I have a DPAC tester I cobbled together that seems to work well with most of my telescopes. However, my Brandon f/15, three inch refractor is a little long for my current set-up, so I tested it with a Ronchi eyepiece instead.
Looking at a bright star, the lines were ruler straight. I adjusted the eyepiece until only one line was visible and slowly moved the line to, and past, the edge of the field of view. On both sides of focus, I could not see the slightest indication of bowing, or hooks that might indicate a turned edge. My Brandon has an internal diagonal mirror, so I was actually testing the optical quality of the objective and the mirror together.
I know that a DPAC test is twice as sensitive as using a Ronchi grating alone, but wouldn't at least some curvature be visible in the Ronchi test as performed above if any significant spherical aberration were present?
I notice in DPAC that as the number of visible lines are reduced and the spacing between the lines increases, any curvature present gets worse. That is why I reduced the number of lines visible in the Ronchi eyepiece to just one during my test.
I have a lens that shows significant bowing of the lines under DPAC testing and if I mentally reduce the bowing by half, it would still seem it would be noticable.
So is a Ronchi test of some value when assessing the quality of an optic? At least for gross defects?
Edited by Steve Allison, 12 February 2020 - 02:18 AM.