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16" f/4 Tensegrity string scope

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#1 mrcotton73

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Posted 12 September 2017 - 12:00 AM

Folks

 

I would like to share details of my 16" f/4 Tenesegrity telescope which I built slowly over the last 4 years and debuted at Oregon Star Party this year. It was also featured in Mel Bartels' Telescope Walkabout..

 

Specifications:

  • 16" f/4 Quartz mirror, 1" thick / 15 lb - by Mark Cowan
  • 3.5"  1/20th wave Antares Secondary 
  • Wire spider 
  • Tensegrity String Telescope design based on Don Peckham's design and Dan Gray's original string scope design
  • Strings are 10 loops each of UltraCam bowstring material and covered with Slink-drifter tubes 
  • Dan Gray's Sidereal Tech Drive System with clutches to manually move scope
  • Optical tube assembly weight - 49.8 lbs (includes paracorr II, 20mm100 ES eyepiece, Stellarvue 50mm finder)
  • Rocker box + ground board + motor drive mechanics - 23.5 lb

 

Lots of people helped with ideas and inspiration that I would like to acknowledge - David Nemo with his 20" string scope, Chuck Dethloff from whom i borrowed a lot of the drive system ideas, Don Peckham, John Delacey and many others at the Rose City Astronomers Telescope workshop for ideas and help with tools, Dan Gray for help with the drive system and Reiner Vogel's excellent telescope making website with lots of ideas and inspiration. Also Albert Highe's books on telescopes and string telescopes are a great resource.

 

In general I am very happy with how it turned out and how rigid the assembly is.. Vibration damping is excellent - something I was worried about. I am posting several pictures below ! 

 

I do need to work on the clutches - the telescope doesn't move as well by hand..

 

Thanks

Sameer

 

Assembled Scope

SAM_3348.JPG


Edited by mrcotton73, 12 September 2017 - 12:34 AM.

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#2 mrcotton73

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Posted 12 September 2017 - 12:01 AM

Scope dismantled with tensioning poles removed:

SAM_3351.JPG

 

 

 


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#3 mrcotton73

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Posted 12 September 2017 - 12:10 AM

18-point Mirror Cell with front collimation (3/8"-24)  bolts (Cell attaches to the collimation bolts using Heim joints - third point is a pivot using a 1/2" heim bolt. Mirror cell / mirror box ideas borrowed from Reiner Vogel's site.

 

Mirror support pads are from Aurora Precision.

 

Mirror box (and rocker) joinery was half-blind dovetails made using a $20 Harbor Freight dovetail jig..

 

Wood finish was General finishes pre-stain conditioner + 1 - 2 coats of GF Merlot water based bye + 1 - 2 coats of GF Cherry gel-stain and Minwax semi-gloss polyeurathane.

 

All wood was baltic-birch.

 

SAM_3341.JPG


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#4 mrcotton73

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Posted 12 September 2017 - 12:11 AM

Mirror Cell (front view):

 

SAM_3339.JPG


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#5 mrcotton73

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Posted 12 September 2017 - 12:18 AM

Secondary cage is a Bicycle wheel rim purchased new on Ebay (2 for $25). Very rigid and light-weight - 1.5lbs

 

Wire-spider was based on Reiner Vogel's design - however instead of using 6 bolts - I substituted the lower 3 bolts with plastic spherical bearings that are epoxied to the outer tube and the inner tube that holds the mirror rides on the surfaces of these bearings - so adjustment can be made using the 3 upper screws only. The wires are 0.014" guitar strings.

 

The secondary ring was chrome plated - so used an adhesive matt black film on the inside - shaped with a heat-gun - these films are used in automobile applications..

 

SAM_3345.JPG


Edited by mrcotton73, 12 September 2017 - 12:18 AM.

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#6 mrcotton73

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Posted 12 September 2017 - 12:20 AM

Secondary holder detail with the spherical bearings replacing the lower bolts:

 

SAM_3346.JPG


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#7 mrcotton73

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Posted 12 September 2017 - 12:26 AM

Rocker box detail - the mirror box / altitude bearings ride on two 1" aluminum rollers -- shaft is driven by the altitude motors; on the back side the altitude bearings sit on two ball-bearings attached to the rocker box sides..

 

The base of the rocker box is a sandwich panel with 3/4" end-grain balsa between two 1/8" plywood sheets - helped shave off at least  5lbs. Weight reduction here only serves to ease back strain smile.gif especially with the added weight of the drive mechanics / gearboxes.

 

SAM_3333.JPG

 

 

 

 



#8 mrcotton73

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Posted 12 September 2017 - 12:28 AM

Azimuth drive system - belt that is wrapped around the ground board - friction drive. a 1/16" AL. plate is glued to the bottom of the rocker box and this rides on 3 vertical bearings in the ground board..

 

SAM_3334.JPG


Edited by mrcotton73, 12 September 2017 - 12:30 AM.


#9 mrcotton73

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Posted 12 September 2017 - 12:32 AM

Altitude drive mechanics -- the servo motors and gearboxes are sourced from Ebay:

 

The clutch (between the gearbox and motor) is an inline slip clutch from Stock Drive products..

 

SAM_3335.JPG


Edited by mrcotton73, 12 September 2017 - 12:32 AM.

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#10 hamishbarker

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Posted 12 September 2017 - 03:42 AM

super job. now I just need to get going on my 22" blank...



#11 Augustus

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Posted 12 September 2017 - 05:57 AM

Truly a work of art. I could never make something this good!



#12 gatorengineer

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Posted 12 September 2017 - 08:24 AM

Beautiful build and a scope to be very proud of, but I don't get the point of the design a traditional 8 pole 0.75 aluminum tube would be within a pound or so and would be a much quicker setup up no?



#13 Pinbout

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Posted 12 September 2017 - 08:38 AM

Some people love string theory...


Edited by Pinbout, 12 September 2017 - 08:46 AM.


#14 mrcotton73

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Posted 12 September 2017 - 12:19 PM

Beautiful build and a scope to be very proud of, but I don't get the point of the design a traditional 8 pole 0.75 aluminum tube would be within a pound or so and would be a much quicker setup up no?

Thanks for all the compliments. The setup is very quick actually -- ~ 5mins and tool less. The 4 Aluminum poles are drop-in into the mirror box (the bolts on the bottom of these poles thread into pronged t-nuts - and these t-nuts sit in a recessed hole at each corner of the mirror box. The secondary ring, the middle ring and the strings are always connected to the mirror box. All that need to be done is the secondary ring is lifted up and placed on the 4 poles and the poles are screwed out - tensioning the strings. Take-down is similar - loosen the tension and then pull the poles out.  It takes longer to describe than to actually do it :)

 

The setup is very rigid and from talking to others who have built string scopes - they maintain pretty good collimation from setup to setup. I just haven't used the scope enough to verify it on mine. I also haven't used a 8 truss scope before so can't really comment on how rigid one might be - but several folks at OSP commented on how rigid this scope felt to them.  

 

Thanks

Sameer



#15 mark cowan

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Posted 12 September 2017 - 10:08 PM

I like the mechanicals quite well.  goodjob.gif 

 

So, uhm, how were the views?   poke.gif



#16 NiteGuy

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Posted 13 September 2017 - 12:14 AM

Hey, great job on your scope! It's obvious that a lot of thought and effort went into it. You'll probably want to paint the secondary edges flat black just so the eyepiece doesn't see the top edge. For public outreach events, you might also want to make sure your scope is not a hazard by protecting all the sharp bolt ends and square tube corners to keep it safe in the dark. Clear skies!



#17 mark cowan

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Posted 13 September 2017 - 01:00 AM

I assume there's a baffle that performs that purpose but I don't see it...

This would just be a lightweight enclosure for the upper ring making it look more like a cage.



#18 hamishbarker

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Posted 17 September 2017 - 05:17 AM

at corners of the floating square of 4 tubes, are the strings fixed or are they free to slide through the (what looks like) loop of heavy guage steel wire?



#19 drneilmb

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Posted 17 September 2017 - 09:13 AM

at corners of the floating square of 4 tubes, are the strings fixed or are they free to slide through the (what looks like) loop of heavy guage steel wire?


My understanding of the tensegrity design is that the top and bottom sets of strings are separate and fixed to the attachment loops on the the corners of the middle square.

#20 bjensen

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Posted 21 September 2017 - 03:44 AM

Very impressive build.  How are the views?  Have you star tested the optics?

Barry



#21 mrcotton73

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Posted 21 September 2017 - 11:42 AM

Very impressive build.  How are the views?  Have you star tested the optics?

Barry

Thanks ! The views are superb - haven't star tested yet - but at OSP the views of the Veil Nebula / Pickering's triangle, etc. were really impressive.

 

 

at corners of the floating square of 4 tubes, are the strings fixed or are they free to slide through the (what looks like) loop of heavy guage steel wire?


My understanding of the tensegrity design is that the top and bottom sets of strings are separate and fixed to the attachment loops on the the corners of the middle square.

 

This is correct - the strings are independent (there are 16 strings). The tensegrity ring is 4 sections of tent-poles connected using a stainless steel 5" long safety pins that are bent at right angles and the loop in between is pried open. Don Peckham illustrates this on his webpage:

 

http://dbpeckham.com...yString12.5.htm

 

Hey, great job on your scope! It's obvious that a lot of thought and effort went into it. You'll probably want to paint the secondary edges flat black just so the eyepiece doesn't see the top edge. For public outreach events, you might also want to make sure your scope is not a hazard by protecting all the sharp bolt ends and square tube corners to keep it safe in the dark. Clear skies!

Thanks for the tips . I will be adding the end caps on the square tubes and a baffle - it was quite a race to get the scope done for OSP :)..  How does one go about blackening the edge of the secondary without getting paint on the face?

 

Thanks

Sameer



#22 NiteGuy

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Posted 21 September 2017 - 01:43 PM

mrcotton73 (the OP) said:   Thanks for the tips . I will be adding the end caps on the square tubes and a baffle - it was quite a race to get the scope done for OSP :)..  How does one go about blackening the edge of the secondary without getting paint on the face?

 

Position the secondary and its holder on some thing "steady" where you can rotate it (horizontal rotation would be best). Maybe make a wooden jig to attach the secondary holder to or a "V" shape that it can sit in and be rotated. Then use one of those very small foam paint brushes (about 1/2" wide) and a steady hand. Keep the amount of paint on the brush small and start by covering the part of the edge closest to the aluminized surface and work your way all the way around, painting in the same direction as the edge and getting the brush right up to the very edge. Second time around, finish the unpainted part. Don't worry about trying to get it all with one coat of paint, better to keep the paint thin and use two or three coats if necessary. It's overkill, but I'd be inclined to paint the back part of the secondary that shows around the holder too. Hope that helps.


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#23 mrcotton73

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Posted 22 September 2017 - 01:23 PM

Thanks NiteGuy - this helps quite a bit. Added to my to-do list!



#24 I forge iron

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Posted 24 September 2017 - 11:44 PM

I want to thank you for sharing your complete build and inspiring some of us that have not fully completed a tensegrity build. You have done very well with keeping tension and compression points very close to each other!

I'm currently changing over to carbon fiber... wonderful stuff... it just takes a boat load of money to get started!

... And.. When you get over the initial shock from start up costs... the shock just continues to roll on!

Edited by I forge iron, 25 September 2017 - 08:24 AM.



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