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A cheap and effective way to correct tube flexure

DIY Collimation Reflector Equipment Dob
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17 replies to this topic

#1 cloudypatio

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Posted 27 February 2021 - 11:49 AM

Since I use a coma corrector in my F5 dobson I try to mantain collimation as perfect as  I can, but I  was noticing some tube flexure (not focuser flexure) when the focuser was loaded with the corrector and heavy eyepieces. It wasn't too much but enough to be readible at the autocollimator unstacking reflections just a bit.

 

Instead of reinforce the tube with an inside metal ring I decided to try another cheaper and lighter way trying to not compromise the tube balance. I have used 3 embroidery circles to aply press to the tube and now I can add 1.5 kg to the focuser and autocollimator reflections stay stacked.

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Edited by cloudypatio, 27 February 2021 - 11:56 AM.

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#2 Bob Myler

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Posted 27 February 2021 - 12:06 PM

Great idea!!!
What size tube and what size embroidery hoops and where did you buy them?
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#3 cloudypatio

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Posted 27 February 2021 - 12:19 PM

Great idea!!!
What size tube and what size embroidery hoops and where did you buy them?

The inner diameter of the inner hoop is 290 mm, it didn't fit in my tube (303 mm) so i cut it as you can see in the first picture. I bought them in a local store, but they are easy to find online.



#4 TOMDEY

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Posted 27 February 2021 - 12:36 PM

Interesting! I'll have to start sorting through my wife's stuff --- when she's not looking.    Tom


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#5 PirateMike

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Posted 28 February 2021 - 07:42 AM

Great Idea. Who would have known that Grandma's old hoops could be so useful. laugh.gif

 

I use those hoops to make my flat frames. I just stretch a few layers of cloth in the hoops and trim the edges a little. When I make flats (not usual for me) it is an easy task.

 

 

Miguel   8-)

 

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Edited by PirateMike, 28 February 2021 - 07:44 AM.

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#6 starcanoe

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Posted 28 February 2021 - 08:59 AM

Interesting! I'll have to start sorting through my wife's stuff --- when she's not looking.    Tom

HAVE to START ?

 

And here I was thinking how smart you are :)

 

Kudos to the OP...just when you think clever ideas are thing of the past somebody come up with something else !


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#7 TxStars

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Posted 28 February 2021 - 09:09 AM

Is that a grab handle I see there ?.. gramps.gif



#8 TOMDEY

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Posted 28 February 2021 - 09:14 AM

Interesting! I'll have to start sorting through my wife's stuff --- when she's not looking.    Tom

HAVE to START ?

And here I was thinking how smart you are smile.gif

Kudos to the OP...just when you think clever ideas are thing of the past somebody come up with something else !

Just checked her craft room... Something tells me I'd better wait.    Tom

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#9 Dick Jacobson

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Posted 28 February 2021 - 12:21 PM

I made a solar filter using an embroidery hoop and solar film. Another local club member, whom I later married, was very amused that a guy would know anything about embroidery hoops.


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#10 MitchAlsup

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Posted 28 February 2021 - 03:00 PM

The stiffener on the outside is stiffer than the stiffener on the inside by (D-d)^3



#11 PirateMike

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Posted 28 February 2021 - 04:28 PM

Just checked her craft room... Something tells me I'd better wait.    Tom

Be careful Tom. She can probably hear you thinking about it. lol.gif

 

But seriously, she does excellent work. Work as beautiful as that has to come from the heart. You should be proud of her talent.

 

 

Miguel   8-)

 

.


Edited by PirateMike, 28 February 2021 - 04:28 PM.


#12 Lee D

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Posted 03 March 2021 - 01:17 PM

The stiffener on the outside is stiffer than the stiffener on the inside by (D-d)^3

Other than the difference in diameters, would a variable-length hoop with the length adjustment hardware on the inside, placed on the inside of the tube, do as well in adding stiffness as a hoop on the outside? 



#13 MitchAlsup

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Posted 03 March 2021 - 01:53 PM

Other than the difference in diameters, would a variable-length hoop with the length adjustment hardware on the inside, placed on the inside of the tube, do as well in adding stiffness as a hoop on the outside? 

two issues::

 

1) The hoop on the outside is stiffer

2) the hoop on the outside does not cause warm air rising off the mirror to enter the light path.

Beyond those two issues, it is all aesthetics.

 

3) A hoop on the outside screwed on from the inside is stiffer still.


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#14 hamishbarker

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Posted 03 March 2021 - 08:57 PM

on my 550mm dob with 600mm diameter  light steel duct tube, I used 27.5" bike rims (two were carbon, damaged by a hot car exhaust, one aluminium) inside to stiffen it at the focuser, and at entry and rear of the mirror box. they were slightly too small, so I cut them at one point and used opposing wooden wedges in the cut location to get a tight fit, then screwed into them from outside the tube. the scope keeps collimation quite well. without the bike rims, the tube was very flexy (only 0.6mm steel).

 

At the top end of the tube, I riveted a 29" alloy rim onto the outside. It was a little too tight so had to crimp the edge of the tube slightly. it gives a nice easy grab location for lifting (when transporting) and for slewing.


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#15 matt_astro_tx

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Posted 04 March 2021 - 06:53 AM

Since I use a coma corrector in my F5 dobson I try to mantain collimation as perfect as  I can, but I  was noticing some tube flexure (not focuser flexure) when the focuser was loaded with the corrector and heavy eyepieces. It wasn't too much but enough to be readible at the autocollimator unstacking reflections just a bit.

 

Instead of reinforce the tube with an inside metal ring I decided to try another cheaper and lighter way trying to not compromise the tube balance. I have used 3 embroidery circles to aply press to the tube and now I can add 1.5 kg to the focuser and autocollimator reflections stay stacked.

That's an amazing idea.  Great job.  If only embroidery hoops were strong enough to be used as tube rings!  They'd be perfect.



#16 bokemon

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Posted 13 March 2021 - 10:03 PM

Hello, late to this thread:

Can somebody explain to me how this wooden loop can increase tube stiffness?

It doesn't really help much to prevent oval-ization since the wood is thin.

I'd think you'd need some kind of hoop that had a bigger difference between inner and outer diameter



#17 MitchAlsup

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Posted 14 March 2021 - 01:07 PM

The wood is 8×-20× thicker than the metal tube.

 

As you add thickness to a round object you gain in the moment of inertia by r^4



#18 starcanoe

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Posted 15 March 2021 - 11:21 AM

That's an amazing idea.  Great job.  If only embroidery hoops were strong enough to be used as tube rings!  They'd be perfect.

 

They probably are if used/modified right.




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