This is my reply to a discussion here:
At first, sorry I don't understand French, just by Google.
On the upper link the users have a disput about the Lyot-Test and the way, I use that test. I know, my way is different to the description, I've found many years ago on the astrosurf pages: http://www.astrosurf.com/tests/contrast/contrast.htm#haut
There you can study the Lyon test in RoC and the image has the shape of a almond and not the full surface, as the Foucault test shows. So I use the Lyot test not in the matter of the astrosurf description, but my way should be a better Foucault test with more resolution as the normal Foucault test. See here:
There you see the difference Foucault test at my personally kind of Lyot test.
This shows the roughness of different Newton mirrors just qualitative, not quantitative. I do not try to calculate any results in numbers about the roughness of a surface. Therefore my test images just show the surfaces better as the Foucault test does it, no more. So my test is not comparable with the description on astrosurf's link here:
So I actually use the Lyot test to show the structure of a surface to give a general information about the manufacturer of an optical system and the differences between optics from the same producer. No more.
This should respected from David Verneth, a mirror grinder with one of the smoothest mirrors surfaces in France
see also here: http://rohr.aiax.de/hp_new/lyottest.php
Da ich doch etwas überfordert bin, auf AstroSurf die richtigen "Knöpfe" zu drücken, hat mir Rolf dankenswerterweise geholfen: