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Optical Craftsmen 6-in OTA with "Mystery Mirror"

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#1 desert_woodworker

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Posted 16 April 2018 - 09:59 PM

Hi, All.  I purchased on eBay a 6-inch Optical Craftsmen fiberglass-tube OTA recently from a seller in Pennsylvania, and found an oddity with the primary mirror.

 

I don't believe it's original.  Its focal length is too long.  Under test at home, focus came almost to the very top of the maximally racked-out focus tube.  Fortunately, the primary mirror cell was not attached at the very bottom of the tube, but about an inch and a half of tube length remained, so I re-drilled the tube for the mirror cell and moved the mirror "down" one inch.  There's only half an inch of tube remaining, so this is as far as I'll reposition the cell downwards.

 

On measuring the fl of the primary, I find 53 3/4 inches!  This does not seem to be a standard commercial focal length for a 6-inch mirror, and comes to an f-ratio of f/8.96 (I'll call that f/9.0).

 

The mirror has a Beral coating, and has Clausing's orange sticker on the mirror back saying so.  The coating was in pristine looking shape, but I washed it and finished with isopropyl alcohol, and find that this removed a bit of "haze of age".  It looks like a freshly-deposited coating now.  I do not know how old the coating is.

 

As I say, I don't believe the mirror is original to the tube, and so I don't believe the mirror originated at Optical Craftsmen.  There's a detail about the mirror that I hope someone here with a long memory or inside information can help me with:

 

Scribed near an edge of the mirror blank back surface are the characters in upper-case writing: "SFB  CO" (pls. see image below).

 

Does anyone have information about what that might indicate?

 

By the way, I star-tested the optics and find the mirror to be excellent, but I have not Foucault tested it.  Diffraction rings on both sides of focus appear identical.  At focus and with a 6 mm eyepiece, the Airy disk is sharp-edged, tiny, and circular, and in good seeing, the host of diffraction rings are beautiful, and many.  It looks like a good mirror.  I am very pleased.  But I hope someone may have some information on what the letters scribed on the back of the mirror may mean.  Thanks for having a look.

 

--Joe

 

mirror_6in_inscribed.jpg

 

 


Edited by desert_woodworker, 16 April 2018 - 10:02 PM.

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#2 tim53

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Posted 16 April 2018 - 10:47 PM

Pretty much all of the 4 OC mirrors I have, have that same "sandy" texture on the back.  I don't know what the SFB CO stands for.  Only 2 of my OC mirrors have anything engraved on them.  Fortunately, indicating they're optical craftsmen and including the date.  The two 8" f/6 mirrors don't have anything on them.  One, I bought new myself in 1972.

 

-Tim.



#3 desert_woodworker

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Posted 16 April 2018 - 11:14 PM

Thank you, Tim.  Yes, the mirror back is a bit unusual in having that "sandy" texture, and yet smoothed, besides, almost as if it's "wet".  I've not seen that on a disk, before.

 

Hmm, so the lack of opticians' inscriptions on the disk by workers at OC doesn't immediately discount the mirror as having been made by OC, from the evidence of your buying one of the two 8-in f/6 mirrors directly from them, new.

 

I'd like to think that this 6-inch is original and made by someone at OC, but I think the focal length casts strong doubt on that: have they produced 6-inch f/9.0 mirrors, to your knowledge?  If so, in this case, I think they would have had to have put it in a longer tube than mine came with.

 

By the way, did OC use Clausing as its coater?  And did they routinely order the Beral coating from him?

 

I'm speculating, now, but maybe an owner of this mirror -- whether made by OC or not -- had the mirror re-coated, and by Clausing with his Beral coating.  AND... maybe the owner had initials "SFB", and lived, moreover, in Colorado ("CO").  Before sending the mirror for recoating, then, maybe Mr. or Ms. "SFB", of "CO", scribed initials and state of residence on the substrate as a bit of insurance against a mix-up at the coating plant.  Shear speculation... .

 

But maybe someone else here knows of an "SFB  CO" of some identity.  And maybe someone knows if OC ever supplied 6-inch f/9.0 (!) prescription paraboloids in too-short fiberglass tubes!  I'd be grateful for information.

 

Image shows tail-end of tube, with coater's sticker on the mirror back.  This image was supplied by the eBay seller.

 

Thanks again, Tim!

 

--Joe

 

Attached Thumbnails

  • OC_6_OTA_tail.jpg



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