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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1365
Loc: salem, OR
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The edge zone was always subtle under the Foucault KE at ROC. It was visible much more easily with my Ross null test. It was turned up, or more accurately, it was pretty much flat (nearly spherical at its own ROC) while the rest of the surface was fully corrected - this makes it look turned up or long. It was also very close to the diffraction edge, which complicates the IFs, the null image (which shows some slow turbulence but I can't be sure), and reading it with Foucault. But it was there and it was symmetrical all around the edge. The mirror obviously had a few other minor issues for which the outside testing was useful.
Just to clarify, these images all document a mirror that was completely reworked in '04, from a sphere, even though James seemed to think it wouldn't be hard to "fix" the edge. My experience with "fixing" edges or any narrow defects has never been satisfying. I have several more IFs from the same test if they're of interest. The two data reductions are from the exact same IF BTW, with just the outer 1/2" left out on the second one.
The test stand astig is similar in nature to every thin mirror I've had tested with IF so far, and James noted it as well as being typical for their setup (vertical support with a sling). He said at the time As mentioned; if not for the TUE the mirror would be quite good so I don't think he saw anything of concern relating to astig. I have 5 IFs that all appear to be taken on the same orientation (unfortunately), when I have some more time I'll trace all of them and average the result as the fringes move a bit and that should improve the accuracy of the analysis.
James was quite helpful in interpreting these results and then waived his usual fee for doing the testing. How cool is that?
In the meantime, here's a Ronchigram in the null test that shows the edge defect as well as the slight undercorrection.
Thanks for your input!
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1365
Loc: salem, OR
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Quote:
the mirror was polished by hand/ only on the oversized lap
Yeah, that works very well, it's what I've done for making spheres when the edge has to be good too. I think if you do it by hand that way with quartz and you rotate it regularly it's unlikely you'll fail to get a good astig free result.
Using oversize polishing laps on top with a machine might be difficult though I expect it could be done. I've used full size tools on top on the machine, but oversized tools only MOT and by hand, just like you're describing. You'd almost have to pitch the mirror to a backing plate to work it MOT on a machine, I suppose. 
Working with the machine and thin mirrors TOT you need a very flat true turntable and a thin flotation layer (the webbed latex carpet-backer pads or shelf liner rolls sold at home improvement stores work well). The tool can't be allowed to get into resonance with the table rotation, and there can't be any wedge in the glass, plus the optical/physical center of the mirror have to match (no off-center grind). Given all that, working by hand MOT on an oversized tool by hand is probably less trouble if you're only doing a couple mirrors.
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1365
Loc: salem, OR
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Here's a Zernike average of the 3 best quality IFs for the full surface for comparison. For whatever reason this is a considerably different result than the first frame I showed, and furthermore agrees with James's result for Strehl. It reproduces the null surface image much more closely now, so perhaps turbulence was to blame. Tracing the fringes is slow stuff. 
The trimmed version just looks like this with the edge cut off but the Strehl goes to 0.921 - which isn't a big change but shows what a tough to see edge zone can do, which was the point.
Best,
Mark
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2474
Loc: Northeast
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"In the meantime, here's a Ronchigram in the null test that shows the edge defect as well as the slight undercorrection."
****
Now is this on James's test rack or yours? the astig is virtually non-existent here. The IF showed it plainly.
M.
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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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Glig
sage
Reged: 10/02/05
Posts: 270
Loc: Baltimore, MD
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This thread is a favorite of mine and before it sinks into the west for good I'd like to address an old issue. I hope this is not a sore point. Mark Harry I hope you'll forgive me for using such an old quote of yours (From my thread called Handle Grinder)but here goes.
Mark Harry: "...Using a fast spindle is called "spin-polishing". There are some who can figure mirrors that way,(large mirror-small polisher scenario) but if you get carried away, you can really rip an edge that'll take forever to fix. Been there, done that, seen others do that, and thank goodness, I know better!"
I responded: "...I'm wondering how tub polishers could rip the edge? I was told that tales of fast spindles turning down edges are a myth..."
Mark Harry: "...Whoever said it can't happen hasn't done any optical work with machines, period. Anytime the polisher has a tendency to "dwell" at the edge, with some overhang as it changes direction while the spindle has a chance to rotate a slight degree, will turn an edge for sure, guaranteed..."
It seems to me that spin polishing has been a lively topic between Mark Harry and Mark Cowan. I'm been trying to follow it all, but I'm not sure where Mark Harry's ideas about spin polishing and TDE is today. Have we reached a verdict on this? Is there still a fundamental disagreement here? 
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Richard Caldwell
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1081
Loc: Sin-sin-atti
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And if you go ahead and remove the spherical and trefoil it'll be a darn perfect mirror! : 
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Ed Jones
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2474
Loc: Northeast
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I haven't changed my opinion on that method one bit, Richard.
***
One provision: I believe Mark C is working with quartz, and a polishing medium that lacks resilience. It -CAN- work under those conditions, if you tweak the parameters correctly. (we used "flower-power" with quartz flats, and the spindle speed wasn't too critical, and within the machine's capability) But I think if he changed substrates/pitch, I'd be very suprised if he'd have that same success with minimal edge issues.
***
Richard, I'd say you were trying to 'stirr the pot'......Am I right?
M.
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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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Glig
sage
Reged: 10/02/05
Posts: 270
Loc: Baltimore, MD
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Quote:
Richard, I'd say you were trying to 'stirr the pot'......Am I right?
M.
  
O.K. You got me, Mark H. Guilty as charged. But also for another reason. This thread and others like it I'm saving to disc and keeping forever. I'm a seeker of knowledge first and foremost - and this is great stuff.
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Richard Caldwell
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Joe Cipriano
Entropy Personified
Reged: 09/03/05
Posts: 3830
Loc: Uh... anyone have a GPS?
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Quote:
...Richard, I'd say you were trying to 'stirr the pot'......Am I right?
Annoy sleeping dogs, throw rocks at hornet's nests, etc.
Let's not.
Things have remained civil (kudos to all involved), which can be difficult for a discussion as rigorous as this one. I'd REALLY like to see it continue in the same manner.
Thanks.
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In the Land of Eternal Light Pollution & Great Pizza (Chicago)
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I may not have gone where I intended to go, but I think I have ended up where I needed to be.
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