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Full frame astigmatism with 8" RC

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#1 avarakin

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Posted 16 October 2019 - 10:45 PM

I recently noticed that my 8" GSO RC has an issue, similar to astigmatism: all stars extend vertically on one side of focus and then extend horisontally on another side. Has anyone seen this with their scope? Is this collimation issue or maybe mirror cell issue? I know that 10" GSO RCs have very poor design of mirror cell, where they have only 3 contact points. Does 8" have the same issue?

 

Alex



#2 jhayes_tucson

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Posted 16 October 2019 - 10:58 PM

That is the definition of an astigmatic error.  Astigmatism varies with the field and anamorphic error is independent of field.  You either have a problem with your components (being either made wrong or warped) or the system is misaligned.  I'd start by checking the optical alignment.

 

John



#3 Todd N

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Posted 16 October 2019 - 11:12 PM

My understanding is that astigmatism is a dominant aberration for RCs along with field curvature so, maybe nothing is wrong. I can witness this phenomenon with my Orion ST80 achromat refractor and a 9mm eyepiece. You're bringing into focus the tangential and sagittal planes respectively. I believe proper focus should be achieved between the two.



#4 avarakin

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Posted 17 October 2019 - 11:28 PM

Todd, I have a 10" RC too and never noticed this issue there. 

John, Any thoughts on which kind of misalignment that could be? It does not seem to fit primary or secondary misalignment patterns, descibed in DSI collimation guide. 

I completed mirror cell mod for my 10" and it did fix triangular stars, thinking of doing the same for 8"

 

Alex




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