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Help identifying a 12.5in f/6 primary mirror

ATM mirror making
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#1 AndresEsteban

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Posted 23 January 2020 - 02:13 PM

Hi dear colleagues!

Here in Brazil a friend of mine found and old reflector built around a heavy metal structure (look like a 2WW tank). He bought very cheap it and found a beautifull 12.5 inches (317.5 mm) f/6 mirror with 74.8 inches focal length. See attached images. My friend trashed to the junk yard the telescope structure and kept the primary and secondary mirrors to build a better project. (Please don't consider the mirror clips touching the mirror surface! I know this is totally wrong but as I've explained, the scope structure was awfull!).

From what the actual owner of the scope, who never fully mounted it, the scope was made from an engineer who brought the mirrors from the USA long time ago and according to him, the primary mirror was made by an ATMer. The question is, we're trying to find who could have produced this mirror...

As you can see it has a "LK" mark at the end of the "12 1/2" F6" spec. We think, perhaps we''re wrong of course, that this "LK" is some kind of signature...

First, I thought about Michael Lockwood. I've contacted him but he kindly answer me and explained that this "LK" i's not his signature and that he engraves the back of his mirrors with his own signature and other specs (date, FL., #number, etc). So our quest continue...

Does anybody have any ideas about what this "LK" may be? Who could be the ATMer who build this mirror? Or, if the original info is wrong, is this one a comercial product?

Did anyone ever find a mirror with this "LK" letters along the "diameter, f-ratio" spec at the border or perhaps at the mirror's base?

Thanks in advance for any help and insights to solve this riddle!


Clear skies for us all!
Andy

 

 

 

 

Attached Thumbnails

  • mirror cell low.jpg
  • 12.5 f6 LK.jpg
  • FL 75 b.jpg

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#2 photoracer18

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Posted 23 January 2020 - 03:36 PM

I am betting its definitely not Cave or Optical Craftsman because I have both of those brands and they engrave on the back of the mirror. I am betting its an ATM DIY setup as the cell does not look commercial either, but it may be a model that had plans available for it.


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#3 TOMDEY

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Posted 23 January 2020 - 04:45 PM

The nubbly back surface looks like native raw Pyrex sheet glass to me. I know the early Caves used the (wonderful!) Corning individually-cast ~full-thickness~ and fine annealed blanks, made specifically for telescope mirrors. I once had such a stash of those blanks (6, 8, 10, 12.5, 16) that my wife made me haul them to the Town Dump. Now those blanks are quite sought after. Far as I know, they are no longer made. Nothing wrong with the sheet glass, though... provided it got properly annealed.    Tom


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#4 Jon Isaacs

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Posted 23 January 2020 - 05:47 PM

Anybody want guess the thickness?  Looks thin to me, not full thickness.

 

Jon


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#5 CHASLX200

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Posted 23 January 2020 - 06:54 PM

It is not a old school blank.


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#6 AvaniSoares

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Posted 23 January 2020 - 08:30 PM

Hello everyone!
I actually bought this mirror, and asked friend Andy to do the post due to my great difficulty with English.
I bought this mirror due to the low price as Andy said, more because of the glass that I also thought was a pyrex, than because of the optics that I will hardly be lucky to be good at.
We are about 90% sure that the mirror came from outside Brazil, but the mechanical part was done later by the former owner who is now dead. Hence the difficulty of obtaining information.
The thickness of the glass is exactly 1.1 "as a friend who received it in São Paulo told me.
I should take this mirror only 2 weeks from now on a trip that I will take to São Paulo, and then I intend to leave it at the home of the great optician Sandro Coletti for tests at the optics.
Any clue or suggestion of a probable origin will be very welcome.
In advance thank you all.
Best Regards!
Avani.
PS: I'm using the translator!


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#7 Don W

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Posted 23 January 2020 - 09:00 PM

One thing to note. The clips should not be touching the surface of the mirror. You should be able to slide a credit card between the clip and the mirror. Same with the sides of the clip and the side of the mirror.


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#8 AvaniSoares

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Posted 23 January 2020 - 09:21 PM

One thing to note. The clips should not be touching the surface of the mirror. You should be able to slide a credit card between the clip and the mirror. Same with the sides of the clip and the side of the mirror.

As stated above, try to ignore the old construction that was very poorly done. It was totally discarded, my only interest was the mirror that looks like a beautiful pyrex plate.


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#9 TOMDEY

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Posted 23 January 2020 - 11:20 PM

Anybody want guess the thickness?  Looks thin to me, not full thickness.

 

Jon

I was wondering too, and got 1.03" sorta scaled off the image. Avani says it's 1.1" Pyrex, so that's consistent with the "sheet Pyrex" that I was speculating. I do note that sheet Pyrex is most often not fine annealed, just cut out round and hope for the best. Like the old Coulter mirrors. When my son worked at Glass Fab (materials science engineer) he configured and ran the annealing ovens... very cool to have the equipment right at hand! My recollection is that the Corning continuous flow sheet Pyrex machine got stressed trying to run the thickest stuff, so they discontinued that and only ran thinner. The sheet can be post-annealed, of course... but rather defeats the intent of ultra-cheap. I don't believe they make the cast mirror blanks anymore... anybody know? Probably some other company would be doing it cheaper now, if at all.    Tom


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#10 Don H

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Posted 24 January 2020 - 01:46 PM

I made a 12.5" f/5.9 mirror with 1" thick Pyrex about 20 years ago. The back had the very slightly rippled texture to it and I left it that way because I liked being able to see clearly through it, and thought it would look neat when it was coated and done. I was (and still am) very happy with the finished product. It gives me very tight, round stars and great detail on planets and DSOs. My edge does not seem as rough as the one you picture, and maybe your back texture is a bit more pronounced. I purchased the blank from an ATM friend from Adler Planetarium, back when they still had the optical shop. Not sure where he got it... I put the ring next to it to provide a reference for the thickness.

 

 

 

 

Attached Thumbnails

  • ring12.jpg

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#11 AvaniSoares

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Posted 28 January 2020 - 08:44 PM

I made a 12.5" f/5.9 mirror with 1" thick Pyrex about 20 years ago. The back had the very slightly rippled texture to it and I left it that way because I liked being able to see clearly through it, and thought it would look neat when it was coated and done. I was (and still am) very happy with the finished product. It gives me very tight, round stars and great detail on planets and DSOs. My edge does not seem as rough as the one you picture, and maybe your back texture is a bit more pronounced. I purchased the blank from an ATM friend from Adler Planetarium, back when they still had the optical shop. Not sure where he got it... I put the ring next to it to provide a reference for the thickness.

Hello everyone!
To keep the post up to date I would like to clarify that the mirror above is not yet in my possession.
It was delivered to a friend in São Paulo where I will pick it up next week.
Even so, I got new information, one of which concerns the thickness that is actually 1 "and not 1.1" as I had been informed before. That is, exactly the same thickness as Don H.'s mirror.
Any information or suggestion from where you could get it will be very well accepted.

Attached Thumbnails

  • c19c6015-db13-474a-8a80-b61343471450.jpg



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