Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Preparing to start polishing and this happens.... 
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Chris
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 13776
Loc: Amargosa Valley, NV, USA
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Oh, that hurts even to look at! 
Is it salvageable? The blank looks pretty thin. Can it be reground on the other side?
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"Since the process of science generates more mysteries than it solves, we can never learn everything, we'll simply generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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Don W
Demi-God
Reged: 05/19/03
Posts: 15144
Loc: Wisconsin, USA
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Eeeeek!! Doesn't look like the edge was beveled enough.
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
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Scott Horstman
Vendor- Backyard Observatories
Reged: 03/11/04
Posts: 8155
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Oh no!! That just stinks!!!
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Scott
www.backyardobservatories.com
www.m1oasys.com
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RRPG
sage
Reged: 01/26/08
Posts: 363
Loc: 42° 25'50" 123° 19'00"
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I am pushing a thin 13.5 blank and feel your pain.
“Only after disaster can we be ressurected.”
Chuck Palahniuk
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Peter
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8 in HO Truss Dob
8 in. Skywatcher SCT
C102XLT
ETX 90 EC
C80ED
CG5-GT
WIP: 14 inch HO Truss Dob
Canon 450D Xsi
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GJJim
scholastic sledgehammer
Reged: 09/09/06
Posts: 907
Loc: Western CO
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That is too bad, especially for it to happen after all that fine grinding. That chip is huge -- did you bump the edge while cleaning?
At this point starting over on the reverse side is probably your best bet.
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gnabgib
sage
Reged: 06/05/05
Posts: 371
Loc: Fall River MIlls Ca.
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Just bevel the edge more (at least 1/8 inch and rember a rounded edge rather then a 45 degree will cause any chip to rise toward the surface quicker resulting in a smaller chip).
The loss is only a few square inches and a HUGE blow to ones pride. When observing you will never know it is there.
Keep on polishing!!!!!
My .02 worth
Kevin
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 920
Loc: Easton, PA, USA
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Chris,
I'm very sorry to see that.
What will be done?
I hope the maker isn't discouraged from continuing.
Gary
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1671
Loc: Fort Worth TX
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Chris,
Kevin's absolutely right. The effect on the image will be minimal, mostly just a few percent in light loss. That chip's not even the size of the diagonal you'll use. Doesn't look like you have enough blank thickness to grind the other side without getting too thin at the center, and it's not worth doing anyway. The only problem is having to trim out the lap so it doesn't flow into the chip during pressing.
Once it's figured and aluminized, a little ultra-flat black and a Q-tip will eliminate any trace of the chip.
Tell you what - take a photo of it at normal incidence and post it, and I'll curve-fit it and run a ZEMAX simulation of the chip and resulting star images so you can see for yourself. Remember Frank Tomazewski's 10" mirror with all the chips around the edge, and the analysis I did for it last year? If you added up the surface area of all of Frank's chips, they probably were about the same total area as this one. The effect on the PSF was insignificant.
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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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Thanks Guys,
... for the supporting words, 'lucky' me...  .. the mirror is not mine. This mirror is Jean-Pierre's. An ATM-friend here in Belgium from the Oostende region. Not to far from our local observatory. He also gives a telescopemaking course in that area.
He will continue polishing and using it. At first he wanted to cut it down to 18" to get rid of the nasty chip. His pride got a bit of a blow for a few hours but I adviced him to continue. And he will. He will cover it up with some black paint once the mirror is done.
The mirror is indeed to thin to start all over at the reverse side, it's only 25 mm thick, but it would not have been an option anyway.
For Jean-Pierre there is no way he will be Flexing his mirror. he's forced to parabolize it the old fashion way.. pitty.
Thanks Mike for his kind offer, I'll ask JP to make a new picture and will post it here
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Chris
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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"For Jean-Pierre there is no way he will be Flexing his mirror. he's forced to parabolize it the old fashion way.. pitty."
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What? Think of it as a learning experience!
M.
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So many projects, so little time!
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F.Meiresonne
Post Laureate
Reged: 12/22/03
Posts: 3632
Loc: Eeklo,Belgium
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What a pity Chris, it's horrible to look at it. But disasters do happen. Here in Assenede a 18" broke in 2 pieces once while grinding...ouch...
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Freddy Meiresonne
Obsession 18 inch #1638
Orion Optics 8 inch F/4.5 -1/8 wave optics -Vixen GP-E
22x85 Helios Apollo (=GO SS)
15x70 TS Marine (=Obie Ultra)
10x60 Helios Quantum 4(= Obie Mariner)
10x50,8x40 Helios Nature sport plus
Eyepieces in use :Pan 35,24,19, N13T6, Pentax 10 XW, N9T6, Ultrascopic 7.5, TV2, BGO 12.5 and 9 mm
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Quote:
"For Jean-Pierre there is no way he will be Flexing his mirror. he's forced to parabolize it the old fashion way.. pitty."
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What? Think of it as a learning experience!
M.
Mark,
JP knows how to parabolize very well, he was going to try his very first Flexed mirror... 
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Chris
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Quote:
What a pity Chris, it's horrible to look at it. But disasters do happen. Here in Assenede a 18" broke in 2 pieces once while grinding...ouch...
I know Freddy, I rememeber it was posted here at CN a while ago.
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Chris
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oldphoggie
newbie
Reged: 01/09/09
Posts: 4
Loc: Ohio
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What were the circumstances that caused it? A bigger bevel would have helped? You have my sympathy.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8681
Loc: Bremerton Washington
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Quote:
Quote:
What a pity Chris, it's horrible to look at it. But disasters do happen. Here in Assenede a 18" broke in 2 pieces once while grinding...ouch...
I know Freddy, I rememeber it was posted here at CN a while ago.
Being familiar with mirror disasters...I feel for you guys.
To get so close to the final product and.........
Rob(been there, done that)
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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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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Mike,
This the picture JP send me. I hope it is large enough to be used in ZEMAX. If not let me know I ask for a larger one.
Thanks....
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Chris
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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I'll almost bet that it was a wicked clunk up against something hard, or it might have hit the floor.
M.
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So many projects, so little time!
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2160
Loc: Boechout, Belgium
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Quote:
Mike,
This the picture JP send me. I hope it is large enough to be used in ZEMAX. If not let me know I ask for a larger one.
Thanks....
Silly me ... 
Forgot the picture.
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Chris
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kfrederick
scholastic sledgehammer
Reged: 02/01/08
Posts: 783
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hi Chris in 1992 i had a 20 inch pressing on a lap[ mirror on top ]with 50lbs of weights on top /it slid off and landed on the floor [carpet over wood floor] i was in another room when it fell / no damage //i was very luckey / i have neaver pressed with the mirror on top sence/ kevin
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