Focal Reducer Information

I need some information. I want to make a focal reducer for a SCT. Something with 50mm aperture. I understand that these can be made using an achromat as long as the correct focal length and spacing is used. Has anyone in the group made one? I have a large number of surplus optics and could proceed by trial and error, but if I could get some basic information it would save me a lot of work.

Thanks
Ray

Ray, what is the SCT, the reducing factor you'd like to get,
and the primary purpose (photo or visual)? SCT reducer has to have quite a bit of astigmatism of appropriate sign in order to keep the field nearly flat. That's because a positive lens w/o such astigmatism "content" tends to worsen the field curvature, which is already quite strong with SCTs. In other words, you need to know specifics of a lens you intend to use (radii, thicknesses, indici) to find out what effect on field curvature it will have. Given location, it's easy to figure out what f.l. will result in desirable f# reduction (counting in necessary defocus to bring it to the eyepiece field stop); it's just that you don't know what kind of aberrations in general an "anonimous" lens would induce.

Hi Ray.

I removed the doublet achromat lens of a finder 6x30 and wrapped it with black Scotch tape around the barrel of an old 1.25" prism diagonal.
I have a Nexstar 5i f/10 and I think I got a focal reduction about x 0.55 because I put a Celestron Ultima 30mm in the diagonal and I could see nearly all three stars of the Orion's belt.
I noticed some field curvature near the edge but the AFOV is really huge.
I estimate 2,2º or more AFOV with the Ultima 30mm.
If you can't reach focus remove the barrel of the diagonal and find out the correct distance to place the lens.
I'm thinking to cut about 1cm of the diagonal barrel to reduce the focus travel but I have to leave enough room for the visual back screws.


Fernando