DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1444
Loc: Hockessin, De
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Bath Interferometers work very well and are easy and inexpensive to make. Here is a picture of mine that cost me under $30 to make and took an afternoon. I wouldn't use it to try to figure a mirror but they work great for determining the final wavefront error.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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colinsk
Pooh-Bah
Reged: 01/17/08
Posts: 1219
Loc: CA
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Why is a bath interferometer not useful for figuring a mirror?
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Mahalo,
Colin Kaminski
Coulter 10.1" Dobsonian
PST
TV-76/Baader White Light Filter/LS50F/BF1200
LS60T/DS50/FT/BF1200
DSM
LXD-75
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1444
Loc: Hockessin, De
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Colin,
Interferometry is time consuming to setup, more so then other test methods. Also for conic sections besides spheres, one either needs other "nulling" type optics or image analysis software to compute the wavefront error and show were the problems are.
Other tests like double pass autocollimation allow for much quicker testing with excellent accuracy. Once other methods show an excellent figure, then it is time to switch to inferometry to measure the fine details.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1369
Loc: salem, OR
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Free and open source fringe analysis software is available, see FringeXP or Open Fringe for two good ones. Check the latter first as Dale is the one doing a lot of the Bath work.
If it wasn't clear in my earlier post, I should point out that Bath doesn't require nulling optics for aspheres at ROC, the fringes are captured very densely and estimates of the SA are used to null the results digitally. There are some limits, as in if you have a LOT of SA and not a lot of resolution on the IFs the results will not carry much resolution either.
Best,
Mark
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grinder65
journeyman
Reged: 12/04/07
Posts: 38
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Made my bath interferometer this morning, frankly i do not know what i am doing yet. Here are my results on a 13.5" f4.1 homemade Reco Lab mirror. One is a picture of ronchi at null and the other is a bath interferometer photo
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grinder65
journeyman
Reged: 12/04/07
Posts: 38
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Bath interferometer photo, 13.5' f4.1
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1444
Loc: Hockessin, De
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You need to adjust the Bath so you have few interference lines showing. If you have a perfect sphere then you'll get perfectly straight interference lines. If you have an aspherical surface the lines will have a curvy pattern to them and this were you need to take an image of them and use software to tell what is going one.
It looks like your close a sphere but the rings are slightly wider in the middle also the pattern is not perfectly round which indicates some astigmastism. It could be coming from the Bath depending how far apart your reference and return beams are.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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grinder65
journeyman
Reged: 12/04/07
Posts: 38
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Thank you for the info, the reason the fringes are not round is because i photographed it from a reflection on the wall and the camera was at 30 degrees to the fringes. The mirror test out excellent on a ronchi ross null test and so does it under the star test. I would not say this reference test was easy, it is very difficult to get larger fringes separation, this may because i use an plano convex lens from a microscope objective. Right now i am just having some fun trying to use it and understand it.
Conrad
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1444
Loc: Hockessin, De
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If I'm understanding correctly, you used the Bath in conjunction with the Ross Null lens ? If so that makes sense that you got round fringes of just about equal spacing. I was puzzled at first and thought that you were testing a spherical mirror since a f/4 parabola without the Ross Null would show a wild fringe pattern.
You might want to try a Biconvex lens ( equal radii on both sides) instead of the Plano-convex lens in the Bath, since it will have less abberation. You also need a good micrometer stage to adjust the positions. That will easily allow you to reduce teh number of fringes.
Bath's are a fun device and having the ablity to do interferometry for next to nothing is a huge plus.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Don W
Postmaster
Reged: 05/19/03
Posts: 13230
Loc: Wisconsin, USA
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Please do NOT change the subject title of this thread.
Thank you.
Don Wyman
Moderator
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2454
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the harbor freight dial indicator is on sale this week for $7.99 item # 623.
http://www.harborfreightusa.com/html/emails/29/RetailA/Images/5a.jpg
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 616
Loc: Easton, PA, USA
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Here's a minor, but maybe useful mod to a Stellafane type Foucault tester.
After use I try to remember to tip the dial indicator up so that there's no pressure on it but that loosens the nut holding it. To save getting a wrench and maybe getting some of whatever metal chips/oil might be on that on the tester and maybe mirror I switched the bolt and nut for a scavenged bicycle seat post quick release. (This one might actually be for another component, but looks the same to me) You can adjust tension easily by turning the nut that's just visible under the dial indicator.
Gary
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2254
Loc: Courtice, Ontario
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Is this the thread we're supposed to put our Foucault tester pics?? I'm so confused.  
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 616
Loc: Easton, PA, USA
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In: Started my Foucault tester Biff wrote Quote:
I use 1/4-20 and it's too coarse.
Nothing against fine threads, but how fine do you need it to be? If I'm doing the arithmetic correctly with a 1" diameter knob and 1/4"x20 thread I can easily get .00025" movement or better.
For really fine nulling movement a light touch on the base or table or even floor seems to work nicely. I read about that technique somewhere. Or think I did anyway...
Gary
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Spoonsize
Vendor/Clothing
Reged: 08/27/04
Posts: 2178
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FYI:
For $14.95 plus shipping, the book by Sam Brown, "All About Telescopes" is available from at this link:
http://scientificsonline.com/product.asp?pn=3009611&cmss=l30090-94
It does a real good job in the back of explaining some techniques to use while grinding and polishing, AND MORE IMPORTANT, has a fairly detailed drawing of how to build a tester. It includes a method of building a dial indicator using a scale cut from the top of one of the pages.
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Steve Durham
www.margesmonograms.com
(Marge made me do it)
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2254
Loc: Courtice, Ontario
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Quote:
Nothing against fine threads, but how fine do you need it to be?
A little more. It might not be so much of an issue when I get to faster mirrors but the shadows are pretty fine on the f/7.25 I'm working on. The pressing on one side of the table method works well (probably you read it in Texereau, great trick BTW) but trying to find just the right setting so I can just leave it and stare or take a pic is a little touchy. It's certainly not a big issue but I think going from 20 TPI to 28 might be just right.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
My house.
DRAACO
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 616
Loc: Easton, PA, USA
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I see what you mean; though my Schief primary is f27 and 4 1/4" and with a scope I didn't have any trouble - then again I was looking for an overall null.
I like the table tapping method for "blinking" the zones and haven't had any problems finding or holding them with the 1/4x20 threads.
I think I'd tend to get fooled staring - which is why I like the blinking - but probably if I'd had the tap I'd have gone finer.
Mostly moot with AC though...
Gary
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2488
Loc: Northeast
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20 threads/inch is a fairly easy number to work with in figuring that 1 turn = .050". 28 threads = .035714.... could be a headache in assessing total travel. My lathe slide has 20 thd screw advance, but is acme thread, and about 1/2"+ diameter.(works very well)
Mark
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Ummm...all this talk about dial indicators and degree wheels got me thinking...so I ran out to the garage, and presto! I have both...and a whole elaborate mounting kit. It was purchased by myself from Comp Cams years ago when I was putting the cam in my truck! I'm just now trying to figure out how I can incorporate the piston stop into my foucault tester.
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Quote:
a really nice way to test for strain:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/strain/strain.html
Hey Dave! I just did a simple strain test on two 8" blanks I received in the mail last week. I placed my laptop on its back, monitor on a blank white screen, popped the mirrors on top of the monitor, then threw my polarized filter on the lens of my digicam. Dunno if that was the right way or not, but check out the results!
Thanks!
Sam
woops...probably need to link you to photos...sorry if they are a tad blurred...I was hand held at 1 second each.
picture 1
picture 2
Edited by yosemitesamiam (11/03/08 12:41 PM)
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