawing was very helpful and thank you very much ed. perhaps there are others who have such drawings and or information that should be placed here.
thanks doc
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Longitude -85.42786 Latitude 39.59153
"Current Build 127mm refractor "Cosmos I" getting closer
"current build 6 inch F6.5 "Little Toe" waining
"if all else fails use duct tape "works for me"
"Remember there is no spoon"
Edited by The bear (10/19/08 04:32 PM)
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madrian
member
Reged: 06/30/08
Posts: 46
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I think you guys would be really better off with a Bath interferometer if your main interests are testing telescope mirrors and not flats . This design would require a 4" reference surface on the null lens which is not so easy to come by .
The beauty of the Bath is that it uses a very small ( and likely good ) area of a cheap commercial lens as a reference surface. If the reference surface of this Fizeau Null in this design is concave then that should not be to hard to make and test, but certainly more expense and hassle than bath type. Ed would be able to confirm the null lens design.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1673
Loc: Fort Worth TX
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Some notes on Fizeaus I have built in the past at work:
1. The interference occurs between the front face of the transmission wedge and the part, as shown in the diagram. The wedge front face is the reference surface and must be very flat, ideally 1/20 wave P-V.
2. The transmission wedge has a wedge angle of about 30 arc-minutes to eliminate visibility of fringes formed by the interference of the two wedge faces. The fringes are still there, but they are so finely spaced that they are not visible to the camera.
3. The collimating objective does not have to be anything more than a simple achromatic doublet. Its only purpose is to illuminate the wedge evenly, and its wavefront error does not matter - it is not in the interference cavity.
4. Ed's using a half-ball lens to expand the beam, which works fine. I've used 20X to 40X microscope objectives to expand the beam, but ideally in conjunction with a spatial filter to clean up any higher modes and general grunge in the beam.
5. The Fizeau can be used to test telescope objectives by using a small return sphere after focus. The return sphere has a short radius and once the curve depth is ground or generated in, it is easy to make very spherical.
I'm really glad Ed has introduced the Fizeau to everyone here. It is definitely worth building, with the only really critical component being the transmission wedge. The Bath works as well, but is an off-axis test that makes me uncomfortable for testing faster spheres such as Maksutov shells. The Fizeau is an on-axis test with no inherent astigmatism.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1673
Loc: Fort Worth TX
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This is the Fizeau test for telescope objectives. The return sphere is a deep, well-made spherical mirror, with its COC coincident with the lens focus.
A small optical flat can also be used if it is placed right at the lens focus, but it is harder to get focused and aligned.
The comments on the Bath are good, and it is a good test for higher focal ratio optics. But I have a 10" Maksutov to build having roughly 13" radii, and will have to use the Fizeau on it to get on axis. The Bath would not work for such a deep sphere unless an auxilary optical assembly called a Shack Cube is used.
Something not discussed yet is the optical table stability, which must not vibrate during testing. The easiest way is to arrange four wheelbarrow tire inner tubes in a square on the floor, with bricks at the middle to support the table until it is aired up. A piece of plywood is placed over the inner tubes, and a few stacks of cinder blocks are placed on top of the plywood, up to a convenient height. Another sheet of plywood is placed on top of the cinder block stack, then the optical table surface, either aluminum or headstone granite, is placed on the top plywood.
I made hundreds of holograms on just such a table during grad school, and it worked great. That's also where I learned the trick about putting the brick inside each tube so the tubes don't get crushed when you let the air out.
Mike
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1477
Loc: Sin-sin-atti
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My friend wanted an interferometer primarily to check astigmatism. A Bath is not as expensive but it has an offset and can introduce astigmatism in some cases wheras the Fizeau is not offset and doesn't have this issue. A Bath is not affected by vibration and air currents like the Fizeau but then it also will need a null lens setup or autocollimating flat with fast mirrors. This interferometer is a 4 inch beam but can be made with a smaller aperture to save money. So to make one like this you would need:
1. Hene laser and power supply Ebay for $45
2. 4 mm half-ball lens Edmunds ~$27
3. cube beamsplitter S Shed L20480 $45
4. collimating lens 4 inch S Shed L3926 $89
5. reference flat or lenses expensive
6. solid base to mount everything on
7. you will need to have or make stages to allow alignment of the interferometer and the test part
The reference flat or null lenses are the most expensive or hardest to get but is something that an ATM can make to save money. If you have an specific question on how I made this you can send me a PM.
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Ed Jones
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Gordon Rayner
Pooh-Bah
Reged: 03/24/07
Posts: 1096
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What about eye safety? I have bought and sold some commercial Fizeau interferometers which were originally equipped with non-laser monochromatic light sources. When modernized by addition of a laser, as seen in the shop of a working optician who formerly worked at the interferometer manufacturer, this person had a piece of diffusing translucent adhesive tape ( Scotch brand 3M) in the laser beam, to break up the speckle pattern. He said that there had been some concern by OSHA about this issue. Another way to diffuse which was mentioned is to slowly rotate, with a motor, a diffusing groundglass or sanded plastic window.
I still have a 5 inch Fizeau for flats, but have not used it, nor laserized it.
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madrian
member
Reged: 06/30/08
Posts: 46
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Ed, Can you show a cross section of a typical null lens that would be used in front of the collimating lens for spherical wave configuration for testing telescopes objectives. Is the reference surface concave ?
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1477
Loc: Sin-sin-atti
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Once the laser is dispersed by the half ball lens the danger to the eye is greatly diminished. Even though this one is very bright it isn't dangerous, I reduce the intensity with a filter.
Here are three useful configurations for the Fizeau.
#1 replaces the collimator with a meniscus reference element. This is Ceravolo Optical's arrangement. It produces a spherical wavefront for testing spheres or the spherical wavefront of a refractor.
#2 is what Mike showed above for testing refractors. You just make the aperture large enough to match the refractor you are testing.
#3 uses 2 plano-convex lenses for testing telescope mirrors. The first flat side is a reference surface.
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Ed Jones
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reodds
member
Reged: 05/04/06
Posts: 76
Loc: Merrick, New York
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Ed, you mention the reference lens or flat. Can you be more specific. For example, if I were testing a 10" f/5 spherical lens (typical of an ATM Newtonian scope), what would the reference lens need to be, and where is a good source for it?
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Roger
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1477
Loc: Sin-sin-atti
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I would say a parabola would be more typical of a Newtonian (#3) but if you really needed to test a sphere then Ceravolo's reference element is better suited for testing a sphere. I think it goes for $410. However to then test parabolas you would need a Ross lens for another $575.
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Ed Jones
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madrian
member
Reged: 06/30/08
Posts: 46
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The problem I see with telescope testing arrangement #3 is that the errors of the optical surface of the convex side of the plano lens and the homogenity of the lens will also contribute to the the instrumental error of the interferometer ( as well as to a lessor extent the small field lens). In the Ceravalo spherical wave Fizeau design only the quality of the convex reference surface has any significant contribution to the interferometer instrumental error.
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1477
Loc: Sin-sin-atti
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Actually you have the same problems either way. #1 needs a null lens when testing parabolas and both surfaces of the null lens and homogeneity must be good.
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Ed Jones
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