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Question about mounting a 2" focuser on a 5.5" OD Telescope tube

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#1 Rustyfl

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Posted 17 May 2024 - 11:36 PM

Spacer for Telescope Focuser.jpg I am looking for the spacer that fits between a Sky Watcher 2” focuser and the side of a Newtonian telescope tube. I want to bolt the focuser onto a 5.5” OD tube. The primary mirror is 4.5“. I have the spacer for an 8” telescope tube (see photo) but It’s too thin because the angle on the outer sides of the smaller tube is too sharp. Does anyone know where I can purchase this part or have it made?



#2 Jon Isaacs

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Posted 18 May 2024 - 05:35 AM

attachicon.gif Spacer for Telescope Focuser.jpgI am looking for the spacer that fits between a Sky Watcher 2” focuser and the side of a Newtonian telescope tube. I want to bolt the focuser onto a 5.5” OD tube. The primary mirror is 4.5“. I have the spacer for an 8” telescope tube (see photo) but It’s too thin because the angle on the outer sides of the smaller tube is too sharp. Does anyone know where I can purchase this part or have it made?

 

You could try shimming it with washers.. I have done that before.  Otherwise, find someone with a 3D printer.

 

Jon


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#3 jp071848

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Posted 18 May 2024 - 07:10 AM

You could try shimming it with washers.. I have done that before.  Otherwise, find someone with a 3D printer.

 

Jon

My thoughts exactly. I recently picked up a pair of Moonlites, one of which was destined for my 16" Lightbridge. Unfortunately, the baseplate was for the previous owner's 6" Newt, hardly a good fit. I fired up Freecad and made up an adapter to go from the 6" diameter to the 19" diameter of the 16" Lightbridge, 3D printed at an unnecessary 100% infill, and it fit perfectly. I know Ron sells correct sized baseplates but this was far cheaper.
 


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#4 Garyth64

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Posted 18 May 2024 - 07:29 AM

I assume you want to use 2" eyepieces, for low power wide field viewing.  If so, your secondary is most likely too small to benefit from the use of 2" eyepieces.


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#5 Jon Isaacs

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Posted 18 May 2024 - 08:15 AM

I assume you want to use 2" eyepieces, for low power wide field viewing.  If so, your secondary is most likely too small to benefit from the use of 2" eyepieces.

 

I have installed 2 inch focusers on 130mm F/5 Newtonians.. I did not increase the size of the secondary, it was not necessary... The existing secondaries were not optimal for use with the 2 inch eyepieces but the views certainly benefited and the vignetting was not bothersome.  

 

The 31mm Nagler was pretty awesome..

 

5901459-SpaceProbe 130ST ALT-AZ at the Starpad.JPG

 

Jon


Edited by Jon Isaacs, 18 May 2024 - 08:16 AM.

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#6 Garyth64

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Posted 18 May 2024 - 08:52 AM

Alrighty then, go for it.  I wouldn't do it, and didn't do it.  I have 1-1/4" eyepieces that have worked just fine giving nice low power wide fields, with no vignetting. 


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#7 Jon Isaacs

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Posted 20 May 2024 - 09:02 AM

Alrighty then, go for it.  I wouldn't do it, and didn't do it.  I have 1-1/4" eyepieces that have worked just fine giving nice low power wide fields, with no vignetting. 

 

And those "wide field views" were not that wide...  You missed out.. My 130mm F/5 would provide a 2.4 degree TFoV with a 1.25 inch eyepiece.  That's it.  An 8 inch F/6 does 2.2 degrees. I can do 2.1 degrees with my 10 inch. With the 31mm Nagler, the 130mm F/5 provided a 3.7 degree TFoV.  That is enough to show the entire Veil nebulae..  Pretty awesome. 

 

The eye is surprisingly insensitive to vignetting..  People use 2 inch eyepieces with 5 inch SCTs with a 1 inch rear port.  It seems like it wouldn't work but it works surprisingly well. 

 

That said, the real reason to upgrade to a 2 inch focuser is to get a better focuser,  really decent 1.25 inch focusers are rare, I know of no 1.25 inch focusers with a two speed.   And a 2 inch focuser allows one to use a coma corrector.. a nice touch with a fast Newtonian. 

 

This adapter was machined but it could have been 3D printed.

 

Skywatcher 130 with JMI focuser CN.jpg
 
"Do not let what you cannot do interfere with what you can do."  Coach John Wooden.  
 
A little vignetting is no biggie.. 
 
Jon

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#8 Garyth64

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Posted 20 May 2024 - 11:14 AM

Thank you for that.  I don't have any fast scopes, and I not do I own a Nagler.  I am satisfied with what scopes I have and the eyepieces I use.


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#9 Rustyfl

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Posted 20 May 2024 - 11:32 AM

Thanks for everyone's advice and your replies. A 3D printer is probably my best option. I don't know exactly where to get access to one. If anyone reading this has a 3D printer and would like to help, I will pay for the cost and shipping. I plan to do astrophotography with this setup and need to get everything precise. I am using an ASI533 color camera. The chip is about 1 inch square, so a wide field is not necessary. Since the telescope's primary mirror is f4, I need the 2-inch diameter focuser tube to insert my field flattener-comma corrector.




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