ATM Astigmatism

["John Sherman" <shermj@netzero.net>]

Dear List,


Recently Jerry had problems with astigmatism, and as a result there has been
some misleading information going around, which I will address. Here are two
facts, which I have determined from much experimentation:

1)  Astigmatism cannot be seen with the Foucault test or the Ronchi test,
         unless it is horribly severe.

2)  Astigmatism can easily be seen with the Spot test.

Jerry described using a Ronchi grating and having straight lines in a few
orientations. He was told that means he has a good sphere. However, this is
not a good enough test to see astigmatism. The problem is that the Ronchi
grating is a one dimensional test and astigmatism is a two dimensional
defect. At each orientation he was performing a 1D test. Some might say that
since the Ronchi covers the entire mirror that it is 2D, but that is not the
case. If you have very severe astigmatism and you rotate the Ronchi grating
you might see a faint ying-yang pattern. Maybe. If you're lucky. I have set
up very severe astigmatism and I have never seen this pattern.  Yes, it is
possible to detect (as opposed to see)  astigmatism with the Ronchi grating
by rotating it to different orientations. You will need many orientations
(not just a few) say 0, 20, 40, 60, 80, 100, 120, 140 and 160 degrees. And
here is how you do it:

1)  When you rotate the grating you must make sure it is EXACTLY the same
distance from the mirror at all times,

---------AND---------

2)  At each orientation you must count the number of lines and spaces and
fractions thereof. Astigmatism will manifest itself as a variation in the
number.

If you cannot satisfy condition 1) then don't even bother with the test 2).

And here is how you can prove to yourself that I am correct. Make your own
astigmatism!! Last night I performed yet another experiment. This time I
used an 8" f/8 128" roc paraboloid mirror. This is an excellent Richard
Fagin mirror known to not have astigmatism by using the eyepiece
Suiter-style star test. Using a pinhole light source and a Spot tester,  I
set up with the spot at the 70% zone. When there is less than one inch
lateral distance between source and spot the ring shaped shadow of the spot
was round, showing no detectable astigmatism in the mirror or the setup. At
two inches lateral separation the ring is very elliptical shaped. This is
the visual manifestation of simple astigmatism, you can see it first hand.
At three inches the astigmatism is so severe that an image was not
obtainable with the 2D Spot tester. Yet with the Ronchi tester I still saw
normal lines at each orientation. Since this is a paraboloid mirror, the
lines are slightly curved to match the curvature of the mirror (with a
spherical mirror I would see perfectly straight lines). Even at six inches
lateral separation between the light source and the Ronchi grating the
Ronchi still showed normal lines at each orientation. No matter how I turned
the gratings (I have two, 100 lpi & 150 lpi), astigmatism was not visible.
Using a slit light source instead of the pinhole might make some small
difference, but the slit has to be aligned with the grating. If they are not
aligned or if something is not square to the optical axis then you might see
a ying-yang effect because of that.

Even though the Ronchi tester makes measurements at every point on the
mirror, it is not a 2D test. Why do so many people insist on using a Ronchi?
It is pre-Hitler technology, found in dusty old books. A month ago I made a
post with this same subject line. You could refer to it for more
information.


Thank you,

John