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Helical Focuser Replacements

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#1 Wire

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Posted 31 July 2019 - 12:01 PM

Hi All

I have and old Meade MTS-SN6 Schmidt - Newtonian. A great scope and tracking mount, but I'm looking to replace the helical focuser to a rack and pinion focuser. Was wondering if any of you have any recommendations on what I should use. I'd like to know who thought of using a helical design on telescopes. They're hard to focus and if you change eyepieces, it's like a major career to get in focus again. I've only seen these on Meade instruments, so I guess they're the inventors of this useless optics.

 



#2 howardcano

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Posted 31 July 2019 - 12:07 PM

It's not a rack-and-pinion focuser, but the Kineoptics helical Crayford focuser works very well, and has a low profile (which might be a limiting factor for your scope).



#3 izar187

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Posted 01 August 2019 - 11:47 AM

Hi All

I have and old Meade MTS-SN6 Schmidt - Newtonian. A great scope and tracking mount, but I'm looking to replace the helical focuser to a rack and pinion focuser. Was wondering if any of you have any recommendations on what I should use. I'd like to know who thought of using a helical design on telescopes. They're hard to focus and if you change eyepieces, it's like a major career to get in focus again. I've only seen these on Meade instruments, so I guess they're the inventors of this useless optics.

A work around for helical focusers (and push/pulls) is par focal rings on ep's, so they're all close to equal focus. 

 

O rings from local hardware can work,

 

These type also: https://agenaastro.c...5-eyepiece.html


Edited by izar187, 01 August 2019 - 11:51 AM.


#4 Pierre Lemay

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Posted 06 August 2019 - 09:06 AM

I'd like to know who thought of using a helical design on telescopes. They're hard to focus and if you change eyepieces, it's like a major career to get in focus again.

It's not a rack-and-pinion focuser, but the Kineoptics helical Crayford focuser works very well, and has a low profile (which might be a limiting factor for your scope).

 

The original helical focusers on telescopes were of the fine threaded type and, indeed, refocusing when changing eyepieces or, simply, when switching observers with more or less myopia can indeed require many frustrating rotations of the draw tube. There is also the problem of slop in the threads when they are of the single thread type, especially for fast reflectors.

 

As howardcano correctly points out the helical focusers of the Crayford Style, like those sold by Kineoptics, are way superior to threaded types in ALL respects. One rotation of the draw tube will move the eyepieces up or down by up to 3/4 inches and there is no slop at all since there are no threads. The high quality bearings make accurate focusing a breeze.

 

If you are willing to give it a try and have access to a drill press (yours or a neighbors) you can even make your own Helical Crayford in one evening and for less than 15$: see this part of a thread where I show how to make a very simple one which will look like this:

 

A simple Helical Crayford.jpg

 

This focuser is very easy to make if you normally use a Paracorr type coma corrector since no draw tube is required to hold the eyepieces (the coma corrector acts as it's own draw tube). I believe Schmidt Newtonians also have some residual coma and owners do use them with coma correctors. If this is the case the design I show in my thread would work as is. If not, a draw tube will have to be purchased or made.

 

If you want to go a bit more fancy you could also build the Dual Speed Helical Crayford I invented last year which is what the thread is all about. You would then have a very accurate fine focuser which also allows a rapid swap of eyepieces without having to rotate the draw tube many turns to refocus non parfocalized eyepieces.


Edited by Pierre Lemay, 06 August 2019 - 09:08 AM.


#5 Eddgie

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Posted 06 August 2019 - 08:11 PM

Hi All

I have and old Meade MTS-SN6 Schmidt - Newtonian. A great scope and tracking mount, but I'm looking to replace the helical focuser to a rack and pinion focuser. Was wondering if any of you have any recommendations on what I should use. I'd like to know who thought of using a helical design on telescopes. They're hard to focus and if you change eyepieces, it's like a major career to get in focus again. I've only seen these on Meade instruments, so I guess they're the inventors of this useless optics.

Have the other responders mi-understood? It sounds like you don't want a helical focuser.

 

Does it have to be rack and pinion?  It is hard to find rack and pinion focusers these days.

 

Your best bet would be to use a Crayford focuser, but if your focuser does not have a fixed base, this could be problematic.

 

Here is the problem though. Helical focusers are usually used where the focal plane is placed rather close to the optical tube, and if this is the case, most rack and pinion or Crayford focusers will be too tall, and will not have the inward focuser travel to reach focus. 

 

That would make the suggestion by izar187 to be probably your best bet.   All helical focusers are going to behave the same, and until you know how far above the outside of the OTA your focal plane is, you really don't know if you have any other options but Helical.


Edited by Eddgie, 06 August 2019 - 08:12 PM.



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