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dane lambson
sage
Reged: 09/18/07
Loc: Western New Mexico (dark sky h...
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fixing astigmatism
#2885080 - 01/24/09 01:23 PM
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Hi
I just posted on Mark Hary's 20" thread about thin mirrors and it seems like a lot of concern and problem arise from astigmatism and thin mirrors, especially since there are a lot of them available. The concern is justified and I am building a large thin mirror of quartz. 20x 1-3/4 to be exact. I don't know if this concept is repeatable for everyone but it is worth a shot. Last spring while polising I had several bouts of astigmatism. I had always read that you had to go back and re-do it and I didn't want to loose that much time so I thought about what was happening with respect to KE and shadows and where the astigmatism was. The astigmatism usually showed up in one session so I thought that if I could do it right I could cure it with an equal amount of polishing. I rotated the mirror on its stand and drew on the back where the long and short focus was. I then shimmed the edges to support the two sides of long focus and polished for a short spell. Every time I cured it in one try! I don't know how well it will work for problems made during grinding. It probably won't. That is what you need a spherometer for.(good to 1/10000) I think it will work in many cases though. You might need to rotate the mirror 60 degrees several times to find the astigmatism. I will post the rest later if any one is interested. It takes a little thinking and I will have to remember how I done some of it.
Dane
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Ed Jones
Carpal Tunnel
Reged: 04/06/04
Loc: Sin-sin-atti
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Re: fixing astigmatism
[Re: dane lambson]
#2885283 - 01/24/09 03:18 PM Attachment (29 downloads)
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I think Mark used a bar spherometer almost the size of his mirror. Mahr makes an indicator that measured down to 1/2 micron that when used on such a bar spherometer should pick up astigmatism as small as 1 wave when checked at many orientations.
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dane lambson
sage
Reged: 09/18/07
Loc: Western New Mexico (dark sky h...
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Re: fixing astigmatism
[Re: Ed Jones]
#2886195 - 01/25/09 12:34 AM Attachment (34 downloads)
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First of I made a mistake in the first post. I stated that I shimmed the long axis. That is wrong. The short axis is to high and shimming it will cause it to be planed off more and bring it into the same height as the other axis.
And Yes that would work for real bad cases but what I am talking about is the real fine stages that show up late in the work. This is why you should rotate the mirror when checking it but at 45 degrees not at 90. If the long and short axis' are vertical and horizontal they will just hide and trade places at 90 degrees but will show up well at 45. If the gap in the shadow is at the top of the parabolic donut and the corresponding section at the bottom is darker the top is obvioulsy short and the bottom long. I have tried to put what I done into this diagram. I wish I had a good camera set-upm so I could get pictures but that is a project for another time. Think about this and try it if the occasion arises (hopefully not).
Dane
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