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figureing fast mirror for a 1.0x coma corrector

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#1 Matt Wallin

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Posted 25 June 2013 - 03:35 PM

I doing some basic computations and thinking about the ins and outs of a very fast (f/3.5 or so) mirror. Of course, with such a mirror, use of a coma corrector is a given. Coma correctors such as the TV ParaCorr act to increase the effective focal lenth of the scope they are used on by 1.15x I have read that the GSO CC is ~1.10x. The added focal lenth is not neccessarily welcome in a richest field scope. It is my understanding that this extender effect is necessary to correct for some spherical abberation that is introduced in the coma correction. My question to all is first: does a 1.0x coma corrector add under or overcorrection to the SA? and second: has anyone made mirrors corrected to work as a package with a 1.0x coma corrector and is there a reason this would not be desirable?

#2 GlennLeDrew

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Posted 25 June 2013 - 05:55 PM

For an RFT, which by definition is used at larger exit pupils, the SA will not likely be resolved.

Have you considered an even faster primary to be used with the TV Parracor II?

What aperture are you considering? Up to moderate apertures it shouldn't be necessary to go as fast as f/3.5.

#3 EyeSage

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Posted 26 June 2013 - 09:45 PM

The Baader MPCC does not change f.l. but I understand it introduces a small amount of undercorrection. If you wanted to make this a permanent part of the optics I imagine you could just overcorrect the primary to compensate.

#4 mark cowan

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Posted 26 June 2013 - 10:03 PM

It is my understanding that this extender effect is necessary to correct for some spherical abberation that is introduced in the coma correction.


Can't say for sure, but it's more likely it's to allow (Paracorr) to fit in a standard focusing scope and still provide for EPs to reach focus downstream. It also makes it slightly easier on those EPs.

Best,
Mark

#5 cjc

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Posted 27 June 2013 - 02:40 AM

The Baader MPCC does not change f.l. but I understand it introduces a small amount of undercorrection...

The latest Mark III model should be free of SA, here.

#6 Matt Wallin

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Posted 27 June 2013 - 01:28 PM

Thanks for the link Chris, I wasn't aware of a new model being out, it seems they changed the focal ratio it is optimized for. I would be curious to know if there are any reports of SA being introduced. It was my investigation of the MPCC with its lack of effect on focal length that turned up the reports of SA being noted, but some users didn't see that issue, perhaps they had some overcorrection that was actually being improved by the MPCC? Maybe it would be enough to *slightly* bias a mirror towards overcorrection if one is planning to use an MPCC such that any introduced undercorrection would be beneficial, but that the mirror was not appricably misfigured without it.

#7 MitchAlsup

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Posted 27 June 2013 - 03:51 PM

The latest Mark III model should be free of SA, here.


Does anyone know if the Paracorr tunable top is threaded with M48 threads?






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