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Fixing a small central 'flat' spot...

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#1 KerryR

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Posted 12 February 2013 - 04:30 PM

6" f5, 90% corrected.

I have a 1" 'flat' spot at the center. Everything else is looking decent. I know this area of the mirror will be obscured by the secondary shadow, and that it's surface area is inconsequential, but if I can fix it fairly safely, I'd like to do so, before I hit ~100% correction.

I'm considering two things:
1) Teeny W's or ovals with the flat spot centered on the edge of the tool, MOT. This option would save the correction I have, but at the risk of doing something funky to the center.

2) COC MOT 'til it's gone, then repair the lost correction. Not sure what put it there in the first place, so it's possible I'd just put another one back...

Any suggestions?

#2 StarDusty

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Posted 12 February 2013 - 10:03 PM

As I understand your condition, you have all zones but the center where you want them and you wish to deepen only the center zone.

If this is the case, you might try making a cold press mask with a small five point star (only 3/4" tip to tip) cut out in the center of the mask. I use thin plastic from a file folder, but thick paper will work too. Cold press with the mask covering the mirror with the missing star shaped area in the center. This should focus more of the polishing in the center. Check if the center is moving down, repeat as needed.

#3 KerryR

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Posted 12 February 2013 - 10:59 PM

Thanks for the suggestion! I appreciate it.

I don't want to deepen the center zone as a whole, at least not more than necessary to approach 100% correction. The area of longer ROC is only 1" diameter that nulls at the same time as the 70% zone, giving it a 'flat' appearance. Still, the modified pitch lap might work.

So close yet so far...

#4 mark cowan

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Posted 13 February 2013 - 12:00 AM

Just leave it. That's an unhealthy obsession to fall into... If you can't resist just use your thumb on it. :shrug:

Best,
Mark

#5 Mark Harry

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Posted 13 February 2013 - 06:20 AM

Fixing a slightly high center-

-VERY- fine, dilute, hi quality cerium or rouge. Ovals COC. first, edge of tool lined up with the major axis of the oval, then 90* to it, very light pressure. Only -ONE- turn around the stand, then look at it carefully on the rack. Can be done MOT, but I prefer a 2-3" subtool, TOT. Don't bother to rotate the small tool.
****
This should be done prior to doing the last 10% on your mirror so that you have a chance to smooth it off if there's any roughness. I never had very much luck with thumbs, just my experience.
Mark, do you mean you never chased a 1/20th wave mirror to make it better??? :lol:
M.

#6 KerryR

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Posted 13 February 2013 - 09:46 AM

Thanks for the further suggestions, guys.

Not sure what I'll do, yet, but, if I attempt the fix I'll do so before further parabolizing.

#7 RossSackett

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Posted 13 February 2013 - 09:46 AM

I'm sure you diagnosed it correctly, but it reminds me that on my first mirror I went after a 1" flat spot. Turned out it was a hole. And I made it a deeper hole. MUCH deeper.

#8 ed_turco

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Posted 13 February 2013 - 10:53 AM

Oh, it is so easy to overdo things working on a flat spot at the edge of a tool. Too easy to get a canyon there. Be so careful.

#9 KerryR

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Posted 13 February 2013 - 11:01 AM

Thanks for the warnings, Ed and Ross.

I checked it (again) with zone 1 nulled (after your warnings), to be sure. It's a hill.

I worked it down on the edge of the tool using VERY light pressure ovals and very dilute CO. Worked 1 rotation, tested, repeated. It actually took several iterations, and I stopped before digging a hole.

It's now very much reduced, but still evident when z1 is nulled. Overall correction increased to %95 (3 zone mask).

But: Zone 3's crest looks a little too 'sharp', so I think I'll work 1/3 COC MOT to hopefully smooth out the remaining slight lump at the center, as well as smooth z3 a bit. I'll finish with a wee bit of parabolizing, depending on where I am after that.

#10 KerryR

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Posted 13 February 2013 - 04:03 PM

I successfully removed the hill and smoothed the overall figure a bit, so this mirror is done 'til I get the chance to star test, which might be a while thanks to the weather...

#11 mark cowan

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Posted 13 February 2013 - 04:44 PM

Mark, do you mean you never chased a 1/20th wave mirror to make it better???


I KNOW we've had this conversation. :lol: I do it, but I don't recommend it. :p

Best,
Mark

#12 KerryR

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Posted 13 February 2013 - 04:50 PM

I'm trying to resist doing anything more to this mirror-- according to FigureXP, it's 1/22 wave, though I'm sure the actual is nowhere near that: Things look pretty smooth, the edge looks alright, but I still feel like the shadow crest on the 70% zone looks too sharp. I'm also sure the wave rating would drop with a 4 or 5 zone mask.

I'll star test before doing anything more... I think... I could work zone2 down just a little bit...

#13 StarDusty

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Posted 13 February 2013 - 05:04 PM

I did not pick up on the fact that the mirror was 1/22 wave before recommending working the center a little more. I would stop at 1/22 wave and leave it as-is.

#14 KerryR

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Posted 13 February 2013 - 05:48 PM

I did not pick up on the fact that the mirror was 1/22 wave before recommending working the center a little more. I would stop at 1/22 wave and leave it as-is.


It wasn't when I first posted the question last night; closer to 1/6 in FigureXP.

The 1/22 thing is where I finished today. It's not likely it's actually 1/22 wave-- that figure is from a 3-zone measurement, and it's unlikely I have the surface or curve THAT smooth; zone 3 in particular looks a little sharper than I'd expect it to look.

I'll test with a 4 zone mask to get a better idea of the slope change near the 70% zone. I'd prefer to star test it at this point, but it's not going to be clear here for a while. Seeing is rarely any good at all here in the winter anyway.

#15 scopemankit

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Posted 25 February 2013 - 04:33 PM

Does it matter? isn't it going to be in the area covered by the diagonal?

#16 KerryR

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Posted 25 February 2013 - 04:52 PM

From my original post:
"I know this area of the mirror will be obscured by the secondary shadow, and that it's surface area is inconsequential, but if I can fix it fairly safely, I'd like to do so".

I probably should have added:
1) to placate my fragile optical ego
2) to learn to better control the surface, which is really the point of this project.

(I still suck at this, but am gradually learning to reduce my ego's involvement in the process. I have since fixed the flat spot, but introduced other issues whose impact or lack thereof I wish to observe in the star test. It's unlikely I'll be seeing clear stable skies any time soon.)






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