Re: [ATM] On Axis Foucault (try this instead)

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

Alfredo,

"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 one of the jaws 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 cubes or pellicles,
either.

I and others (most notably John Sherman) have been preaching this for
decades; and yet we just can't
make it catch on.

Dave

----- Original Message ----- 
From: <aneves@snowsoft.biz>
To: <atm@atmlist.net>
Sent: Tuesday, June 14, 2005 9:29 AM
Subject: [ATM] On Axis Foucault




Hi All,

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. Any
Ideas?

Regards,

Alfredo


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