ATM 6" Mirror/Binoscope

Currently, I have a polished but not parabolized 6″ f/5 plate mirror made. I may get to test it later this year but I need to work out the bugs of the Foucault tester.

Grinding it went well as can be expected without any major problems, with no pits showing up after polishing. I used a tile tool, rather than the more standard glass tool as I wanted to use this small piece of glass for practice.

In other news, I will go ahead with the roughing in of the two pieces for a binoscope. I plan on using two tools, working each one for a set amount of time, switching, then continuing on. This should give me a pair of blanks with the same ROC, provided I switch them often enough. The blanks are both pyrex this time from Willman Bell and being tougher than plate, the ROC will only move with a lot of effort.

I was going through old S & T magazines (a great resource BTW, a much better magazine then than it is now) and found an article [Dec 1979] by Sinnott on a 6″ binoscope. I had seen it before and was going to file it but its unusual optics caught my eye.

It featured an unobstructed light-path that allows the pencil of light to exit the tube through the side into prisms that rotate and invert the image. With 3 additional reflections (all internal, less light loss) you end up with a normal view like a true binocular.

The only problem is that while you grind them like a 6″ f/6, you need to parabolize them like a 12″ f/3. This means each mirror has to be deepened in an offset manner. How far offset depends on how much you want the light pencil from center. This also means a very difficult Foucault Test.


I have an idea what to do with this mirror: Build a scope out of it in a Dob base, and leave it in a case at the Elvin Lakes shelter in Garabladi Park

Lots of people with kids go up there and what better place to enjoy the wonders of the night sky than a dark place like this?