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Edmund Scientific 12 1/2" F6 Mirror

Started by duo-glide98, 01/01/2012 09:48AM
Posted 01/01/2012 09:48AM Opening Post
Hi all, this is my first post in 12 years. I'm an old atm who built dobs back in the 90's. I left the pursuit of this and sold everything in 2000. I'm now back and starting over. A guy on e-bay had this mirror for sale and I made an offer of $200 bucks and he took it. The mirror arrived last week and it is brand new, perfect coating. The guy had it listed as a spherical mirror but his company is an industrial surplus outfit and I know he did not know what he had. I'm of the opinion that this is a parabolic mirror. If I remember faster mirrors were usually parabolic. I don't remember how to test this mirror, or can I find any info on this. It's marked "12 1/2 F6 FL 75 inches". The tag on the rear also states "pyrex first surface mirror". I've built a couple of solid oak (not plywood) dobs that were award winners at Stellafane. My 12 1/2" F6.4 won first place for craftsmanship back in'96. That same year my late partner won first place for optics on his 16" f6 dob. Before I get to it I'd like to know if anyone has any imput on this glass. Happy New Year.

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duo-glide98's attachment for post 54687

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Posted 01/01/2012 10:38AM #1
At the f-ratio listed on your mirror, it would have to be parabolic to produce a decent image in a dob. You could send it to a couple of opticians to have it tested:
or, if it needs a recoat:
I'd also try a google search for an amateur astronomy club in your area. Some members may have access to testing equipment. I'd also try contacting a local university with a physics/astronomy department as they may have testing equipment.
Posted 01/01/2012 06:42PM #2
John, you could build a quick and cheap test stand and do a star test on it (real or artificial). You could also get a ronchi grating and see right away if it's a sphere or parabola. Since you are apparently not far from Stellafane, you could just run it up there to test. BTW, nice score!
Posted 01/09/2012 11:13AM #3
I checked my 1962 Edmund Scientific catalog and the only 12.5-inch mirror was parabolic. In my 1975 catalog the same is true -- however, the focal lengths are 100-inch. But the only spherical mirrors are 4.24-inch and 6-inch diameters.

Terry Atwood
Posted 01/09/2012 11:33AM #4
If you are ever in California, give me a call, and we'll test it on the Focault tester.

Posted 01/25/2012 10:01PM #5
I saw that mirror on ebay and said to myself, "Too bad it's spherical!" Good catch!!!


In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm