> Page 29 of Richard Suiter's book shows an example of an undercorrected
> start test, more examples are on pages 182, 184, one 186, pages 183 and
> 185 show two undercorrected and one overcorrected set of images.
> In all of these undercorreced pictures:
> the outside ring is brighter inside of focus,
> the center is brighter outside of focus,
> the rings are more distinct inside of focus.
> It does seem kind of counter-intuitive.
I guess this is another proof that star testing is not the simplest way of rating the miror (IF you want to know what, where and how much). Unnecessary mental exercises aside, central obstruction and atmospheric effects will much more complicate extra-focal star tests than good ol' Ronchi. Moreover, complex zonal errors are virtually impossible to "reverse engineer" from a set of extrafocal images, while such thing is plainly visible in a Ronchi test. I don't think that investing in a simple transparency photocopied or laser-printed Ronchi grating is a big $. Star test _is_ an ultimate test for a telescope optics. It _is_ much more sensitive than anything else amateur has access to. But in judging optics it is just not as simple to interpret, nor as easy to quantify as Fucault or Ronchi tests. Well, at least for me, I should add.
Bratislav