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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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Ed -
I recall, from a previous thread, that you were working on a test procedure for testing large flats using fringes under diffuse monochromatic light. As I recall the outcome was in determining the number of rings you would see from a certain viewing distance to establish that a flat was flat. Did you finish that analysis??
Specifically, if I am making a 12.5 inch flat, how many rings would I see at 6 feet viewing distance and would the flat being tested appear concave or convex with the flat being tested under the reference flat. If you would, could you please provide the ring count for a few distance choices, or the method for me to make that determination.
Thank you in advance
Dick Parker
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Dick,
Well it is a Zemax file. If you input the viewing distance, the flat diameter and the water thickness Zemax will calculate the number of fringes. In your case at 6 feet viewing distance and assuming 1 mm water thickness you will see 20.14 fringes concave. Way too many I think. However if you view it from 15 feet away (doable I think) you will only see 3.142 fringes with mercury green light. You do need to measure the water thickness accurately (see my water thickness gage I posted).
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Ed Jones
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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I personally think you'd be better off to get hold of, or make a 6" flat, and fab a 12"er by the contact method. It has the advantage of manipulating the pattern to see all defects, as well as surface smoothness. Kelly made his that way, so did I. They are all highly accurate, and all certainly suitable for AC work.
Somehow, having a large mountain, or valley ajacent to the test area, while making a 12"er with water test doesn't give me any warm fuzzy feeling about the overall accuracy to dead-nuts flat surfaces. I heard recently that around undersea mountains, the water can be 3 feet higher than a similar area encompassing a flat plain. Don't know what that would equate to in fringes, but certainly wouldn't equate to absolute flatness. (the height variation of water level was due to mass proximity, not wind or currents.)
Mark
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So many projects, so little time!
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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I suppose it depends on what you needed the flat for. For AC work leaving it 3 fringes convex is no problem, probably 20 as well. Using a small flat to measure a large one loses accuracy.
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Ed Jones
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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Using a 6 reference to make a 12 changes the sensitivity of the flat test in fringes by a factor of 8. Same for the AC test vs KE with fixed source as far as longitudinal measurement is concerned. So what? If you can be 20 fringes off, make it 100+. If you want to -quantify- the error capability of an AC flat test, the fewer fringes it's off absolute flat, the better.
M.
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So many projects, so little time!
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Again I don't know what purpose Dick has in mind. If I wer making a 12 inch AC flat and had the choice of using a 6 inch reference or the water test I'd pick the latter. You would be much more likely to miss some astigmatism testing with the 6 inch flat instead of seeing the whole optic. If it needed to be dead flat I would be using a collimator with it.
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Ed Jones
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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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Ed -
Thank you for your response. I also did look up and found the thread where you originally discussed this back in July. If the ZMAX file is still available, I would appreciate a copy, if I may. I will perue this off list, give me a couple days, I'm travelling.
I intend to offer this test as one possible interm test so a friend can monitor his progress while figuring a flat. Once he gets close, more rigerous tests will be used.
Thanks again
Dick Parker
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Sure PM me your email address and I'll send it.
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Ed Jones
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