Re: ATM Astigmatism

[Marty.T@t-online.de (Martin Trittelvitz)]

Hallo,
You are right, with a paraboloideal mirror it is difficult to impossible to see astigmatismus with Ronchi. But Jerry did
not have a paraboloid, he tested a sphere. Thats a completely other thing. 
For a sphere a Ronchi test is a 0-test. The lines have to be strait. And when you have a small amount of astigmatismus
the lines appear bent round to one side, not like ying yang. It is necessary to turn the grating to 45° and 90° to avoid
to test parallel or perpendicular to the direction of the astigmatism. 
I often see ying yang without any distortion of the mirror, it appears when the slit is not parallel to the grating. 
Thanks Marty


John Sherman schrieb:
> 
> 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

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