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GSP RC6 Star Issue

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#1 Jhillphotos

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Posted 12 June 2019 - 11:15 PM

Hello!

 

So, I recently received the GSO RC6 that I purchased and decided to take it out to my front yard to do some test shots. Upon loading the images onto my computer the next morning, I saw a few interesting things happening that I am not sure how to identify. I saw field curvature like behavior (streaked stars) in the top left and right corners (as well as some artifacts coming off bright stars). I also saw what looked like out of focus stars in the bottom left, and the bottom right was nearly perfect. After re-collimating the telescope myself two nights later, I found that the exact same issue was happening, just slightly less. 

 

Does anyone know what this could be and how it could be fixed? (preferably not a field flattener or focal reducer – I am on a tight budget lol)

I have attached single image frames of the problem on M13 and the Trifid Nebula. 

 

I also talked to someone at Agena AstroProducts who gave me some great insight but said that it couldn't be the optics and that it must have something to do with my camera or focuser not being aligned properly. Even after carefully triple checking that my camera and guide scope/its camera were all aligned properly and securely, the issue still appeared. I would really appreciate some help as I don't want to have to return it and go telescope hunting again tongue2.gif

 

https://flic.kr/p/2gdijEc

 

https://flic.kr/p/2gdi6wu

 

Also, I am using a Nikon D5500 with a T ring and the 1.25" adapter with a Baader moon and skyglow filter attached to it.

Here are the two full versions of the images above. I would post the version I stacked in pixinsight, but my polar alignment was off so there was some image shift over the night and stacking it made the corners go crazy hahaha (so it is probably not accurate to the issue).

 

https://flic.kr/p/2gduJu9

 

https://flic.kr/p/2gdugao


Edited by Jhillphotos, 13 June 2019 - 08:47 AM.


#2 RJF-Astro

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Posted 19 June 2019 - 02:49 PM

Hi Jonathan, welcome to CN! This does not look like field curvature to me. More likely it is a collimation issue, perhaps astigmatism from a misaligned secondary. Issues with the primary result in on-axis coma, issues with the secondary result in off-axis astigmatism. 

 

What does your collimation procedure looks like? Did you follow the DSI method? See this document. The thing with the (GSO) RC scopes is that they can appear collimated through a cheshire, but in reality they are not. I collimated mine with the DSI method and the stars look really tight now, but when I look through a cheshire everyting appears misaligned.


Edited by RJF-Astro, 19 June 2019 - 02:50 PM.

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#3 Jhillphotos

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Posted 20 June 2019 - 03:59 PM

Thank you for the kind welcome and for the great insight! 

 

When I collimated my telescope, I used the out of focus star method for a roughly accurate collimation. It did the job for a bit, but there was still that star issue in the same corners. I then performed a series of tests that a great employee at Agena AstroProducts told me to try and the tests concluded with the error being probably within the focuser. I then ordered a linear bearing focuser from GSO and I am hopefully going to try it out soon and see if it corrects the errors I was seeing (weather permitting obviously hahahaha).

 

Thank you you so much for the DSI recommendation. I'll absolutely give that a look and use it next time I do collimation. 

 

I checked out your Astrobin btw and you have some fantastic images! Thanks again for the help. 



#4 RJF-Astro

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Posted 21 June 2019 - 02:56 AM

Thanks! I hope it works. The linear bearing focuser looks nice and it is always good to prevent flex with this scope. Like the DSI-method says, the RC is not like an SCT. The two hyperbolic mirrors react differently to collimation problems.  


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#5 B. Hebert

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Posted 18 August 2019 - 08:11 PM

Has there been any more progress on this issue?

 

I have a 6" RC with a similar problem; sharp focus in the upper right corner and poor focus in the lower left.

 

I suspect it is flex from the focuser and camera (including a field flattner) but I don't yet know how to fix it on a 6" RC..



#6 Lead_Weight

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Posted 20 August 2019 - 11:58 AM

This has been the best tutorial for collimating an RC that I have found. I was able to remove all tilt effects that appeared in my 6” RC. See if this makes a difference for you. 




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