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Tube rigidity for long OTAs

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#1 25585

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Posted 25 May 2024 - 04:49 PM

For refractors like 80mm F15, 100mm F11 etc, what is their tube rigidity like. They seem to come with relatively short dovetails and 2 rings, is that enough to stop flexing and kinking? Perhaps a lightweight "splint" of some sort might be good?


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#2 DLuders

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Posted 25 May 2024 - 05:03 PM

There is not nearly enough weight hanging on the telescope tube to come even close to bending or flexing it.  Tubes are extremely strong shapes, used for Roll Cages, etc.  Even hollow trees can stand up quite a long time against the wind.


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#3 RichA

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Posted 25 May 2024 - 06:32 PM

There is not nearly enough weight hanging on the telescope tube to come even close to bending or flexing it.  Tubes are extremely strong shapes, used for Roll Cages, etc.  Even hollow trees can stand up quite a long time against the wind.

True.  More likely to experience minute flexture of a vixen rail if the ring spacing was too much.  I know that happened on some larger, heavier scopes.


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#4 Jethro7

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Posted 25 May 2024 - 06:39 PM

For refractors like 80mm F15, 100mm F11 etc, what is their tube rigidity like. They seem to come with relatively short dovetails and 2 rings, is that enough to stop flexing and kinking? Perhaps a lightweight "splint" of some sort might be good?

Hello 25585,

My Altair Starwave 102 ED F/11. Is. Pretty long and the scope tube is quite stout no flex or kinking. Here is where problems with long scopes occur The instability was with the mounting hardware not the OTA. In my case the Vixen dovetail bar that came with the scope was very short and not adequate for the long OTA that creates a long momentum arm resulting in stability issues. I purchased the longest  Losmandy D style dove tail bar that I could find 15" to sepperate the parallax rings as far apart as I could. This effectively stabilized the scope.

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

 

20210816 130328
 
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Edited by Jethro7, 25 May 2024 - 06:40 PM.

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#5 macdonjh

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Posted 26 May 2024 - 01:29 PM

You can look up a Hargrave (Hargreave?) strut if you have concerns.


Edited by macdonjh, 26 May 2024 - 09:09 PM.

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#6 Polyphemos

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Posted 26 May 2024 - 06:37 PM

I’m not so sure a very long and narrow scope tube wouldn’t deflect enough to measurably change collimation.

 

You’d never notice such a minute deflection just looking at the scope, or probability through it either, but a triplet within a collimatible lens cell and hanging at the end of an f/15 focal ratio might deflect enough to show a change in collimation in a magnified view through a Cheshire eyepiece. It’s a simple enough experiment to perform: check straight through collimation with a magnified Cheshire eyepiece and the scope oriented vertical, then repeat with the scope mounted horizontally. I’ll bet there’s a visually detectable, albeit very small, difference in some scopes.

 

Edit: while all of the foregoing is true, some quick and dirty back of the envelope calculations about tube beam deflections plus some basic trigonometry shows a lens cell loaded optical tube is almost certainly not something most of us need to ponder at any length. You might or might not see it in the experiment proposed, and even if you did it’s not something you’re likely to feel the need to correct for. There are exceptions.


Edited by Polyphemos, 26 May 2024 - 07:41 PM.

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