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How long is too long? refractor visual back end

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#1 HIbill

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Posted 27 January 2021 - 03:56 AM

what is a reasonable limit for a 2 1/2” focusser regards max length 2” visual setup sticking out the back of a refractor?  for example- a 2x powermate coupled with a 3.5 apm xwa on a 2” diagonal-  it seems really long and rather precarious sticking out of my little AT92.  Do people combine barlows with long eyepieces?

 

 



#2 sg6

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Posted 27 January 2021 - 04:21 AM

Suspect no specific answer, and people will add a barlow to anything in the idea of some magic number close to or exceeding infinity.

 

From use I have found that some focusers just have too little travel for comfort, my 72ED is a prime example. On the other side is my ZS61, where if I wind the focuser in and look down the front that lump of metal gets close to the objective.

 

Personally I do not use barlows, they are not as useful as is made out - little use on short focal length eyepieces and barlowing a 25mm Paradigm means I might as well use my 12mm Paradigm. So I have never been drawn to barlows.

 

My extent or requirement therefore would be travel sufficent to take a 2" diagonal and therefore possibly a 2" eyepiece or 1.25" eyepiece and have sufficent travel to accomodate whatever combination. So I guess around 60mm travel.



#3 Astrojensen

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Posted 27 January 2021 - 04:40 AM

 

Personally I do not use barlows, they are not as useful as is made out

Barlows can be super duper useful. They may be more or less the ONLY way to reach focus with something, for example a binoviewer on a telescope with limited back focus. 

 

 

Clear skies!

Thomas, Denmark 


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#4 Astrojensen

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Posted 27 January 2021 - 04:45 AM

what is a reasonable limit for a 2 1/2” focusser regards max length 2” visual setup sticking out the back of a refractor?  for example- a 2x powermate coupled with a 3.5 apm xwa on a 2” diagonal-  it seems really long and rather precarious sticking out of my little AT92.  Do people combine barlows with long eyepieces?

How many accessories you can hang on the end of the focuser depends more on how solid the focuser is constructed than anything else, really. 

 

The 2.5" R&P focuser on my Altair 102/1122 ED is very solid and doesn't seem to mind a 120mm extension, plus T2 diagonal, Zeiss binoviewer and eyepieces. The 3.5" focuser on my APM 152ED is even more robust. 

 

 

Clear skies!

Thomas, Denmark


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#5 HIbill

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Posted 27 January 2021 - 02:51 PM

Thankyou.
Good point on focussing range.

astroJensen how many millimeters total from The back of your focus are to the end of the eyepiece?

#6 SeattleScott

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Posted 27 January 2021 - 05:03 PM

I don’t think this is much of an issue with respectable telescopes. It is more of a problem when trying to use a Powermate and Ethos in a Celestron 70AZ. But not many people use $1,000 of glass in a $100 telescope.

There can be weight issues that cause the focuser to slip when pointed near zenith. Particularly with Crayford focuser.

There can be balance issues with the little 60-70mm Apos, but let’s face it those are mostly intended for AP.

But a 92mn Apo with a robust 2.5” R&P should be just fine.

My Vixen 103 has a 2.3” focuser with 4.5” focus travel.

Scott

Edited by SeattleScott, 27 January 2021 - 05:04 PM.


#7 rkelley8493

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Posted 27 January 2021 - 05:18 PM

You can get focuser extension tubes to help with equipment that requires a lot of back focus. I just ordered some M68 extension tubes for my FC-100DF so I can use my new convertible barlow with minimal flex.

I'll post how it looks with the extension tubes when they arrive, but here's what I'm currently working with.

Attached Thumbnails

  • ffc-optical-train.jpg

Edited by rkelley8493, 27 January 2021 - 05:25 PM.


#8 HIbill

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Posted 27 January 2021 - 08:22 PM

Thanks again. yes my focuser can handle alot but after 55mm of focus tube is out (95mm is max out) the focuser tube begins to develop almost a little flop regardless of how tight you make the tightening screws. turning focuser in slightly doesn’t eliminate the free play but helps. The tube can be moved side to side or up and down making a clanking noise. This has been remarked on in other posts on this scope. I am not aware of it causing problems with visual or imaging, but I may need to manage my visual and imaging setups to try and keep the focuser tube from flopping. That is part of my reason for asking the question about how long is too long.

I got a 1’” 68mm extender, not used it yet. I haven’t had a clear night since the At92 arrived. I at least corrected for cone error during the daytime with a .75 mm shim in front.

Edited by HIbill, 28 January 2021 - 12:21 PM.



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