mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1369
Loc: salem, OR
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Chris,
Quote:
Correct but not necessary to partially cover up the LED at all, we always use a bare LED, works just fine. We did some thourough testing using a bare LED compared to old fasion pinhole, nope no difference at all exept for lots brighter using a LED. (Focograms..!)
Are you using the KE so that it cuts across both the LED and return image or just across the return? The first is a slitless tester, the second is a pinhole setup with the LED serving as the illuminated pinhole. The source doesn't have to be very small to work like this, so long as it has a ~sharp edge, as the KE eventually cuts across it as well.
Best,
Mark
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Mitchell
member
Reged: 05/19/08
Posts: 82
Loc: USA
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Gary,
Yes indeed too long. And yup, a great shot (by you guys!)
Glad this has stirred so much conversation!
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Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1953
Loc: Boechout, Belgium
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We use a bare LED only cutting the returning rays. I know MarK, sounds very strange but it works. I think I told this before : we've tested this during one full course-year. Parallel with an old fashion pinhole: all the time we compared it with a fully illuminated powerLED, no difference at all. Try it, you'll see...
We needed that bright powerLED to make our focograms.
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Chris
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Quote:
We use a bare LED only cutting the returning rays. I know MarK, sounds very strange but it works. I think I told this before : we've tested this during one full course-year. Parallel with an old fashion pinhole: all the time we compared it with a fully illuminated powerLED, no difference at all. Try it, you'll see...
We needed that bright powerLED to make our focograms.
I think this works because a clear LED is actually a pinpoint source (the size of the LED chip is quite small.)
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half meter
Postmaster
Reged: 05/05/04
Posts: 12517
Loc: Great Lakes
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Somebody mentioned not to use white LEDs because of their broad spectrum. Both blue and white LEDs are in reality invisible ultraviolet LEDs with phosphors to make the colors visible. Red, yellow, and green LEDs all generate their colors by selective doping of the semiconductor material (no phosphors) so their colors are much more "pure".
I'm not sure if this makes a difference here but thought it should be noted.
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Gary
Collins I3 (Thin Film) Image Intensifying Eyepiece
Coronado Maxscope DS 90 <0.5A w/BF30
152 mm f/8 TMB/A&M Carbon Fiber APO; f/5 with 4" Borg ED Field Flattener/Reducer
20" Obsession/OMI Mirror/Servocat/Argo Navis
 First Light for the 30" Obsession at BEOTS!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1369
Loc: salem, OR
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Chris,
No not strange at all! I know why it works - if you have a wide slit source for Foucault it works as a narrow slit because the KE eventually cuts it down to one side in operation. If your power LED has a sharp definition to the edge of the source it'll work just like a slit would, and that's why it works as you describe.
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1369
Loc: salem, OR
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The color doesn't matter - the Foucault test is strictly geometric. Whatever color or type of source that makes you happy is more than adequate. Foucault used a candle...
Best,
Mark
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1953
Loc: Boechout, Belgium
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Quote:
The color doesn't matter - the Foucault test is strictly geometric. Whatever color or type of source that makes you happy is more than adequate. Foucault used a candle...
Best,
Mark
Correct, we've tested different collors LED's all gave the same result. Bright white gave the best result for focogram purposes. The brighter the LED the shorter the exposuretime for the camera.
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Chris
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 618
Loc: Easton, PA, USA
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Quote:
The brighter the LED the shorter the exposure time for the camera.
With the 12000mcd (red) LED I could easily get decent handheld exposures.
Gary
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Not all white led's are UV with a phospher to emit white light. Some actually have three chips, one for each primary color and have the currents balanced to give white light. This kind of white LED would be more expensive.
And most blue led's are NOT UV with a blue phospher. Most of the blue leds that the company I work for uses in their products are clear lens types with a real blue emitting chip.
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half meter
Postmaster
Reged: 05/05/04
Posts: 12517
Loc: Great Lakes
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Thanks for adding those further details about LEDs, Ken.
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Gary
Collins I3 (Thin Film) Image Intensifying Eyepiece
Coronado Maxscope DS 90 <0.5A w/BF30
152 mm f/8 TMB/A&M Carbon Fiber APO; f/5 with 4" Borg ED Field Flattener/Reducer
20" Obsession/OMI Mirror/Servocat/Argo Navis
First Light for the 30" Obsession at BEOTS!
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Alan A.
super member
Reged: 02/08/07
Posts: 249
Loc: San Francisco
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this is a great thread. any simple ways to build a simple phase contrast apparatus to test the fine smoothness? - a mirror maker suggested using candle soot on a slide held at the radius used in a similar fashion to a knife edge test - anyone every try something like this?
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Alan A
17.5" f/5 discovery truss
10" f/6 royce dob
SV115(TMB) f/7 /unistar mount
Apex 102 Mak-Cass
XT 4.5 f/8
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1369
Loc: salem, OR
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Try this post from the ATM List: Phase Contrast and Foucault image puzzle , and this detailed article: Phase Contrast .
High resolution Foucaultgrams (in recent comparisons) appear to resolve the same level of detail as phase contrast images, if you allow tweaking the gamma and contrast to bring out low level contrast. This is not completely unexpected, though it appears to have been unanticipated by many.  Kodalith is not supernatural...
Best,
Mark
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2490
Loc: Northeast
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I think what Gary was saying, was that it may not be wise to take the chance of using a UV led on account of exposing the eyeball to short wave light. It can lead to cataracts later on, years afterward. I note that at least one of my led flashlights had a specific warning about looking directly into the beam.
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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Alan A.
super member
Reged: 02/08/07
Posts: 249
Loc: San Francisco
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Thanks Mark - very helpful articles. 
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Alan A
17.5" f/5 discovery truss
10" f/6 royce dob
SV115(TMB) f/7 /unistar mount
Apex 102 Mak-Cass
XT 4.5 f/8
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Several questions:
The older way to set up a foucault tester was with the light source fixed and only the knife edge moving. In this case the light was usually always displaced horizontally from the KE. In the newer testers we now have the KE above the light and both move together. The total movement required in the latter case is half of what is required in the former.
1: Isn't the measurement of the KE movement less critical in the older style tester since it must move twice as far to get from zone to zone?
2: With the KE above the light why aren't the screens in front of the mirror arranged with the zones stacked vertical instead of horizontal? (I think it really doesn't matter since the light source would light the entire mirror evenly no matter where it was located in reference to the KE).
3:Wouldn't it be better to move the KE (or both the KE and light together) in a horizontal movement rather than adjust the angle of the KE (tilting it into place) as is most commonly done? This would keep the KE and light slit parallel.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1369
Loc: salem, OR
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1) The absolute positioning is twice as sensitive to the shadow movement, but the measurement accuracy itself (.001" even with a simple dial indicator, I use a mic that reads to .01mm or .0004") is always much better than the error in manual zone estimation. So no, not really, the increments are just smaller.
2) The KE would have to move vertically then, and the mirror is subject to test stand errors in the vertical direction but not the horizontal.
3) Sure. But the difference is very small.
Best,
Mark
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1953
Loc: Boechout, Belgium
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I think Roddier fits nicely in this thread...
Quote:
Quote:
Bratislav,
It has been discussed before ...
Yes, I see lots of discussions but no results. People still get attached to subjective ways of testing (ubiquitous star test) and endless discussions about Foucault (which is BTW still a great tool, but we are in 21st century, not 19th).
What gets me cranky is that magazines like S&T still hang onto "I give this X stars" when truly objective tool is just around the corner. And like properly setup interferometer, it takes absolutely no prisoners. It should be a mainstream tool for an ATM today, not something you read about. I am absolutely sure that had Texereau have written his book today, he'd have a chapter about optical testing with Roddier. Foucault would still be in there, no questions about that, but as a final quantitative test, Roddier has no peers as far as ATM (and even small manufacturers) are concerned.
Foucault would still be in there but I doubt if Texereau would mention Ronchi again in his book...
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Chris
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 618
Loc: Easton, PA, USA
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Maybe I missed it here, but how 'bout the double pass autocollimation test that for example Dave Groski and Dick Parker advocate... AC test on CN
Gary
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1369
Loc: salem, OR
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If we're adding good stuff check out the common-path Bath Interferometer. The Interferometry group on Yahoo has a lot of discussions and there's a Wiki on it as well. Relatively easy and cheap to build, the computer decodes closely spaced fringes without any reference elements required. The only limitation seems to be accuracy for very fast aspheres...but what else is new?
Best,
Mark
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