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Old 03-10-2012, 07:48 PM
Brub58 (Bruce)
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Brub58 is offline
 
Join Date: Dec 2010
Location: Melbourne
Posts: 6
8" mirror refigure complete

I'll add my 2c to the 'any mirror makers' thread. About 14 years ago I made an 8" f/6 mirror as my first project. It was OK but ended up with a TDE and a rough zone inside that where I'd tried to fix it.

The coating has had it, so I decided to have a bother go before I got it recoated. I tried fixing the edge itself, but in the end went back to a sphere and reworked it from there. To parabolise I used a 6" star lap - not really necessary for this mirror, but there's an unfinished 14" next and I was trying a few things out on this one.

The biggest difference since my first effort was that I built a Bath interferometer and used that for monitoring the figuring progress. I have to say, it's a wonderful thing to just take a photo of the mirror, load it on the computer and see the exact shape of the surface. I'ma bit surprised I haven't seen more mention of them on here.

The final result is that I've gone from a rough surface, -0.8 conic and 0.75 strehl to a smooth surface, -0.98 conic, 0.95 strehl, and 1/29 wave RMS. the only downside is a small scratch. You can hardly see it, but it shows up in the photos, and I'm betting it will stand out when the mirror gets coated. I have no idea how it happened, and 2hrs polishing didn't improve it. I decided in the end that I wasn't going back to grinding to get it out, so it's going to get a bit of texta after it's coated.

I've attached the before and after images and reports. I'd like opinions from the experts here, but I think it's done.

Bruce.
Attached Thumbnails
Click for full-size image (image.jpg)
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Click for full-size image (Zernike-Report.jpg)
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Click for full-size image (IMGP2842.jpg)
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Click for full-size image (IMG3106 Report.jpg)
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Last edited by Brub58; 03-10-2012 at 08:02 PM. Reason: Adding photos
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