greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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using the water test my 1.2" flat now has an x fringe pattern, is this convex?
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (10/16/09 12:14 AM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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You mean a fringe in the shape of an X? No that would be astigmatism.
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Ed Jones
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Just curious, is there a write up on the "water test" for flats somewhere? I am not familar with this test at all.
Google doesn't seem to be much help.
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Gleanings for ATMs, Sky & Telescope July 1990 by E Jones.
Also The Best of ATMJ Vol. 2 page345 by Dick Parker.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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Now it has 4 fringe rings convex = 3/8 times 4 = 1.5, I put the 4 inch pitch lap on my record player and held the flat on it . The flat isn't getting better though. I'm cold pressing a piece of 4mm plate glass on the tool.
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10" f/5 dob unf.
7x50 Tasco binos
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3127
Loc: Northeast
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How do you know it's convex? How big is your polisher?
Mark
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So many projects, so little time!
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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I put a weight on the water pan to tilt it slightly and the center of the ring pattern moves away from the weight so I suppose its convex. My polisher is 4" diameter and I'm polishing it with a 4" flat to adjust the pitch tool. The 1.2" flat is now 3 rings convex in water and the center is more flat than the edges.
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10" f/5 dob unf.
7x50 Tasco binos
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3127
Loc: Northeast
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I think perhaps you're right??? We need Ed to confirm this. But you may want to rethink whether the edge is a different ROC. Convex fits are "dangerous" as far as interpreting certain defects are concerned. Could be that it's closer than you think as far as regularity is concerned.
I'd be really interested if you can polish such a small piece by hand the way you're doing it. The method makes sense. I've never been able to do such a small piece by itself and have it come out reasonably flat to say 1/8th wave surface error. I'm rooting for you!
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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Is it convex or concave? - Turn the screw closest you clockwise (when it is perpendicular to the fringes). If the fringes get more open then continue. If you turn the screw clockwise and the fringes become closer together then they represent a cross-section of the part measured in fringes. Remember that 1 fringe =3/8 wave.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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Here's a picture of the 1.2" secondary neon light water test. The edge is turned a bit.
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (10/24/09 01:23 AM)
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Ed Jones
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Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Looks like the left edge is turned about 1/3 of a fringe X 3/8 = 1/8 wave error. You seem to have the water test down pretty good, good work! 
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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Thanks for the help, the turned edge is at the thin end, the thinner end might be bending? I polished it on a 4" pitch tool and 4" flat to adjust the pitch, holding the secondary by the top edges and the middle pushing it lightly on its long axis for 1 minute going around the tool couple times then test it. I must have tested it 50 times. (When it was quite flat I could adjust the mirror to level in the water test and see no fringes or 1 fringe covers the flat, and why do the fringes sometimes appear to be expanding or moving across it? water evaporation? Moontides? Scoring the tool with a razor helped since there were no channels. I never needed to put the TOT of the 4" flat for some reason, maybe the smaller glass was keeping the pitch from going convex.
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (10/24/09 10:04 PM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Quote:
why do the fringes sometimes appear to be expanding or moving across it? water evaporation? Moontides?
I've seen this too, not moontides but it must be evaporation mostly or possibly the wood changing under the weight.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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It must be the tester tilting, evaporation wouldn't change the water angle. I left the tester a few hours and the fringes went from 3 to 15
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10" f/5 dob unf.
7x50 Tasco binos
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Yes, if the number of fringes changes then the tilt is changing. If the fringes just appear to be moving but the number stays the same then that has to be evaporation.
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Ed Jones
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