If the ronchi lines show a fair amount of curvature then the mirror is not parabolic but hyberbolic or an oblate spheroid and maybe strongly so. A 6" f/8 parabola would show only slightly curving Ronchi lines and if one wasn't careful could mistake them for straight. You said that the lines were convex in curvature. Were you inside or outside of focus when they showed this pattern since this would determine if the mirror was an oblated spheroid vs a ellipse/parabola or hyberbola ?
Correct. In other words, I should really do a Foucalt test, and measure the radius of curvature of the radial distances ("zones"), to determine the conic section most appropriate.
If the other 6" mirror looks to be a sphere you can use it to test the corrector by placing it in front of the mirror while your testing the mirror. If it is an optical window with no optical power the ronchi lines will stay unchanged.
Well, Dave, thank you for your gentle prodding to "get the numbers" on all the mirrors/correctors.
This required resurrecting the 6" Foucalt tester my dad made for me when we were doing a telescope (http://jolitz.telemu.../benjamin/scope
). I've carried it along with me since, and it was completely rust encrusted. Took a lot of work and PB Blaster.
The result - both 6" f/4 and f/8 are on the numbers (24.5" and 50.25") exactly as parabolas. Very different results from my sloppy ronchi tests. Next step is to confirm by star testing with a diagonal mirror.
As to the "correctors", putting them in front of the mirror(s) under test (again, not spheres), revealed different results.
The one that had been mounted is striated - a bright line slightly off center is prominent, there are other lines more ghostly, a wavery pattern overall, and a "wedge" on one side of the line. Perhaps the symmetrical "D" back to back of slightly different diameters, is now explained, as with it the selective fogging?
The other one is worse - larger blotchy-ness, roughly in irregular highly off centered rings like a weak lens.
There is no way these optics work this way, with these fittings.
Oh, and in the center of the f/4, there's a ... hole. Right where you'd either cut a physical one, or have a secondary shadow. Almost like a "drill here" mark ...
Here's the tester in action with the f/4 mirror: