Re: [ATM] Slit and source arrangment part 3.

["Bob May" <rmay@nethere.com>]

Actually, the only way to eliminate the false astigmatism
alltogether is to use a beam splitter and put the source and KE
at the same optical axis.
Your idea will only complicate the alignment of the test to a
point where it becomes very difficult to do.  In addition, you
will still have an offset between the source and destination and
will have false astig. in that direction.  The error occurs
because you're not testing at the optical axis but rather a close
to spherical elipsoid with the two focii being the two places
where the light is concetrated at the tester.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Arjan te Marvelde (hetnet) <arjan.te.marvelde@hetnet.nl>
To: <atm@atmlist.net>
Sent: Monday, April 28, 2008 1:38 AM
Subject: Re: [ATM] Slit and source arrangment part 3.


> The issue is that evaluation with a KE is neccessarily breaking
the
> rotational symmetry of the (intended) conic: it really only has
meaning in
> the direction perpendicular to the knife edge (i.e. usually
horizontal).
> This is why offset in this direction will influence the
measurements and
> offset along the KE won't.
> This holds for the horizontal diameter (equator) in theory, but
in most
> practical cases also for areas nearby. I can imagine though,
that an error
> may be introduced when using a Couder mask on very fast
mirrors, with
> considerable vertical offset between source and KE.
>
> If you want to make a measurent without breaking the rotational
symmetry of
> the conic, the lightsource must be a point and the KE be
replaced with a dot
> of a size similar to the point source. This could be
constructed with a
> glass reticle or photonegative or so, but it will be very hard
to achieve
> zero offset between source and dot. Maybe an optic fiber could
serve as a
> source here.
> Theoretically you would see a ring on the zone under
evaluation.
>
> - Arjan
>
> > Actually, you are wrong here.
> > A slitless tester will not show errors in the direction of
the
> > line between the source and reading edge.  Typically this is
in
> > the vertical direction.  It will show accurately errors in
the
> > horizontal direction tho.  Putting the source and the KE
> > horizontally won't show any problems in the vertical
direction
> > but will show all of the error in the horizontal direction
and
> > those, assuming a vertical alignment of the KE, will
definitely
> > be in error.
> > Only the alignment where the distance between the source and
the
> > KE from the mirror will show a proper null for a figure of
> > revolution properly.
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

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