On Mon, 1 May 1995, Bratislav Curcic wrote:
> Date: Mon, 1 May 95 11:48:50 EST
> From: Bratislav Curcic <epabcc@epa.ericsson.se>
> To: atm@best.com
> Subject: Re: Star Test Question
>
> Chuck Grant wrote :
>
> > 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
>