JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
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: 5023
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: 3529
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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: 206
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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: 219
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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: 14618
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: 299
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: 3529
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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: 3529
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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: 8306
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: 2156
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: 659
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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: 4933
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: 659
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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: 659
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costs nothing to use it as is very nice blank
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