Any equations are assuming you have exact data on the mirrors, which you don't. You don't have exact Radius of Curvature data for either mirror, just rough design points.
I just ran through this with a set of old 10.25" DK optics and built an adjustable truss for the front ring and secondary holder using telescoping tubing from www.dxengineering.com with hose clamps on the slotted ends, and assembly with star nuts (tube connecting nuts) from www.mcmaster.com. I was then able to get everything dialed in, and actually the open tube is great for cool down!
This is probably the best way to go, and the woodwork does not need to be pretty to get spacing down.
John MacDonough said:
Good morning all,
I've about "trial and errored" myself to death, so I'm asking for some help. I purchased a classical Cassegrain mirror set hoping to assemble a telescope. What I'm told by the person who sold the mirrors to me is this:
Primary: 200 mm diameter, f/4
Secondary: M = 5
EFL: 4000 mm, f/20
My problem, assuming the above is correct, is mirror spacing. I was told to target between 628- 630 mm for a back focus of either 275 mm or 210 mm. That hasn't worked out. For one thing, with the mirror cell, focuser and diagonal I have, I need approximately 339 mm of back focus. I've also consulted the VanVenrooij book and calculated a mirror separation of 614 mm for the back focus distance I want. I've currently got the scope assembled with 611 mm separation and have experimented with separations all the way up to 635 mm without being able to achieve focus.
Can someone help me out?