I was asking about M25x0.75 threaded tubes because I have an ancient University Optics 8x50mm right angle finderscope that lost its crosshairs at some point. (It was part of an ungainly 17.5" Dobsonian originally constructed by several Boise families as a shared astronomy thing for their kids. The scale of the project suggests Jack Daniels. They eventually tired of moving a 300 pound monstrosity, and gave it to the Boise Astronomical Society. I bought it at auction for $600--a bargain beyond belief. Some kid or Jack Daniels-influenced adult may have touched and broken it.)
So, replace the 25mm eyepiece with a new crosshair eyepiece. But it is .965" barrel diameter; the old Japanese telescope/microscope standard. The only one that I could find was 18mm focal length at Telescope Warehouse, who seem to have all the obscure parts. Instead of 8x50 it would be 11x50--still a very useful finder. But I forgot that 18mm focal length means it needs to be 7mm closer to the objective and thus deeper in the diagonal's eyepiece tube (which is threaded into the diagonal: M25x0.75). Not enough inward room for that to get it focused. So I measured carefully and concluded 4mm off the the eyepiece tube would put the 18mm at the 25mm's focal point. (The 25mm was actually 3mm out of the top of the tube.)
Removed 4mm (.157") from the top of the eyepiece tube, and the 18mm eyepiece now focuses perfectly!
Everyone needs tools like this at home. A machine tool tax credit and machine tool stamps are needed! :-)
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