So to sum up, a Ronhci test in one color (usually green, for which wavelength the optics is typically optimized) can tell you if there are any gross errors such as a flipped lens assembly or elements, incorrect rotation or spacing of elements, incorrect collimation, tilt or figuring (TDE, coma, astig).
It can also give you a rough estimate of the wave front error for the test color (again assuming green, which is supposedly the primary figuring and configuration target of the optical design), and a low wave front error in green is one major requirement for an excellent lens.
BUT the error for green cannot in itself be used as an overall quantifier of how excellent the lens is (I never said that, btw ), because every lens design shows both longitudinal and spherical chromatism, and these have to be estimated to get an overall figure of how the total wave front is corrected as a function of focal length. The problem here is, that it’s hard to measure and quantify the combined effects of longitudinal and spherical chromatism in a Ronchi test.
We do know however that longitudinal chromatic aberration (the change in focal length with wavelength) is typically reduced in a doublet to a fraction of the FL as follows: BK7|F4 achromat 0.05%, ED APO 0.012%, Fluorite APO 0.006%. Spherochromatism (change in spherical aberration with wavelength) is typically minimized in green and the figure corrected in blue and red to counteract the residual longitudinal chromatism. We then end up with the chromatic error plots, which most of the high end suppliers provide for their telescope optics. Example from Vixen (20 micron / division):
Such data has been provided by Zeiss Jena from when Abbe designed the first astro-objectives, and also by Vixen since at least the 1980’ies. It was not available from Nihon Seiko when I bought my first refractor (a 3” Unitron 142C) back in 1965, so I had to rely on their guarantee of the optics being “diffraction limited” (which it was).
As I do not have a fully equipped optical bench, I will not attempt to reconstruct this kind of data for my telescopes. The visual experience primarily (along with an occasional star test and a simple Ronchi test) is quite adequate for my use. I did have one of my Vixen objectives professionally tested though, out of curiosity. I’ll return to that later in this thread.
Edited by AllanDystrup, 28 December 2017 - 01:25 PM.