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GSO Coma Corrector Question

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#1 vdog

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Posted 16 September 2019 - 08:58 AM

Hello everyone,

 

I have a question / concern regarding the use of my recently-acquired GSO Coma Corrector.  I'm using it with the SW 10" collapsible. The CNer from whom I purchased it graciously included a high-hat 2-1.25" adapter that enabled me to use it without the 19mm spacer (although I do have the spacer).

 

At first, this setup worked fine, but it seems like all of a sudden the backfocus is not nearly enough and I have to back the CC out of the focuser an inch or more to find the focal point.  I've tried resetting the collimation on the primary mirror from scratch three times but this made no difference.  I tried adding the spacer but it just made the problem worse.

 

Is this normal?  It seems like I have to slide it out a little bit more every time.  And doesn't backing it out increase the added magnification? 

 

Thank you for any info / advice you can offer.



#2 KerryR

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Posted 16 September 2019 - 02:00 PM

This is certainly not normal!

 

How about the strut extension-- are you certain the struts are remaining fully extended?

The only other thing I can thing of to check out is to ensure the lens cell is fully threaded on to the bottom of it's tube-- perhaps it's been gradually de-threading?


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#3 vdog

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Posted 16 September 2019 - 02:58 PM

How about the strut extension-- are you certain the struts are remaining fully extended?

You know, that might just be it.  I've been storing it with the truss extended and the scope pointed straight up.  I'll bet it slipped a bit.  I'll check it out.

 

I would not have thought of that. Thank you!


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#4 KerryR

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Posted 16 September 2019 - 03:05 PM

You're welcome! I hope that's the culprit!



#5 Jon Isaacs

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Posted 16 September 2019 - 07:15 PM

At first, this setup worked fine, but it seems like all of a sudden the backfocus is not nearly enough and I have to back the CC out of the focuser an inch or more to find the focal point.

 

 

Hopefully Kerry spotted your problem.

 

One technical point:

 

Back focus refers to inward focuser travel.  It's confusing but back focus comes from photography and means focusing behind the object which means moving the focal plane closer to the mirror.

 

I try to use inward and outward focuser travel to avoid this confusion. 

 

Jon


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#6 vdog

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Posted 17 September 2019 - 08:33 AM

Yup, that was the problem.  Works perfect now. Thanks again, KerryR!


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#7 KerryR

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Posted 18 September 2019 - 09:09 AM

Yup, that was the problem.  Works perfect now. Thanks again, KerryR!

Very cool! Glad to help, and that it was something simple!


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