2800 = b x D/d
where d = secondary diameter, D = scope aperture at the corrector and b is the backfocus from the vertex of the secondary to the focal plane.
The aperture stop for SCTs is the corrector aperture not the primary mirror diameter - the latter is slightly oversized in SCTs to reduce the effects of vignetting off-axis across the field of view. Likewise the secondary mirror is usually oversized by 1-2 mm than simple geometry would suggest.
Mirror spacing is closer to 410 than 445 mm.
The complete solution requires accurate measurements of the maradi of both mirrors.
By the way your figures are so rough the 2 decimal places are a bit meaningless...
Excellent information jim, david and luxo. I feel like the fog is clearing a bit.
Yes, wrong precision for sure. That's just Autocad since I had not adjusted the precision from the previous drawing. It's good for mocking things up though. And it's easy to modify.
Corrector aperture "D" is 276mm and the secondary "d" is 78mm. But I have no idea what the "miradi" of a mirror is . . lol.
2800 = b x D/d or 2800 = b x 276/78 and b = 791
So we've got the "b" based on the actual measurement of "D" and "d". I know that the mirror travel is 37 turns lock-to-lock which corresponds to about 28mm. And I can more accurately measure the distance between the primary and secondary with the focuser all the way forward or back.
Also, I've done the primary mirror focus spring shift modification so my lock-to-lock travel is limited rearward about 4-5mm. If I measure this distance it should be with the primary all the way forward.
So here is a 2nd draft of the schematic drawing. And one of things I had hoped to figure out with this exercise was the placement of the UAGII telecompressor so that its 35mm aperture matches up with the C11 light cone.
The two formulas that seem to be applicable are:
b = (560 - x*)(5) and 2800 = b(D/d)
Where x* is the distance between the primary and secondary.
Using both we end up with an x* = 402mm
Using an online Cassegrain calculator (I have no idea what I'm doing):
Primary mirror diameter: 279.4
Enter field: 1(deg)
Primary mirror focal length: 560
System focal length: 2800
Back focal length: 390
Primary mirror diameter 279.4000 mm
Primary mirror focal length 560.0000 mm
System focal length 2800.0000 mm
Back focal length 390.0000 mm
Distance secondary from system focus 791.6666 mm
Distance secondary from prime focus 158.3333 mm
Distance secondary from primary 401.6666 mm
Radius of curvature of secondary mirror 395.8333 mm
Primary mirror conic constant -1.0000
Secondary mirror conic constant -2.2500
Extension factor 5.0000
The bold numbers seem to be pretty darn close.
Edited by MarMax, 04 March 2021 - 06:25 PM.