Author Topic: Thin 18" Mirror Blank  (Read 194 times)

Adam Washington

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Thin 18" Mirror Blank
« on: December 29, 2017, 11:44:55 PM »
Recently got an 18" glass mirror blank, 3/4" thick. It came with a bunch of other astro stuff, was in a Coulter box and "Crown" was hand written on the box. Seems doubtful this is a Coulter product, probably just a handy box at the time. Newspaper that is was packed with is dated 1978.Perhaps the 'crown' refers to the glass type? Little to nearly no green cast to it. Surface on both sides is pebble like, appears to have been cast in, if it hadn't had been with the other astro stuff and had been a bit thinner I would have guessed it was a top for a small porch table.

Seems rather thin to be an 18"+ mirror, is this common? And is the pebble finish common to mirror blanks?

Thanks

Tom DuncanAttached Thumbnails






David Allen

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Re: Thin 18" Mirror Blank
« Reply #1 on: December 30, 2017, 07:44:23 PM »
Not common at all, especially in the time period you mention. Note, that at say f/4.5, a third of the thickness would be hogged out at the center. It could be supported properly - I think the biggest problem would be in working the mirror as it would difficult to not flex it (or impose a thermal gradient) while working it. Not impossible though. Fortunately it would come to equilibrium quickly, as I would expect a very substantial change in spherical correction when subject to a temperature change - it could get pretty wonky if night time temperatures change quickly.

malphandrafsadd

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Re: Thin 18" Mirror Blank
« Reply #2 on: January 01, 2018, 01:22:43 AM »
Put it in front of a LCD screen, laptop and look thru a camera polarizing filter to see if it has a good anneal or worse ...temperedhttps://youtu.be/FnZb3FiZxaA

Jeremy Swaine

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Re: Thin 18" Mirror Blank
« Reply #3 on: January 08, 2018, 10:05:30 PM »
That pebbly finish makes it look a lot like a....glass table top...to me

acbanlota

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Re: Thin 18" Mirror Blank
« Reply #4 on: January 11, 2018, 04:48:37 PM »
As Pinout says, check it with a polarizer, see if it has any internal strain. If it is strain free, well annealed, it might be possible to work it into a good mirror, with great care taken to avoid astigmatism!

Note, John Dobson used all kinds of random glass - portholes, tabletops, whatever surplus they found cheaply, and made them into reasonable mirrors

linghetade

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Re: Thin 18" Mirror Blank
« Reply #5 on: January 13, 2018, 04:28:32 AM »
You can't used tempered glass unless you run it through annealing first. It shatters... Dobson warned about this in his classes.

swadadanun

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Re: Thin 18" Mirror Blank
« Reply #6 on: January 13, 2018, 03:54:42 PM »
What sort of edging has it had? Beveled corner?

ruesonecrai

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Re: Thin 18" Mirror Blank
« Reply #7 on: January 13, 2018, 11:17:00 PM »
The edging can be seen in the second photo, beveled and with a rough finish, typical to a telescope mirror in my experience.

Tom

luseatcidood

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Re: Thin 18" Mirror Blank
« Reply #8 on: January 15, 2018, 09:59:40 PM »
Quote
You can't used tempered glass unless you run it through annealing first. It shatters... Dobson warned about this in his classes.
It doesn't just shatter, it can explode violently, which happened to me while running a diamond saw in an optics shop in 1973. If I hadn't had safety goggles on, I could not have remained in this profession/hobby.

Chuck Johnston

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Re: Thin 18" Mirror Blank
« Reply #9 on: January 21, 2018, 02:54:02 PM »
Worst case, absolutely. There's a lot of tension baked into tempered glass.

Jeff Swan

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Re: Thin 18" Mirror Blank
« Reply #10 on: January 23, 2018, 05:38:12 AM »
I'll do the polarizing filter test next week when I get some time.

So bottom line: is this a viable mirror blank or should I just recycle it?

Tom

stalafovkith

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Re: Thin 18" Mirror Blank
« Reply #11 on: January 26, 2018, 03:39:09 AM »
and that's why you do the polarizer laptop trick...

tempered glass would give you straight black lines btw

Michael Robinson

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Re: Thin 18" Mirror Blank
« Reply #12 on: January 30, 2018, 12:35:17 AM »
Or you could just scratch it and stand back...

Test with the polarizer. The rest is speculation.

Ben Marquez

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Re: Thin 18" Mirror Blank
« Reply #13 on: February 03, 2018, 06:15:05 AM »
Quote
and that's why you do the polarizer laptop trick...

tempered glass would give you straight black lines btw

No, polarized light would just give a colorless black null across the full blank aperture. Strain in glass shows up as rainbow-like colors in polarized light. Highly tempered glass looks like a CME aurora.

Here's a shot of an 18" f/8 fused silica mirror for the McDonald Observatory wheelchair scope, taken by Carl Zambuto. You can see the blank is strain-free except right at the bottom, where the concentrated load is straining the silica a tiny bit.

John Abreu

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Re: Thin 18" Mirror Blank
« Reply #14 on: February 08, 2018, 03:13:21 AM »
I see your pic and raise you a strain (this is a 12" piece from an old "piepan mirror" folder).

This may be the kind of pattern Danny's talking about, it looks like tempered glass:
Here's what two samples look like in ordinary light:
Here's the smaller one, Pyrex without that much strain in the base, but some patterning: