Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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I bought a 5" blank a while back and after checking it has some stress, classic 'cross'. I complained about it but that didn't get me anywhere so that user got some bad feed back over a few $$$.
Anyway, is there anything I can do with it or should I just toss it?
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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: 3548
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send it to zambuto and have him anneal it with his next batch of blanks
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5028
Loc: Mobile, Alabama, USA
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The cure is to put it in an oven and heat it until the glass softens and flows, which eliminates stresses that became locked into the glass. No, a household oven doesn't get nearly hot enough, the glass is heated up until it's literally red hot then allowed to cool back down slowly to avoid locking in new stresses. The glass needs to be re-annealed, which will fix the problem.
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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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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I used to make mirrors from old ships porthole glass, these are generally 3/4 to 1 inch thick, there was a ship breakers yard five miles from where I lived on the river Medway in kent, I could pick these up for nothing. They made quite good mirrors for general use and I never had one that was stressed, and I have a theory that 60 or more years bolted into a bronze bezel and worked by the twisting and working of the ships hull had the effect of relieving the the stresses in the glass, work annealing in effect, has anyone got any comments on this?.
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Feidb
super member
Reged: 10/09/09
Posts: 127
Loc: Nevada
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Crayfordjon,
That's in interesting concept. Work annealing.
I've heard of lots of people making mirrors from portholes and have never heard of any problems with stress. I wonder if the portholes were annealed before they were installed, just to make them last longer. I've seen them annealed for use as view ports in storage tanks.
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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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David Castillo
scholastic sledgehammer
Reged: 09/09/06
Posts: 882
Loc: Carmel Valley, Ca
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I think Newport glass may be able to help they do offer annealed glass.
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Dave
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George Kiger
member
Reged: 05/19/06
Posts: 29
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The old ATM books (no. 1, I think) recommended finding a fire hydrant and hurling the mirror against it. Hopefully the needed annealing won't cost too much.
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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It may be that it is perfectly OK. I've seen many a lens made that had stress in the form of a Maltese cross and their surfaces polished free of astigmatism. If this is a hot molded blank the stress is probably concentric and should polish up fine. If it was cut out of a sheet and the stress is uneven then that's a different story. You say it has some stress but optical glass often has some allowable spec for birefringence. If you see colored rings it definately has too much.
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Ed Jones
Edited by Ed Jones (10/14/09 05:14 PM)
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John Jarosz
Astro Gearhead
Reged: 04/25/04
Posts: 2287
Loc: Chicago area, IL
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You can anneal it if you know how to do it. Heating any glass to those higher temperature always comes with a risk of breakage. The cooling rate must be correct or you will simply lock in new stresses/strains. This is one of those things that you either know how to do or you don't. I don't, but I will admit to annealing a lot of molded television glass in the 80's & 90's.
I did read that fire hydrant comment in ATM as well.
John
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6" F4.6(w/Paracorr) GEM reflector, 8" F11 Dall Relay Scope
6" F5 RFT Refractor, Garrett Gemini 20x80 LW
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Porthole glass blanks were cut from huge plates cast onto cast iron tables. The newly cast sheet was then run through a 'lehr' or annealing oven to releave it of stresses. The sheet was then ground and polished both sides to produce the 'plate'. This was the only treatment the glass received. The discs were broken out into a rough edged circle and left that way. The portholes were then sealed into bronze or brass rebates with putty, and then bolted on to the hull of the ship. Nowdays, we use float glass, this is made by casting molten glass onto molten tin alloy, and when it was hard enough it went into the lehr. the glass is flame polished by this process, and is much brighter, also the quality control is so good that the glass is nearly perfectly transparent, and I have found that it can be used for refracting optics. Plate glass on the other hand was full of striation and changes of refractive index.
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neo
professor emeritus
Reged: 02/12/08
Posts: 623
Loc: Iasi, Romania
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John, your post came jast at the right time. I was actually asking myself how those portholes were made. A guy from our local astronomy forum, living near the Black See in Constantza, a portuary city, promised me he can get me a 400 (~16") 30mm thick piece of glass from an old ship. He said the glass is in one piece, not tempered. Would be worth the trouble to consider pushing the ol' chunk of glass into a god parabola?
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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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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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I'd suggest getting one, and doing a quick stress check...
Have fun
Owen
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Ditto,get one and make a mirror, what can you lose,it's the cheapest option, you gotta speculate ( pun) to accumulate. Good luck. John.
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gregj888
sage
Reged: 03/26/06
Posts: 303
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Biff, You could just use it as a tool...
Greg
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2273
Loc: Kent, UK
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any stained glass artists in your area? they might pop it in the kiln next time they're firing? most modern kilns can be programmed for staged cooling.
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
PST
DSI 1
and a curious mind
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1165
Loc: Gateshead.NSW Nth Coast,Austra...
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If you can find someone who has a temperature controllable kilm , you could heat the blank to just under softening temperature (slowly ramping up the temperature over several hours) and then hold it there for a day or so and then very slowly in step , bring it back down to ambient temperature over a week or so.
That'll anneal it.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 372
Loc: UK
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Would a ceramics kilm do? lots of hobbyists making pots these days.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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If I remember correctly, the temp to reach with pyrex is around 900-1000 degrees, and to have a cooling schedule roughly a day or so long(?) for a good anneal. Zambuto could tell you a realistic schedule.
M.
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So many projects, so little time!
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dave brock
super member
Reged: 06/06/08
Posts: 165
Loc: Hamilton, New Zealand
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We are talking about a $50 piece of glass here, right?
Dave.
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20" homebuilt truss dob
6" watson refractor
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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$50? SOLD!
No it was ~$20 IIRC, or maybe even less. I don't think I'll be sending it away to have it annealed as that wouldn't be cost effective. BTW Ed, I'm seeing some colour rings in the outer edges so I'm assuming it's not usable as is. The fire hydrant solution seems like the best solution but I guess I'll just file it away. In the event I need to send something to someone that has the facilities to anneal it then I'll throw it in the same box. Until then it can collect dust.
Thanks for the replies all.
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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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