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Seeking advice: focuser drawtube length

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#1 mjgillen

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Posted 24 November 2020 - 10:02 AM

Hello all,

 

Looking for advice on "draw tube length". I've searched around however didn't really find anything definitive. I have a older 12.5" Portaball with the aluminum ball and it has a decent older JMI focuser that I am thinking about upgrading because I would really like a dual speed focuser. When I use my Nagler 31mm it does not reach focus - there is not enough in-travel. So I am considering the Starlight Instruments Feather Touch lightweight focuser as it has a lower profile than my current focuser and as I understand it that should allow my 31mm to achieve focus (if I'm incorrect please educate me). The FT has options for 1.5", 2.0" and 2.5" draw tube lengths so I need to understand how to figure out which one is best for me.

 

I don't use binos however I do have several Naglers from 31mm to 3.5mm and I also have a couple of Powermates in 2" and 1.25". I will be doing visual observing only of planets, the moon, and deep sky objects.

 

Opinions appreciated!

 

Clear skies,

Michael

 



#2 Couder

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Posted 24 November 2020 - 10:18 AM

I have quite a collection of eyepieces, including many antique ones, plus some custom ones I made. I just have to accept the fact not every eyepiece will work in every scope. You may get yours to work by changing the focuser, only to fine something else doesn't work out or look like you think it should. 

One of my scopes I just got has the Unitron Super Focuser. It has a drawtube of (without going out to measure it) maybe 10 to 12". Another scope I have only has 1-1/2" travel. I could go on, bottom line is try not to make drastic changes to your scope to make 1 eyepiece work. Maybe you can take the old focuser off, and simply hold the new one in place to see it it meets your expectations before attaching it permanently. If some of your other eyepieces do not work, you can get extensions to hold your eyepieces further away from the tube.



#3 kathyastro

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Posted 24 November 2020 - 10:47 AM

The draw tube length is determined by the difference in focus position between eyepieces.  If the Nagler is the extreme in the inward direction, what eyepiece is the extreme in the outward direction?  The difference between those extremes determines the draw-tube length you need.  We don't have enough information yet to determine that.

 

If you are unable to achieve enough inward focus, what you need is not (necessarily) a longer draw tube but a shorter body.  It is the body of the focuser that is preventing you from achieving focus with the Nagler.

 

The FeatherTouch focusers are excellent, worth every penny in my opinion.  I upgraded the GSO LF focuser on my Orion astrograph to a FeatherTouch, and I am very happy with it.  But they are expensive, so before spending the money, you should take detailed measurements.  Know what is the maximum body depth that you han handle and still be able to focus the Nagler.  Measure the difference between that focus position and the position for focusing the eyepiece that is the outward extreme.  With detailed measurements, you will be able to order the right focuser.


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#4 Jon Isaacs

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Posted 24 November 2020 - 11:11 AM

Looking for advice on "draw tube length". I've searched around however didn't really find anything definitive. I have a older 12.5" Portaball with the aluminum ball and it has a decent older JMI focuser that I am thinking about upgrading because I would really like a dual speed focuser. When I use my Nagler 31mm it does not reach focus - there is not enough in-travel. So I am considering the Starlight Instruments Feather Touch lightweight focuser as it has a lower profile than my current focuser and as I understand it that should allow my 31mm to achieve focus (if I'm incorrect please educate me).

 

 

I have both JMI focusers and Feathertouch focusers.  I just measured the height of a Feathertouch at 1.6", my recollection is that JMI focusers are also 1.6" tall. 

 

If you are going to use the 31mm Nagler, the lightweight Feathertouch may not be a good plan. 

 

I am not familiar with your truss tube design and how the tubes mount but have you considered shortening the tubes 1/4 inch or so?  

 

jon



#5 Eddgie

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Posted 24 November 2020 - 12:20 PM

While it is not a Feathertouch, the Orion low profile focuser is decent.  It has two speeds so that is what you wanted, but it also has a built in extention tube so that eyepieces that need more outward travel can reach focus without having to use extensions. 

 

Antares also sells a similar focuser.

 

https://agenaastro.c...dual-speed.html

 

Just an option you might want to consider. I would not compare these directly to a focuser like a Feathertouch and if you were going to be using a particularly heavy binoviewer configuration, where I would recommend the Feathertouch as the absolute best you can do, but for most standard duty applications, one of these might work well for you.  They are inexpensive, have two speeds, and solve the problem of needing more height with the built in extension tube. Mine works quite well. Not a Feathertouch, but I don't feel compelled to upgrade to a Feathertouch either. 



#6 MitchAlsup

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Posted 24 November 2020 - 06:28 PM

A 1.5" draw tube travel is fine for most EP collections.

A 1.0" draw tube requires very small constraints of optics placements and build accuracy; but can be done with some minor restrictions on EP selections.



#7 mjgillen

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Posted 24 November 2020 - 09:01 PM

I have both JMI focusers and Feathertouch focusers.  I just measured the height of a Feathertouch at 1.6", my recollection is that JMI focusers are also 1.6" tall. 

 

If you are going to use the 31mm Nagler, the lightweight Feathertouch may not be a good plan. 

 

I am not familiar with your truss tube design and how the tubes mount but have you considered shortening the tubes 1/4 inch or so?  

 

jon

Jon, I measured my JMI and I measure 1.7" (without base) so your 1.6" sounds about right. With the base its ~1.9". According to the Feathertouch specs website, the FT is 1.4" "racked in from mounting face" so I'm guessing the mounting plate adds 0.2". It appears that the FT may be a drop-in replacement, adding the dual speed I am looking for.

 

Why do you say it might not be a good plan for the 31mm Nagler? 

 

Shortening the tubes may be a good idea. However that still doesn't get me the dual speed I would like. Fine adjustments at hight power are going to be a challenge...



#8 mjgillen

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Posted 24 November 2020 - 09:03 PM

The draw tube length is determined by the difference in focus position between eyepieces.  If the Nagler is the extreme in the inward direction, what eyepiece is the extreme in the outward direction?  The difference between those extremes determines the draw-tube length you need.  We don't have enough information yet to determine that.

Good point. I will take measurements once my mirror gets back from being re-coated...



#9 mjgillen

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Posted 24 November 2020 - 09:03 PM

While it is not a Feathertouch, the Orion low profile focuser is decent.  It has two speeds so that is what you wanted, but it also has a built in extention tube so that eyepieces that need more outward travel can reach focus without having to use extensions. 

 

Antares also sells a similar focuser.

 

https://agenaastro.c...dual-speed.html

 

Those two look the same. I'll keep that in mind for an option. Thank you for the link.



#10 mjgillen

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Posted 24 November 2020 - 09:04 PM

A 1.5" draw tube travel is fine for most EP collections.

A 1.0" draw tube requires very small constraints of optics placements and build accuracy; but can be done with some minor restrictions on EP selections.

Thanks for the info. I'll take some measurements however I think you're probably right...



#11 CrazyPanda

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Posted 25 November 2020 - 01:03 PM

You will want to factor in the use of a binoviewer if you plan on it. A BVer, even with a barlow, requires quite a bit of in-travel, which means you will need more out-travel for use with normal eyepieces.

 

 

 

Why do you say it might not be a good plan for the 31mm Nagler?

 

For what it's worth, I have the light weight FT, and I find it's not as rock solid as my normal FT.

 

However, it can also handle this absurd cantilever: https://i.imgur.com/OjciDjz.jpg

 

And I've also done 2x PowerMate -> Paracorr (which sticks a mile out of the PowerMate) -> 17mm ES92 and it also handles it. Some flex, but nothing that drastically breaks the view (at least for my skies). Frankly, unless you've deliberately built a thick, stiff focuser board into the design of the scope, there will be more flex at the connection point of the focuser and tube than in the focuser itself.

 

Still, if you want absolute best stability, the non light-weight FT may be the better solution.


Edited by CrazyPanda, 25 November 2020 - 01:04 PM.


#12 ad701xx

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Posted 25 November 2020 - 02:47 PM

I have two of the standard weight and one LW FT. I don't see any difference between the three in actual use.



#13 mjgillen

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Posted 25 November 2020 - 04:21 PM

@crazypanda @as701xx Thanks for the info.




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