greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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i think they have stress
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10" f/5 dob unf.
7x50 Tasco binos
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madrian
member
Reged: 06/30/08
Posts: 46
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Yikes, you've got a load of stress in those blanks. They look a bit like the moulded blanks from Schott which are not fine annealed.
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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Quote:
Quote:
a really nice way to test for strain:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/strain/strain.html
Hey Dave! I just did a simple strain test on two 8" blanks I received in the mail last week. I placed my laptop on its back, monitor on a blank white screen, popped the mirrors on top of the monitor, then threw my polarized filter on the lens of my digicam. Dunno if that was the right way or not, but check out the results!
Thanks!
Sam
woops...probably need to link you to photos...sorry if they are a tad blurred...I was hand held at 1 second each.
picture 1
picture 2
The pattern your seeing might not be in the glass but caused by the blank resting on the LCD computer screen. First, with the glass not in place and by looking through the polarizing filter you should see the computer screen go from even white to even black with no pattern when you rotate the polarizing filter. Next see if you get the strain pattern when the blank is not in contact with the screen. Also rotate the blank to be sure any pattern rotates with the glass.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Maybe you'll get lucky and that pattern was cased by the blank on the screen but they look too text book 'nasty strainish' to me to be anything but.
From what I hear trying to get a good figure on a strained piece of glass is an exercise in frustration and even if you do get a good figure it won't be temperature stable. Sorry.
They'll have to be annealed.
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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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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Unsure there. The pattern did subside some once left indoors for an hour. The pattern was still there, but pushed closer to the corners. What I do know about the blanks is that they are Pyrex (so what at this point), and were formed some time ago. I don't know anything about pooring vs moulded blanks...that said, they do have a bit of material on the surface that looks like a blob of melted ice cream...dunno how to explain it here...and also has a stamped "Pyrex, made in USA" on it, as well as a hefty squared off edge. The box is labeled "Telescope Mirror blank" 8" x 1-3/8" I will try again in a bit without the components touching.
Thanks all...sounds like might be better to sell off, and find something else to work with? What is the annealing process consist of? Sounds like a bunch of extra work!
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Wait...1049 degrees F??? Did I read that right here...
http://yarchive.net/chem/pyrex.html
HAHA...not doing that at home! Let's hope I goofed the test, otherwise I'm looking for new blanks.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Looks like strain in both but do what David suggests, or put the screen edge up, stand the mirror blank on edge in front of it, and check it that way. You need to be sure the screen blacks out completely without the glass in place to test it.
Best,
Mark
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Will do...so test the screen before the glass...then put the glass in front. Going to do it that way...will see what happens. It is officially cooler outside in the garage where the glass is now, than it is indoors here...but I will bring them in and have a go. I will report back asap.
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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OK, this is my finding...really weird. First off, the edge pattern is almost gone. The whole blank(s) show black all the way through, except for the top, bottom and sides. When I turn the blanks, it makes no difference in what I'm seeing. All but the very last 1/2" of edge is black against the screen, standing up on edge. I wonder if it is the filter that is doing this? Or maybe the heavy edge of the blank? Don't know if it is normal or not, as I've never seen one like this...but the edge is not cut at a 90 degree angle to anything...the face is smaller than the back of the blank by 1/4"...the edge does in fact taper..could that be creating this visual? The computer monitor shows pure black through the filter, monitor only.
Thanks guys.
Sam
OK, adding on to post...a big also...these blanks are old...I mean OLD.
markings on the box as follows:
Gert Mullen
Main Plant Guards
Office Walnut St
Phone 7743
Glass
Code 634080
1pc. 8" x 1-3/8"
Telescope Blank
Judging by the way they list their street and phone info, this is going way back to when one would call an operator to be patched into a phone #. Way before area codes, etc.
Anyone heard of these folks possibly? The only "Glass and Mullen" hits I had on Google looked like a custom blown glass company. Dunno, maybe related... 
Edited by yosemitesamiam (11/04/08 02:17 AM)
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Wow that's great news! I test mine the same way - white laptop screen and polarized glasses with the blank standing on edge infront of the screen. I've tested a few and got perfect results, i.e. equally greys out. Except one 5" molded pyrex blank I got, showed the same pattern as yours did but more pronounced that's why I figured you were in the same boat. Glad to see you're not.
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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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madrian
member
Reged: 06/30/08
Posts: 46
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Quote:
First off, the edge pattern is almost gone. The whole blank(s) show black all the way through, except for the top, bottom and sides. When I turn the blanks, it makes no difference in what I'm seeing.
Sounds like you have good anneal but not right up to the edge.
DaveG is incorrect in stating that the pattern you see for *regular* strain ( a maltese cross or in your case just 4 bright lobes near the edge ) should rotate when you rotate the glass. Only *irregular* strain will appear to change the patterns as you rotate the glass.
Regular stress means that the strain and hence glass density increases from the center to the edge. It is a condition that increases uniformly and radially in all directions pointing out to the edge. It does _not_ mean that you have 4 symmetrical lobes of high stress that would rotate with the glass as you rotate, which is what first glance of the maltese cross might demonstrate. If you would like me to photograph a blank with strain with a reference mark to demonstrate this I will, but only if this is an issue. I just looked at a piece with regular strain as a sanity check before I posted.
Its interesting that the strain pattern seemed to diminish when you let the glass settle. If you have even cut-off at least most of the way to the edge then your glass is a 'goer'.
Edited by madrian (11/06/08 08:04 PM)
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Wow, interesting for sure. I'm going to trust what everyone is saying at the bottom line...and that is that I'm fine to grind. I'm not totally probably ever going to understand stress, except for the part where my wife might be pregnant...but other than that...I'm good. LOL
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Beri
sage
Reged: 06/29/05
Posts: 294
Loc: Croatia
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Here is (well at least the bottom half) of my new tester.
It came out much better than the old one. Made out of misc junk, like a printer head rail, some setting screws from an old mitre saw...
This time I bought a proper micrometer , and intend to use a bright blue LED and a small 3x telescope to see the surface and zones better....
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15x70 Binoculars
10" truss Dob
http://www.scopemaking.net
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Beri
sage
Reged: 06/29/05
Posts: 294
Loc: Croatia
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some more bits added.....
front side with small scope made from a old russian 50 mm camera lens and a 15 mm ortho eyepiece. Conveniently, it has a focuser and iris already built in
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15x70 Binoculars
10" truss Dob
http://www.scopemaking.net
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K. A. Scharf
member
Reged: 09/02/08
Posts: 35
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Wish I hadn't thrown out those bad printers in my junk box!
Well, cheap ones show up at garage/yard sales!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Ok, here's a couple tester pics I originally posted into another thread, but I don't want to do the hijack thing...
This is (obviously?) a slitted tester, built massive on purpose (a) because that's the size of parts I had and (b) to be stable and resist bumping.
The light source is a cut down LED with maybe an 800 grit finish behind a 150-micron slit. Visible on the tower is the washer protecting a 50micron pinhole on the near side. It rotates to the same position as the slit for astigmatism testing. In critical instances the return beam can pass through the two holes in the tower.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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The L-bracket with the KE is held on with magnets to the transverse stage and can be replaced with other heads, and easily removed to use a Ronchi screen, etc. The transverse stage moves like a linear stage in that it keeps the vertical alignment. It's made from stainless sheet metal but had to be perforated due to being too stiff as originally built. The main stage rides on Teflon arcs that were lapped with sandpaper to the stainless carriers until there was no play at all, and there's a low-tension spring under there that you can't see to return the carriage. And the micrometer head seats on a ball-bearing in the carriage, also not visible.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3120
Loc: Northeast
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The KE stage looks springy. Is that to use that way on purpose? (with slight pressure to "cut in"?)
M.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3120
Loc: Northeast
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Never mind, I see the second pic is self explanatory. Keeps the KE parallel with the slit.
(I put my LED within about 1/8th inch of the edge of my tower.)
Nice looking tester.
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: 2156
Loc: salem, OR
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Thanks Mark,
If I rebuild the tower (I didn't when I made the rest of it last year) I would use a narrow top (or not center the sources). I'd literally worn out the previous tester.
My main goal in doing the transverse stage was to make it sensitive and stable at the same time, which this one is. 
Best,
Mark
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