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Extension tubes

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#1 Billboard

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Posted 09 December 2023 - 03:31 PM

I just received my new 2 inch diagonal.  When i look thru the scope my heads and face is too close to the tube. I want to set the diagonal further away from the tube. I see all kinds of sizes for extension tubes. From as little as 8mm all the way to 90mm. Is there a limit to how far back you can set your diagonal?   I have a meade 8 inch SCT.  



#2 Michael Covington

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Posted 09 December 2023 - 03:34 PM

With a conventional SCT such as that, feel free to add two to four inches.  Such telescopes can be focused to put the image quite a distance behind the end of the tube.


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#3 Billboard

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Posted 09 December 2023 - 03:52 PM

Thats good to hear. I was looking at the 90mm tube thats in multiple pieces. 



#4 vtornado

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Posted 09 December 2023 - 04:30 PM

Because the secondary mirror magnifies the primary image, as you move the eyepiece backward the focal length is increasing. For planets it is fine, since they are small.  Larger objects like moon or open clusters this may be a problem.

 

https://www.cloudyni...ngth-of-an-sct/



#5 Michael Covington

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Posted 09 December 2023 - 10:31 PM

Because the secondary mirror magnifies the primary image, as you move the eyepiece backward the focal length is increasing. For planets it is fine, since they are small.  Larger objects like moon or open clusters this may be a problem.

 

https://www.cloudyni...ngth-of-an-sct/

Not likely to be a problem with adding just two or three inches though.



#6 Billboard

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Posted 10 December 2023 - 11:20 AM

Which one of these extensions tubes would screw directly to the T threads of the visual back of the optical tube of a SCT.

http://agenaastro.co...-extension.html  

Or

https://agenaastro.c...ter-93633a.html

 

Or will both work as a extension between the telescope and diagonal? 


Edited by Billboard, 10 December 2023 - 11:47 AM.


#7 Michael Covington

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Posted 10 December 2023 - 01:32 PM

You should tell Agena Astro (or Astronomics, our sponsor) what you want to do and get them to recommend the best way to achieve it.



#8 Billboard

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Posted 10 December 2023 - 03:43 PM

I did not know or think of that. Thanks thats the best way to go.


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