JohnH
professor emeritus
Reged: 10/04/05
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Loc: vancouver near the wilds of B...
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I picked up this blank with a bit of luck. Now it seems I made a mistake.
Being 3 1/4" thick it is some 50 lbs and may be too much to do entirely by hand. I think I need to do one of three or four things.
One: Send it out to be diamond generated to the rough sagitta. Who does this? I would prefer someone near enough to drive to, take a short vacation and pick it up on my way back. This possibility means it needs to be done in Washington State.
Two: Send it out to be cut in half into two 1 1/2" 16" blanks. Who does this sort of work?
Three: Tough it out by doing some body-building for a year or two so I can move it around.
Another possibility is to slump it into a new mold to make a larger, thinner blank or even a cellular blank. Who does this kind of work?
Edited by JohnH (09/26/09 12:21 PM)
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5025
Loc: Mobile, Alabama, USA
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How in the world did you get a blank that thick? Normally 2 and 1/8-inches was the maximum thickness sheet glass came in, at least until Corning stopped making it for good in the U.S.
Taras
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15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
Two curious cats
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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cut it in half, or have it slumped into a 20" blank.
a mirror that thick will not cool down in time to use it to its full potential before dawn.
OR make a testing flat out of it.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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John -
FYI (probably a minority opinion) consider yourself lucky. There probably are not many of them left. I made 4, 16 inch mirrors out of those blanks by hand. I have a cart on wheels to move it about and worked TOT with a 10 inch tool. Yes, they took a while. One mirror was a flat. For a test flat, you should have a blank that won't bend under its own weight when mounted vertically.
Dick Parker
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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Quote:
How in the world did you get a blank that thick? Normally 2 and 1/8-inches was the maximum thickness sheet glass came in, at least until Corning stopped making it for good in the U.S.
Taras
This is a blank from United Lens UL 38 in an old catalogue. It is a full thickness, cast pyrex blank. I can see mold marks on the edges.
True, cool down is a problem but the low-expansion characteristics of Pyrex should keep those reasonable, especially if I store it in a cool spot and make a cooling system for the mirror.
PS The seller claims to have a SECOND identical blank available.
Edited by JohnH (09/26/09 01:01 PM)
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dpwoos
sage
Reged: 10/18/06
Posts: 218
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Some years ago I purchased a 16" x 2.875" United Lens precision annealed and blanchard ground blank on Astromart. I subsequently became convinced that it was too thick for a mirror, so I paid GlassFab in Rochester, NY to bandsaw it into 1" and 1.875" thick blanks. They chipped the edge in the process, which is not a huge deal but it was disappointing. I think I paid $400 for the cut.
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Nathan F
sage
Reged: 10/10/08
Posts: 221
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Quote:
True, cool down is a problem but the low-expansion characteristics of Pyrex should keep those reasonable, especially if I store it in a cool spot and make a cooling system for the mirror.
As I understand it, it isn't so much the expansion and contraction of glass that cause issues as much as it is thermal disequilibrium with the environment. The glass should expand and contract fairly evenly, but the temperature difference will cause air turbulence around the mirror as the thick piece of glass radiates heat throughout the night.
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12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount
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Don W
Postmaster
Reged: 05/19/03
Posts: 14621
Loc: Wisconsin, USA
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Yup.....looks like the old full thickness standard of 6:1. A friend of mine ground a 20" full thickness back in the mid '60s. Now that was a big blank.
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
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oldtimer
sage
Reged: 11/13/08
Posts: 300
Loc: NW Illinois
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One of the clubs I belong to has access to a 16" full thickness (6 to 1) F 8 newtonian. The mirror sits on a piece of rug in its cell as it doesn't need eleborate support.
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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no matter what the mirror is made of (even zero expansion material like ULE), you need the mirror to be at ambient temp before it can perform.
a quick calc shows that after 4 hours with a starting temp of 70* and an ending temp of 40* (local weather this time of year) the mirror would still be at 48* with some very powerful fans.
if the mirror was 2" thick you would be a 41*
newport glass could lay it in a 20" mold and slump it into a 2" thick 20" diameter blank
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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Quote:
newport glass could lay it in a 20" mold and slump it into a 2" thick 20" diameter blank
I came up with a similar number myself. It would be a fair fraction of the price I paid. For a bit more $$$$$, OWL could recast it to a cellular blank. You might be able to get a 24" one out of it.
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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interesting, you could get a bubble free cellular....
how much did OWL and newport quote you?
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
I picked up this blank with a bit of luck. Now it seems I made a mistake.
Being 3 1/4" thick it is some 50 lbs and may be too much to do entirely by hand. I think I need to do one of three or four things.
One: Send it out to be diamond generated to the rough sagitta. Who does this? I would prefer someone near enough to drive to, take a short vacation and pick it up on my way back. This possibility means it needs to be done in Washington State.
Two: Send it out to be cut in half into two 1 1/2" 16" blanks. Who does this sort of work?
Three: Tough it out by doing some body-building for a year or two so I can move it around.
Another possibility is to slump it into a new mold to make a larger, thinner blank or even a cellular blank. Who does this kind of work?
If you want me to hog in your sagitta..then bring it down to Bremerton...
If you want to save some money...... 
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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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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Quote:
interesting, you could get a bubble free cellular....
how much did OWL and newport quote you?
I have sent a request to Newport. I think a 50 lb piece of glass would make a primary too big (and too pricey) to justify for a mere Newtonian, so I haven't sent an inquiry on to OWL.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
Another possibility is to slump it into a new mold to make a larger, thinner blank or even a cellular blank. Who does this kind of work?
Yes, Newport can do that and it won't cost more than half your fingers (no arm and leg) but be SURE you emphasize that it must get a good anneal or you'll send it back until it does. It won't make quite as thick a larger part as you expect because the "slumping" is more like squashing and there's always some rough result that has to be removed. In the quote they'll tell you what the yield will be. I've had them do this kind of thing before.
BTW it's just standard thickness for a cast 16". You can have them made if you want.
Best,
Mark
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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IT is a classic not as much worry on astig /by the time you pay shiping both ways and the risks of problems / i would use it as is /and as for the weight it will weigh the same as a 20 in/or you could cut in half and make two stigey mirrors out of it /love to work one of those classic old full thickness blanks
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4940
Loc: MA
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>>>mere Newtonian<<<
???
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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if it were mine a CHief! As long as ED answers his phone /
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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I have thought about this a fair amount more, and idea of cutting the blank in half doesn't seem reasonable. My reasoning is I have looked through a fair number of 16" scopes with thin mirrors and most display some astigmatism and a less than ideal figure. The question of this being either a support problem or inherent to the substate itself is unclear. The few 16" scopes with at least 2" mirrors were much better once cooled off.
Quote:
>>>mere Newtonian<<<
???
I made this crack without explanation. What I meant to say was if I went to the time and expense to get it slumped to a cellular form, it would probably be a 24" mirror. A "mere newtonian" would be a waste of material. An RC would be an optimum use for such a blank.
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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costs nothing to use it as is very nice blank
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polaraligned
sage
Reged: 12/26/08
Posts: 229
Loc: P. R. of New Jersey
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And the Titanic was a very nice boat...
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Can't be "slumped to cellular" SFAIK. The glass is heated until just soft enough to flow under pressure and then, in a scene Cary has described as "something out of Mad Max" an enormous press is used to squash it down to the larger size. To do the cellular the glass has to flow well to fit the webs, and that requires a different process and a lot more heat, and $$$ as well. The glass itself is the smaller part of the expense, and it doesn't have to be one piece to start - consider how the Arizona Mirror Labs make big cellular blanks (tons of chunks).
Best,
Mark
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gregj888
sage
Reged: 03/26/06
Posts: 301
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John,
It takes a little time but you can hog it out with a pipe flange as a tool. Tool on top... so you you dont have to pick up the blank very often.
You can also diamond generate a curve with a hand grinder (I watched Swazye do it once) but you need to find some instructions for that.
If it were mine I would make a machine, hog with a flange then tile tool for fine grinding and polishing... Of course I've had a 20" in the works for a loooonnnggg
time :-(
Just a thought...
Greg
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4940
Loc: MA
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>>>A "mere newtonian" would be a waste of material. An RC would be an optimum use for such a blank. <<<
Boy, there's been a heck of lot Pyrex wasted on big (and small) Dobs then. I'll take the mere 24" Newt (Dob). Certainly much more practical and useful than a 24" RC for my purposes.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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i would grind it into a f10 offaxis you could have a 16 inch sitdown dob / cost no more than a newtonian
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polaraligned
sage
Reged: 12/26/08
Posts: 229
Loc: P. R. of New Jersey
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Amazing how people don't recognize that a 3-1/4" thick 16" mirror would be a potential disaster for any optical configuration depending where (climate) the mirror is used. I guess no one has ever studied thermal effects.
Mike Lockwood is making 20" F/3 mirrors that perform excellent and are only 1.25" thick. Quality cellular mirrors have a face thickness of about 0.3".
I would not waste my time on such a beast. It will never perform as well as a mirror that is 1/3 it's thickness. Never.
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polaraligned
sage
Reged: 12/26/08
Posts: 229
Loc: P. R. of New Jersey
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I made this crack without explanation. What I meant to say was if I went to the time and expense to get it slumped to a cellular form, it would probably be a 24" mirror. A "mere newtonian" would be a waste of material. An RC would be an optimum use for such a blank.
So only an RC is worthy of a cellular. 
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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a friend of mine made a 16inch f 6 from such a disk in the 1950s or early 60s great images! love to have it.Clamed he saw the spokes in the rings of Saturn with it .
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perfessor
sage
Reged: 12/30/07
Posts: 499
Loc: Northern Illinois
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I have nothing substantive to add to the optical considerations ... but ... If the 16" blank is "too heavy to move," (this was the main complaint, as I recall) then having it recast as a 20" or 24" - it will STILL be too heavy to move!!
OK, carry on.
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Tom
"Don't always know what I'm talkin about"
8" f/7
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Likewise I have nothing to add on the optics side of things but moving a scope that gives you ~2x the light collecting and will cool faster may be enough motivation.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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Quote:
Amazing how people don't recognize that a 3-1/4" thick 16" mirror would be a potential disaster for any optical configuration depending where (climate) the mirror is used. I guess no one has ever studied thermal effects.
i tried to tell 'em.....
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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gregj888
sage
Reged: 03/26/06
Posts: 301
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I think we realize the cooling issues, but he has the blank. It is the "old" standard and can be used. If it's to heavy to move it would an observatory and shouldn't get sun baked...right ;-)
My 20" is conical and weighs in at maybe 30lbs, less after it's cored... I have not idea what recasting would cost but a new 20" quartz blank is about $1200 if I remember right. If it's $500 to cut in half and $100 to ship each way... OK you get 2 blanks...
I really like the idea of a cellular blank if that helps any :-)
Greg
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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I got one of those 16" x 3" Pyrex blanks from Optica B/C back in about 1980 and made it into an f/6. Mike Marcario and I tag-teamed on hand-parabolizing it, and we got it down to about 1/18 wave RMS. It gives spectacular images that support the Foucault and wire test data, and it only takes about 2 hours to thermally equalize any time of the year. As you can see in this picture, with our big ol' Corgi boy Rockit sniffing the mount (in bad need of paint), the mirror is completely open to the environment, no fans needed.
I would buy another 16x3 blank in a heartbeat, and even trade for a 17.5" f/4.5 Coulter I have. As long as they are well exposed to the night air, thicker mirrors equalize in a very reasonable period of time. You don't have to worry so much about flexing during figuring, and the support system can be simpler.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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HI MIKE what is the dog doing IN THE PIC?
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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He's a VERY curious guy, plus it didn't hurt to toss some bread crumbs up on the mount so he'd get up on it and I could get some shots.
Yes, the mount was dry when he got off 
Mike
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4940
Loc: MA
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The largest monolithic pro mirrors, like the 8m Gemini scopes, are ~ 7.25" thick. The only active temp control they have is air conditioning the dome during the day to the mean anticipated observing temp. Active optics (control of mirror figure) is open loop and does not compensate for temp changes. Adaptive optics, when used, is conjugated at altitude and does not correct for mirror seeing. They seem to get by with mirrors thicker than 2". This is not to say that amateurs can't get by with 18"+ mirrors thinner than 2".
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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Hummm - I have a friend with a full thickness 16 inch f/4.5 mirror. Final figure by a pro. Gives the best images at star parties. Another friend has a full thickness 16 inch f/6. I helped figure that mirror. Excellent performer. He loves it and is a very experienced observer. The 16 inch mirror in my Cassegrain is full thickness. After a short cool down period with fans it is fine. Can be a bit troublesome on evenings when the temp changes a lot.
This weekend I attended a lecture by Ronald Florence, the man who wrote the Perfect Machine. In that talk he discussed the 100 inch Hooker telescope on Mt Wilson. That is a big, full thickness plate glass mirror. He described how initially the telescope performed poorly. They thought due to thermals. He described what efforts they went to to addess that. Covering it in blankets. Even to the point of taking the mirror out during the day and cold storing it. Then, someone discovered a problem with the mounting structure. They fixed it, without any fanfare due to politics at the time, and the problems went away. The mirror apparently performs great since and to this day. They did not have (serious) thermal problems after all.
Dick Parker
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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The 100 inch Hooker is a beautiful telescope .
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lightfever
Pooh-Bah
Reged: 09/27/04
Posts: 1277
Loc: Macomb Michigan
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Quote:
Amazing how people don't recognize that a 3-1/4" thick 16" mirror would be a potential disaster for any optical configuration depending where (climate) the mirror is used. I guess no one has ever studied thermal effects.
Mike Lockwood is making 20" F/3 mirrors that perform excellent and are only 1.25" thick. Quality cellular mirrors have a face thickness of about 0.3".
I would not waste my time on such a beast. It will never perform as well as a mirror that is 1/3 it's thickness. Never.
I have to agree. For most of the year in Michigan I would not get a mirror that thick to perform. I have enough problem with my 12.5" Discovery.
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Mark
Tasco 15-TE 76mm
Sky Watcher 80mm ED
AT-111 Triplet
XT8i (with Woden re-figured mirror)
Discovery 12.5" f/5 Premium DHQ (PDHQ Split-tube Dobsonian)
12.5" f/6.3 Dob (Underconstruction)
Celestron CG-5GT EQ Mount
Celestron C4 EQ Mount
"Life is not about waiting for the storm to pass, but learning to dance in the rain" unknown
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Gregory Rohde
Vendor (Superior Optical Supply)
Reged: 05/02/07
Posts: 53
Loc: Portland Oregon
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"Two: Send it out to be cut in half into two 1 1/2" 16" blanks. Who does this sort of work? "
I can do this with my custom built slabbing saw.
www.quartzmirrors/images/SOS_shop.jpg
www.quartzmirrors/images/cutting_16.jpg
Greg
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Scott Milligan
super member
Reged: 09/08/05
Posts: 114
Loc: MA
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I'm a little surprised that no one has mentioned the (to me anyways) obvious ideal use for such a blank; make an autocollimating flat. Used indoors in a temp controlled environment, the larger thermal inertia will be of little consequence, and the figure as used will be less sensitive to the details of how the flat is mounted.
Scott M. 
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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I think a couple people already suggested making a flat (on the first page of the thread).
Considering the problem with the weight, though, can a good flat be made working strictly tool-on-top, and if so, can anybody describe how to go about it? This is interesting to me, as I could work up a 20" flat on fused silica to any thickness I'd care to order, but the A-B-C method also calls for swapping the parts top and bottom, does it not?
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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"can a good flat be made working strictly tool-on-top, ..."
==========================================
Yup, certainly can. Done many of those. ABC is used when you don't have any good reference pieces available.
M.
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So many projects, so little time!
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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I didn't really want a huge flat out of it, as I would need 2 other pieces of similar size. I may need such a flat at one point but not at the moment.
I put the blank away on a bottom shelf for the time being, as I don't have time right now to start on something else. Good thing too, as my back is acting up again now and I need to use it to get through work for the time being
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Scott Milligan
super member
Reged: 09/08/05
Posts: 114
Loc: MA
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Re-joining the conversation again after a couple of weeks (a bad habit of mine, sorry Mark). While I think the most reliable method for polishing flats is via a well-adjusted CP machine, very few of us will ever have access to one of these. I myself have limited experience making flats, although what little I have suggests that working MOT can become necessary when the work is especially prone to bending under the weight of the tool (ex. backside of an already perforated Schmidt corrector being reworked), but is otherwise less of a requirement when working a thick and un-or-minimally perforated substrate.
Working TOT, both stroke parameters and lap diameter can be varied to keep the figure simultaneously smooth and flat, provided that the lap backing is made stiff enough to prevent appreciable flexing during the stroke excursion, otherwise predictable figure control will be elusive.
The ABC problem can be avoided using the Ritchey-Common test, where the concave sphere is, of course, self-checking. ABC also suffers from self-weight deflection issues in diameters above 8 inches, not to mention a statistically significant increased risk of myocardial "events" when trying to separate the plates safely. Another workable test method is to check against a known good paraboloid in auto-collimation, although this method suffers the drawback that the errors in the flat are viewed single-pass, while residual errors in the paraboloid are, of course, impressed twice.
Quite helpful is a "pressing plate", polished flat to within a fringe or two, and of a diameter large enough to match the largest lap size you will use (typ. 80-85% of the workpiece diameter). Hard laps work best I've found, provided they can be "persuaded" to press out before dinner is ready.
Even better would be two such plates, one about a fringe convex, the other a fringe concave (I think there is an article in ATM II extolling the virtues of this luxury), but I suspect many reading this will not want to take the trouble to go this route unless making flats becomes an ala carte addiction (or else perhaps a business proposition).
In fact, I am currently making a 14.5" flat for a client using TOT + R-C test, and thus should be in a position to comment further shortly. Meanwhile, if Mark H. is still following this thread and finds a loquacious moment, I'm sure he could add more detailed and varied wisdom to the subject.
Best Regards,
Scott
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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I believe the ABC method is highly underrated. I made larger flats by using smaller 2-3" reference pieces. It has to be kept in mind that a reference half the size of the flat being worked shows only 1/8th the error across the worked flat. But usually the accuracy can be assessed pretty closely to 1/4 wave or a twitch better, with very good smoothness using careful deliberate work. Experience using contact IF is valuable.
We used to have planetary polishers at work, but with exposed gears, they weren't exactly "legal" with OSHA- so as a comsequence they were 'filed'.
What we generally used after that, for a certain size flat, was a specified size polisher used to effect little to no change with a set stroke of the polishing machine. As the surface got smooth and regular, we made slight changes to correct for a bit of power very slowly over a period of perhaps several hours. If careful, and with greatly thinned compound, you could get some beautiful pieces.
The polishers were a whole science to themselves. Could write a book just on pitch polishing for flats alone. Suffice to say, flower-power polishers was used extensively with production, but I found it a bit too hard for microfinish specification. I personally like used #73 with a high quality compound for most glasses. Quartz needed an agressive compound to make any significant changes.
M.
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So many projects, so little time!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Speak on, I'm now saving these posts for reference. 
Best,
Mark
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12223
Loc: Los Angeles
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Over 3" is going to be a problem for thermal equilibrium, but it can be put in a scope with fans both underneath and on top.
I start the one rear-mounted fan on my mirror (2-3/16" thick) about 1/2 hour before sunset and let it run. By 1/2 hour into the twilight, the mirror is +2.5 deg. above ambient. By the end of twilight, usually +1 degree above ambient. Because the temperature falls quickly where I observe, the mirror rises to +4 degrees above ambient an hour later and then, because the temperature drop where I observe always levels out, the mirror is down to +1 degree above ambient a couple hours later and stays there till dawn. Yes, I do run the fan continuously.
Now if I had 3 rear fans, the mirror would stay close to ambient and it would cool faster. With top fans to blow away the boundary layer (they could even be run periodically), I see no serious problem with a 3"+ mirror.
And, if steps were taken to prevent the mirror from reaching too high a temperature (mine is stored in the coldest place in the house, and the mirror averages 56-59 degrees when I pull the scope out), some carefully-placed fans would work fine.
The advantages of a full-thickness blank have been stated. Yet another advantage would be a low COG on the scope, so the scope might be quite a good performer.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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charles genovese
super member
Reged: 02/04/06
Posts: 132
Loc: Madisonville Louisiana
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Cutting a 3" or so central hole would probably dramatically increase cooling
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12223
Loc: Los Angeles
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If he uses a 4" secondary, the "umbra" of the secondary is only 2" or so, so a 3" perforation would be too large.
Also, removing the center would make collimation more difficult later.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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I had wanted to go the route for a big Dobsonian. I am currently just about to go the 25 15 9 5 micron grits on it. When I bought this blank in 2006, it was partly ground with an off center depression. After doing some 100+ wets with 46/70 and 80 grit with a weight of 20+ pounds, it made me think about going any larger with a full thickness blank.
I looked through most of the large Dobsonians at Table Mountain in 2007 and 2008, and found that the limiting factor was the seeing. I came to the conclusion a 16" mirror would gather plenty of photons, could be kept under 120 pounds and under really steady skies, get close to its potential more often.
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