Indianajones
member
Reged: 09/16/07
Posts: 25
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Hi,
I recently purchased a old 17" flat mirror on ebay. I hope to grind it into an f/5 or f/6.
The seller also has a 26" mirror 1.18" thick, ebay id: 180195023756, which might be of interest to someone. I have no interest in the deal, but I'd appreciate if you let him know that I pointed you his way.
Clear skies
IJ
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Stadius
member
Reged: 06/15/08
Posts: 27
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HOw much would it cost to have this mirror ground into an F/5 and reacoated?
Could that be done?
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Indianajones
member
Reged: 09/16/07
Posts: 25
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Hi,
I have no idea of the costs, myself being a novice. I forgot to mention that the 17" I purchased is my first attempt at grinding a mirror. I'm sure there are more qualified members here in this forum who would be of more help.
Have a nice day
IJ
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helpwanted
Carpal Tunnel
Reged: 07/04/07
Posts: 1575
Loc: Phoenix, AZ
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shouldn't you practice on a few 6" mirrors before trying a 17"
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XT8 (8" f6)
EPs: Hyper, Pan, SPL, TMBs
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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17" is a bit large for a first time attempt. But John Dobson DID do his first mirror as a 12"
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greenglass
sage
Reged: 01/22/06
Posts: 394
Loc: Ontario,Canada
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You got a grinding machine?
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7x50 Tasco binoculars
4" f/9 reflector
12.75" f/5.4 reflector
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Dick Parker
super member
Reged: 08/17/07
Posts: 127
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You did not say much about the 17 inch flat. Is it pyrex?? is it optically flat?? if so, it is much much more valuable as a flat than ground to some curve.
Dick Parker
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HiggsBoson
professor emeritus
Reged: 02/21/07
Posts: 706
Loc: Kal-li-fornia
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I would suggest making a 6” to learn the process. It is quick and the shear weight of the thing does not limit the learning process. Next do a 12” because you will need to understand what it means that the weight increases with the cube of the aperture. Much more grinding and lifting of much more weight. One will also learn things that are only relevant to larger mirrors. With something as large as 17” one would want to be past the simple mistakes.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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tatarjj
scholastic sledgehammer
Reged: 04/20/04
Posts: 887
Loc: Auburn, AL
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How would you keep such a thin mirror from warping severely? A 45-point mirror cell?!
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John T.
Auburn, AL
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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A 27 point should be good enough.
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sixela
Postmaster
Reged: 12/23/04
Posts: 9244
Loc: Boechout, Belgium
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Quote:
How would you keep such a thin mirror from warping severely? A 45-point mirror cell?!
You'd be surprised at how good an 18 point cell is even for relatively thin mirrors. The thing that usually bites you on such mirrors, though, is the edge support - anything that exerts a force perpendicular to the surface (even if it's just through some stiction, or because attachment points for a sling aren't *exactly* in the correct plane) will warp the mirror big time.
http://www.cruxis.com/scope/mirroredgecalculator.htm
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400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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While a strap sling is the prefered method, a wire 'sling' or two posts spaced 90 degrees apart also have been used.
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Indianajones
member
Reged: 09/16/07
Posts: 25
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Hi,
I apologize for not responding to the questions several of you had posted. I wanted to wait to have the blank in my hands before answering!I finally took delivery of the blank this morning, and I'm pretty impressed at the sheer size. For those with more expertise at discerning the type of glass, the color seems to be a silvery grey.
I'll try to answer the questiions I've been asked:
1. Perhaps I should have tried a 6" mirror first, but the price for this mirror was irresistible, costing what a 6" pyrex blank would have!
2. Grinding machine: I've asked help from the local astronomy club. I find out if they have a machine, otherwise I'll build one myself. I've got to put that Mechanical Engineering Masters Degree to some use sooner or later!
3. This mirror was used in an optical comparator, so I'm guessing it should be optically flat, it ground on the back and coated on the front although the coating's seen better days.
4. I'm considering an 18 point mirror cell for the support although that could change.
I'm not planning on starting the grinding process immediately, got a 10" f/4.73 telescope to assemble!
By the way, how would I get the existing coating off this mirror? (It's a flat mirror that I'd like to grind to a f/5 to f/6.)
A big thank you to everyone who pipped in with their comments, you've helped me look at this problem from more than one angle.
Clear skies
IJ
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1203
Loc: salem, OR
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If it's actually a decent flat you'd be insane to turn it into a mirror, as it's worth far more than it cost. Any number of people in this forum could test it for you if you don't know how.
For that matter, if it's a decent flat, any number of people in this forum would be happy to buy it from you at cost... 
Best,
Mark
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Indianajones
member
Reged: 09/16/07
Posts: 25
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Mark,
Thanks for the advice, I'll take the mirror to the local astronomy club here. I understand that they are a knowledgeable bunch. I'll try to find out if they can test the mirror for me. Since this is a mirror from an optical comparator, I do expect it to perform decently. I'll ask for help in this forum if the mirror(flat) can't be tested here.
By the way how much would a decent 17" flat cost? and is there a way tell if this is a pyrex flat by looking at it?
Thanks for your advice once again
IJ
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1203
Loc: salem, OR
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Easily $1,000,000 or so.
Really, it depends on the quality, and to a much lesser extent, the condition. It's the flatness that matters, not whether or not it has scratches from use. If its genuine 20th wave surface figure around $10K (anybody else?) unless you got it at a real fire-sale. They are very hard to make. HOWEVER, to be valuable to anybody else it has to be verified for accuracy.
As to what it's made of? If it looks like Pyrex it probably is. If it's clear like acid it's probably quartz or fused silica. If it's greenish it's probably plate, if it's yellowish or amber it's probably ZeroDur or similar.
I'm trying to find the original auction - ah I love the way they keep changing eBay to make it harder to do things - nope. Do you have the auction number, if it was an auction?
Best,
Mark
Edited by mark cowan (07/04/08 02:31 PM)
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Indianajones
member
Reged: 09/16/07
Posts: 25
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Dick, Mark,
I wrote to the comparator manufacturer to try to find out more about the mirror. I received a reply from them today and according to them a mirror with a thickness of about 1" (I have a 1.2") is float glass. I also asked them for the surface finish and their answer is: "Our specification for the quartz glass was around 4 light bands. For the float glass, it is closer to 8 bands.". Unless I misunderstood the answer, this means that the surface finish is 8 wave!
Soooo looks like I generated a lot of excitement for nothing  . Anyway just for the heck of it, I'll still take it to the local astronomy club next Friday and see what the club members think. At the least I can still use the blank for it's original purpose... make a telescope!
I've attached a picture I took this evening. I tried attaching one with better detail but the attachment size limitation wouldn't allow me!
Have a great weekend,
Clear skies
IJ
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1203
Loc: salem, OR
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17x1.2" float should work well, do check the anneal or have it checked before you start. Plate is usually quite good but I've seen some atrocious examples and you won't know for sure without checking.
Best,
Mark
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