From: "Richard Schwartz" 
To: "ATM List" 
Reply-To: "Richard Schwartz" 


Astigmatism is something that most ATM literature overlooks in mirror
testing.   At the local college last Saturday I visited the mirror
workshop,
and nobody had a clue as to how to test for astigmatism.   So I guess that
means there is no astigmatism.    According to authoritative publications,
in the unlikely event that you have astigmatism, you can fix it by hurling
the mirror against a hydrant and walking away.

Astigmatism testing requires a pinhole; a slit or slitless tester will not
do.  The appearance of astigmatism depends on the figure of your mirror. If
your mirror is spherical and the knife edge is near the center of
curvature, the shadow edge will be straight and tilted with respect to the
knife edge.   But if the mirror is corrected with a good figure, the shadow
edge will assume a monad form like the yin-yang on the Korean flag.   If the
mirror has a lot of raised zones and valleys, the shadow edge will probably
be incomprehensible.

If your figure is good, you can measure astigmatism.   Use a mask for the
edge zone tilted 45 degrees and take a reading on your foucault tester.
Then, without disturbing the tester or the mirror in its test stand, rotate
the mask to -45 degrees (perpendicular to the first position), and take a
second reading.   The difference between those two readings is your
astigmatism.   Conversion of this difference in readings is left as an
exercise for the student.

What you have measured is one component of astigmatism, the "twist".   This
is the twist of the top half of the mirror with respect to the bottom half.
There is another component that I call "potatoe chip" that this
test cannot detect.   That is where the top and bottom edges of the mirror
are bent
forward, and the left and right edges are bent back.   However, if you
rotate the mirror 45 degrees in its test stand, the potatoe chip that was
undetectable is converted into twist and can be easily measured.

Yes, there are higher orders of astigmatism, such as "trefoil",
but those are seldom a problem with amateur mirrors.

. . . Richrad
----- Original Message -----
From: "Douglas Braun" 
To: "John Swenson" 
Cc: 
Sent: Wednesday, December 18, 2002 8:36 AM Subject: Re: ATM Re: Aplanats II
via troublesome Juno X-mailer


>
> Hello,
>
> Perhaps this is sort of a naive question..
>
> By "astigmatism", do we really mean "off-axis
astigmatism"?  I do not see
> how any properly built and collimated scope could have
> astigmatism in the center of the field (except for off-axis designs).
>
> Doug Braun
>
> At 09:55 PM 12/17/02 -0800, John Swenson wrote:
>
> >The Lurie Houghton design has essentially zero coma, zero spherical
aberation and an almost perfectly flat field, it does have a little
astigmatism. The basic geometry is a newtonian with spherical primary and a
corrector right before the secondary (the secondary is usually attached to
the corrector). The corrector consists of two weak lenses that are NOT
achromats, the same glass type can be used for both lenses.
>
>

--- BBBS/NT v4.00 MP