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An offset wire spider design

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#51 Daniel Mounsey

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Posted 17 January 2017 - 12:53 AM

By request, attached are some close up shots and the design plans for my offset wire spider. The telescope is a 16" f4.5 with an 88mm secondary and I rebuilt my spider as part of an UTA rebuild to lose weight. I'm not sure that anything I've done here is terribly original, but I'm pleased with the overall result - the spider is rigid and doesn't suffer from vibration - and I thank the many contributors on this and other sites that have provided many of the ideas I have implemented together successfully here.
 
gallery_217007_4746_88360.jpg
 
Key features include:-
 
1. low profile UTA, total height 162mm. I occasionally pack my telescope into the boot of an ordinary sedan. The photo below is my original (heavier) UTA but the rebuild (above) is dimensionally identical.
 
gallery_217007_4746_59539.jpg
 
2. Guitar strings and lockable guitar machine heads make installation and adjustment quick and easy, and permit collimation. Below are two views...
 
gallery_217007_4746_16563.jpg
 
gallery_217007_4746_7931.jpg
 
The spider uses 8 x #2 guitar string wires tensioned with lockable guitar machine heads. This technique made construction very simple; I found it easier to build a wire spider than a normal vane spider. I didn't even use a jig; I simply placed the central support upside down on the table and the UTA around it (upside down) and threaded each wire from the center out through each head loosely, locked the heads, cut the wires and began to tension. When it was roughly right, fit the laser and fine tune. So there is no reason to be intimidated by wire spiders being too fiddly or difficult.
 
3. wire vane fixture points offset from center to edge providing stable geometry preventing rotation (yaw). See discussion of this important feature in these threads here and here...
4. spider fixing plate offset behind mirror to minimise overall height of structure and by extension minimise UTA
5. wires are crossed vertically to maximise stability

gallery_217007_4746_52021.jpg
 
5. secondary is designed offset to optimise beam while maintaining parallel vanes to minimise diffraction.
 
Below is an image of the pdf file attached for anyone who wants to build their own. If you print it on A1 paper it will be drawn to scale full size. attachicon.gif16inch-newTER.pdf
 
gallery_217007_4746_282741.png



Wow such beautiful work.
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#52 Jeff Morgan

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Posted 17 January 2017 - 09:27 AM

What are those eyelets where the wires connect to the diagonal holder?

 

I tried searching McMaster Carr for those, came up empty.



#53 Tempus

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Posted 17 January 2017 - 11:21 AM

What are those eyelets where the wires connect to the diagonal holder?

 

I tried searching McMaster Carr for those, came up empty.

Jeff,

 

Those are what are normally at the end of a guitar string. Buy a pack of guitar strings (or google it) and you will see that is the 'body' side termination of the strings. 


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#54 Oberon

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Posted 17 January 2017 - 05:06 PM

Yes, the eyelets come prefitted to a guitar string. Thats what makes them so easy to use. You don't have to tie anything off, you don't have to twist wires...just slip the clean end of the wire through the hole on the secondary support and thence to the machine head (tuner), cut off excess wire and adjust tension.



#55 Pinbout

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Posted 17 January 2017 - 08:24 PM

What are those eyelets where the wires connect to the diagonal holder?

 

I tried searching McMaster Carr for those, came up empty.

can't get everything from McMasters. :p



#56 I forge iron

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Posted 17 January 2017 - 08:58 PM

HI Oberon,

Use a small flat blade screwdriver and tighten the little screw on the top of the tuning knob. There are little plastic brake parts that go into compression and cease the tuning worm gear shaft. You will be unable ... or have difficulty turning the tuning knob after tightening the center screw. (I have not found that necessary).

Yes I like the knurled wheels that capture the guitar wire. Like the look of not looping the wire under itself and it makes life tremendously easier with the Tensegrity build... especially with the machine heads inside compression poles on tensegrity build.

Was away for supper... not sure if this will post in correct location?

Fred

Edited by I forge iron, 17 January 2017 - 09:44 PM.


#57 I forge iron

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Posted 17 January 2017 - 09:45 PM

Oh! This did not go as planned!

#58 I forge iron

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Posted 17 January 2017 - 09:58 PM

Hi Oberon,
Yes it looks like we have the same exact tuning machine heads (something about the give away price?) If you take a small Philips screwdriver and tighten the small screws in the center of the worm drive tuning shaft hand knob it will place the small white (nylon?) washer in compression and lock the worm drive shaft in place ("Brake"). I have not done this since taking the machine heads out of the shipping carton.. I'll try it tonight with the tuning head firmly mounted in the upper optical assembly.

#59 I forge iron

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Posted 17 January 2017 - 09:59 PM

This didn't go any better!

#60 I forge iron

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Posted 17 January 2017 - 10:01 PM

One more time? Philips screwdriver tightens white nylon(?) washer on the hand nob end of worm gear shaft locking worm gear shaft securely.

Post in correct location?

#61 I forge iron

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Posted 17 January 2017 - 10:13 PM

Oberon,

Yes, original intention was a Stewart Platform build.

That was changed to a 'Tensegrity' OTA and ... WAY too complex for Stewart Platform. The mirror cell was built for SP and had to be altered for Don Peckham's example of a 'Tensegrity' build. http:/dbpeckham.com (yeah I'm going to need some experience in this site to look better!


The Tensegrity build reminds me of a double helix... Well! That's what it is add in two floating rings! (and... Yes, I deeply miss the simplicity of the SP!)

Clear skies, and uncomplicated builds!
Fred

#62 I forge iron

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Posted 17 January 2017 - 10:21 PM

Also had to go with four tube adjustment assemblies instead of three as used in the Stewart Platform for a development yet to be disclosed until build is complete. ... So I had to redirect the plans of the 3 adjustment points on the upper ring as in a SP and add a fourth pole never adjusting into correct compression, rocking if not built to exact length. The pleasure of the SP.

AND! Tensegrity OTA can be built with 3 poles... but... the undisclosed development demanded 4... IF I get tired of all this horseplay with 4 poles I can pitch it all and build a 3 pole Tensegrity and do the SP with cable adjustment! ... It's NOT OVER YET!

Fred

#63 I forge iron

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Posted 17 January 2017 - 10:39 PM

What are those eyelets where the wires connect to the diagonal holder?
 
I tried searching McMaster Carr for those, came up empty.


Next time you see a steel wire guitar look (I'm not a guitar player) were the wires attach to the base end of guitar. There will be a steel bar with small oval holes that the little brass thimbles anchor in by threading the wire and brass anchor assembly pulling wires toward tuning pegs or machine heads.

I would suggest buying the piano wire from McMaster-Carr you will get more than you can ever use, but you will need to buy guitar strings to get the little brass donuts.

I passed guitar string from bottom hole to top hole and the sharp bend anchored the wire center on the secondary body. Used half the guitar strings cut the brass donut off and threw them away, quietly shed a tear knowing I was well past half the price of piano wire from McMaster-Carr and would then own a vast amount of piano wire for all the build and tear-downs the 'Tensegrity' build will requite.

My dirty blacksmith hands in the soup!
Fred

Edited by I forge iron, 17 January 2017 - 10:43 PM.


#64 Oberon

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Posted 17 January 2017 - 11:24 PM

 

By request, attached are some close up shots and the design plans for my offset wire spider. The telescope is a 16" f4.5 with an 88mm secondary and I rebuilt my spider as part of an UTA rebuild to lose weight. I'm not sure that anything I've done here is terribly original, but I'm pleased with the overall result - the spider is rigid and doesn't suffer from vibration - and I thank the many contributors on this and other sites that have provided many of the ideas I have implemented together successfully here.


Wow such beautiful work.

 

Thank you. Here are some more gratuitous photos showing off the UTA and spider...
 

gallery_217007_5817_17242.jpg

 

gallery_217007_5817_529632.jpg

 

gallery_217007_5817_320420.jpg


Edited by Oberon, 17 January 2017 - 11:26 PM.

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#65 Daniel Mounsey

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Posted 17 January 2017 - 11:26 PM

This is really a work of art and quite unique.
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#66 Jeff Morgan

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Posted 17 January 2017 - 11:41 PM

 

What are those eyelets where the wires connect to the diagonal holder?

 

I tried searching McMaster Carr for those, came up empty.

can't get everything from McMasters. :p

 

 

:lol:



#67 Jeff Morgan

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Posted 17 January 2017 - 11:44 PM

Thank you. Here are some more gratuitous photos showing off the UTA and spider...

Gratuitous?

 

More like telescope ****!

 

But since we are all on topic, what size/diameter wire would you recommend for a 4" diagonal?


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#68 Oberon

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Posted 18 January 2017 - 12:02 AM

The 0.4mm wire I've used should be adequate provided you improve/increase the angle of the wire coming from the top of the UTA, as mine is a little shallow and thus borderline. If you stick to 30 degrees you should be OK, as my 88mm is only supported at something like 15 degrees and its OK.

OTOH I'm building another spider for another project at the moment and experimenting with SS cable ties. These are only 0.2mm thick - half the thickness of wire - and if I keep them aligned (no crossover) they'll provide even less obstruction, yet be much stronger and stiffer than wire. They are only 4.6mm wide, and cheap. Of course they are more fiddly and won't be as easy to use as guitar wire, but I like experimenting so will let the forum know when its done. If I can put these together as simply as a wire spider then they should be a very attractive option, aka less diffraction even than a wire spider, and stronger and stiffer to boot.



#69 Oberon

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Posted 18 January 2017 - 12:18 AM

The stiffest designs are on the left, but they take up a lot of space because the wires are at 45 degrees.

The weakest designs are in the centre, the wires are at 15 degrees or less.

The best compromise designs are on the right, each has the wires set at 30 degrees.

Merope is bottom centre, so plenty of room for improvement.

 

gallery_217007_4746_47488.png


Edited by Oberon, 18 January 2017 - 12:18 AM.

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#70 Oberon

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Posted 18 January 2017 - 12:20 AM

Also...

gallery_217007_4746_95039.png


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#71 Oberon

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Posted 18 January 2017 - 12:23 AM

Consequently these 3 spider and UTA concepts offer ideal solutions for spider geometry; full-size, compact and UL.

gallery_217007_4746_60444.png


Edited by Oberon, 18 January 2017 - 12:24 AM.

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#72 Arjan

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Posted 18 January 2017 - 09:45 AM

I copied this from the new ES dob design. they're angled plate is a little different.

 

gallery_106859_3508_139075.jpg

This type of spider works extremely well, and can be made very low weight. I've used this in my first travel scope around 2000:

 

result_sec.jpg

 

Just two pieces of aluminum, three screws and a ball bearing:

 

b_spi2.jpg


Edited by Arjan, 18 January 2017 - 09:48 AM.

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#73 Bob4BVM

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Posted 18 January 2017 - 04:38 PM

Jonathan,

DO I recall in your early build notes on Merope, that you dispensed with all diagonal adjustment screws, and just used the tuners to set pitch as well as center ?

How did that work ?

If it's a go, I'd like to use that idea to set a 35* secondary angle for my low rider setup !

 

CS

Bob



#74 Oberon

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Posted 18 January 2017 - 06:31 PM

Bob, you remember correctly, there is no need for mirror adjustment screws with a wire spider configured as I have illustrated, as the tuners can take care of that. However in my case I never need to adjust the tuners either, as my Stewart Platform is all I need for collimation after initial construction set up.



#75 Jeff Morgan

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Posted 18 January 2017 - 09:55 PM

I have no direct experience with wire spiders, so I have to ask the question (assuming a standard non-Stewart truss configuration):

 

Wouldn't secondary collimation (or more accurately stated, Focuser Axial Error) elimination by the tuners be fairly tedious?




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