Author Topic: Optical flat mirror for double pass Ronchi test from ebay?  (Read 190 times)

Fred Lafever

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Re: Optical flat mirror for double pass Ronchi test from ebay?
« Reply #15 on: January 31, 2018, 08:00:18 AM »
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To test for astigmatism as an example my understanding is that the lines of a Ronchi test will actually flip 90 degrees

No, the presence of astigmatism is noted if the tilt of the Ronchi bands changes ever so slightly when going from inside the focus to outside the focus, and vice versa. Any change in tilt is due to astigmatism and the tilt is proportional to the amount of astigmatism, i.e. the more astigmatism the more tilt.

Mladen

In what way is it tilting? Are you saying that if the lines are going from 12 to 6 o'clock that they would change to maybe 1 to 7 or 11 to 5? That kind of tilt?

Yes.

Jim Snyder

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Re: Optical flat mirror for double pass Ronchi test from ebay?
« Reply #16 on: February 08, 2018, 06:51:59 PM »
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I have worked on number SCT over the years. I'm refiguring a Schimdt corrector for Dynamax 8 right now. I have thread in the Classic Telescope Forum about it. I can tell you, you'll see two types of zones in the corrector when testing via double pass, asymmetrical and symmetrical. Both of which you will not correct by rotating the corrector. When people rotate the corrector and say the image improves what they are doing is correcting for misalignment in the system. You can put the corrector in any position you want if the optics are mechanically centered and collimated you'll get the same results.
 If you want to test the corrector by itself, you can place it directly in front of spherical mirror and do a Ronchi test. You should see envyingly bowing lines like what one sees when testing a parabola.

        - Dave

What if both the corrector and the mirrors have astigmatism or zones?

Rasheed Grayson

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Re: Optical flat mirror for double pass Ronchi test from ebay?
« Reply #17 on: February 08, 2018, 07:34:03 PM »
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I have worked on number SCT over the years. I'm refiguring a Schimdt corrector for Dynamax 8 right now. I have thread in the Classic Telescope Forum about it. I can tell you, you'll see two types of zones in the corrector when testing via double pass, asymmetrical and symmetrical. Both of which you will not correct by rotating the corrector. When people rotate the corrector and say the image improves what they are doing is correcting for misalignment in the system. You can put the corrector in any position you want if the optics are mechanically centered and collimated you'll get the same results.
 If you want to test the corrector by itself, you can place it directly in front of spherical mirror and do a Ronchi test. You should see envyingly bowing lines like what one sees when testing a parabola.

        - Dave

What if both the corrector and the mirrors have astigmatism or zones?
If the optics have zones and astgimatism , then you need to refigure those surfaces. The odds of having two surfaces in which zones and/or astigmatism in one can cancel or even partially cancel the other are very low. In a typical commerical SCT you have 4 optical surfaces. It also means that the optics are being made to very poor quality to start if you have these problems and your trying cancel one or more errors by using defects in another surface. When it comes to zones they are usually radially symmetrical for example a turned edge. So rotating that element doesn't cause it to move position in the light path.
 When you actually test the optics via double pass autocollimation you'll see the problems and see what the actual effect is of rotating elements.

      - Dave