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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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The 26.25 inch mirror needs more correction in the center and the bad edge.
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Feidb
super member
Reged: 10/09/09
Posts: 127
Loc: Nevada
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My 16" was 1 1/4" thick and I had trouble with astigamtism. I used a full-sized tool for all grinding then switched to I think it was 10" and a 6" tools for polishing. I had to go back to fine grinding three times because of the astigmatism and never did get it all out, but I got it minimized. It probably would not pass muster with you masters, but it sure made me happy when I finally got a look at Jupiter!
My tool was 1/4" plate glass epoxied to two round pieces of 3/4" plywood. My polishing tools were plywood sealed at the edges. I never had a problem with sleeks.
During the first go when I put a Ronchi strip on it, the patterns looked like a butterfly. If not for that Ronchi strip, I might never have got it down to where my Focault readings would make sense. At that time I was a bit rusty!
I worried about smoothness and in the final figuring I switched from Cereum Oxide and Barnesite to rouge. The slower bite made for a smoother surface.
Whatever the case, I'm happy with it even now, 22 years later.
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Present gear:
16" Meade LightBridge
Meade 50mm straight through-finder
Lumicon green laser pointer
Orion Q-70 26mm, 32mm, and 38mm
Parks 2X 2" Barlow
Hyperion 17mm, 8mm
1 1/4" 18mm Russell Optics Bertele
1 1/4" 12.5mm and 6mm Coulter Optical Orthoscopics
1 1/4" X 2" 32mm Edmund Scientific war surplus Erfle
Tirion star atlas (white stars, black background) hand-laminated
Megastar
And a partridge in a pear tree
To nudge or not to nudge, that is the question
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Rough surfaces can be quite eye catching. Here is the initial surface of a 13"f/5 I'm currently working on. Pushing back the central sphere to join with the outer zones.
quick work leaves quite a bit of roughness......I'll smooth it all out when I get the right shape.
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
Talking about strokes and smooth mirrors...I hope I'm not to off topic with this, but just a question....what is the best approach for a 16", 1.18" thick mirror?
Depends - what f/ratio do you expect this mirror to be?
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
Worked my last mirror a 20 f 8 with a full sized polisher/ tool on top / long stroke all on one side /manual overarm on a tub grinder/last time i polished it, I polihed for 2 hrS continus very thin ceo /tested with a ronchi and knife edge THEN EDS ZYGO .No small polihers needed on that one .With a f 8 you can do that . Knife edge very strong at 27 ft away /
Say Kevin, have you experimented at all with masking that edge?
Best,
Mark
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4968
Loc: Zuni Mtns, NM
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I have to say.. Some of the foucault and/or interference photos on here look pretty much Ok to me and I am sure would produce decent images. But I no longer desire to make mirrors, many on this site that are posted as having issues look much better than my best attempt.
Saves me money in the end that I am not so picky. 
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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looks better
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neo
professor emeritus
Reged: 02/12/08
Posts: 618
Loc: Iasi, Romania
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Quote:
Quote:
Talking about strokes and smooth mirrors...I hope I'm not to off topic with this, but just a question....what is the best approach for a 16", 1.18" thick mirror?
Depends - what f/ratio do you expect this mirror to be?
Best,
Mark
Well..I guess F/5 would be nice...otherwise it gets way to long for 16".
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Russian 15x50 binos
Home made 8" f/5 Newton on eq mount
Home made 70mm f/6 (Rodenstock Rotelar lens) Apo refractor
www.astronomy.ro
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Feidb
super member
Reged: 10/09/09
Posts: 127
Loc: Nevada
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I specifically went for f/6.5 or f/7 just because of the thickness of my mirror (1 1/4"). I ended up with f/6.4 and the beast is 9 feet long. That is one reason I'm now using a commercial f/4.5.
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Present gear:
16" Meade LightBridge
Meade 50mm straight through-finder
Lumicon green laser pointer
Orion Q-70 26mm, 32mm, and 38mm
Parks 2X 2" Barlow
Hyperion 17mm, 8mm
1 1/4" 18mm Russell Optics Bertele
1 1/4" 12.5mm and 6mm Coulter Optical Orthoscopics
1 1/4" X 2" 32mm Edmund Scientific war surplus Erfle
Tirion star atlas (white stars, black background) hand-laminated
Megastar
And a partridge in a pear tree
To nudge or not to nudge, that is the question
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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You mean the sag? I consider a 16x1.25" mirror on the thick side. 
Best,
Mark
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