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