Re: [ATM] On Axis Foucault

["David Harbour" <scarab2@cox.net>]

John,

You are perfectly corrrect.

"False Astigmatism", the so called appearance of the "monad", "Ying and
Yang", COMPLETELY DISAPPEARS for any focal length mirror when one of the
jaws of the slit is extended vertically to form the knife edge. The knife
can only see and measure aberations lying across it at right angles- thus,
if the slit is located laterally to the K. E. it will show false astigmatism
(most pronounced for short mirrors). If it is located as a vertical
extension (or rather, more conveniently, if the knife edge is located as a
vertical extension of the slit), it will be "blind" to aberations PARALLEL
to it. I was set up this way for years. I figured an f/2 sphere (12") for a
Schmidt camera, and was pleasantly surprised to note that the null (or the
appearance of any anomolous zone) was always PERFECTLY symmetrical all the
way around. No need to mess with beamsplitters, either.

I and others have been preaching this for decades; and yet we just can't
make it catch on.

Dave
----- Original Message ----- 
From: "john sherman" <atm@mail.johnspics.com>
To: <atm@atmlist.net>
Sent: Tuesday, June 14, 2005 3:35 PM
Subject: Re: [ATM] On Axis Foucault


>
> Alfredo,
>
> >I am trying to make an on axis focault tester for
> >testing F 2 and faster optics, and would like some
> >ideas on how to achieve on axis alignment of the slit
> >and knife edge. I was tempted to use a beamsplitter, but I
> >am unsure because of the spherical aberation it might induce.
>
> I would also hesitate to use a beam splitter, but I'm not an expert.
>
> I think the main problem using a knife edge tester with fast optics is the
distance separating the source and the knife causing apparent astigmatism.
But you're in luck! The knife and slit are one-dimensional. Put them both in
the same dimension and the error diminishes greatly. Like when you use the
same knife for the slit and for the analyzer.
>
> In the good axis, the one you can test, both slit and knife will be
"on-axis".
>
> Good luck,
>
> John
>
>
>
>
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>


_______________________________________________
ATM mailing list http://www.atmlist.net/