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Amanda's 8" f/5 Dob

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#26 SeaBee1

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Posted 10 January 2019 - 09:17 PM

When I built my 10 inch, 3 pole scope, I used a clamping system that works very well, for all intents and purposes, zero flex. Collimation holds tight from horizon to zenith... but it is not light on its feet. If I ever rebuild it, it will be a traditional truss scope... and lighter...



#27 jtsenghas

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Posted 10 January 2019 - 09:58 PM

When I built my 10 inch, 3 pole scope, I used a clamping system that works very well, for all intents and purposes, zero flex. Collimation holds tight from horizon to zenith... but it is not light on its feet. If I ever rebuild it, it will be a traditional truss scope... and lighter...

CB, when you built that scope I discussed these same formulas with you about the stiffness of thin walled tubes varying with the fourth power of the diameter.  Your tubes are adequately large. What are they  1 3/4" diameter?  Zane's, unfortunately as we now know, are a mere 1/2".  Even if the ends are clamped as well as you did with widely spaced wooden clamps, I'm afraid it will suffer serious collimation shift with altitude. I was trying to put it delicately....

 

Those 1/2" poles would do just fine as a truss in tension and compression on a small scope. In bending, probably not well enough on a strut scope. 

 

Zane, I encourage you to plug in some numbers into that calculator I sent you to get a feel for this.


Edited by jtsenghas, 10 January 2019 - 10:17 PM.

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#28 Oberon

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Posted 11 January 2019 - 12:50 AM

Thanks.

 

I'm going to try to re-use the existing rocker, but if not I can at least re-use the ground board.

 

How wide (interior and exterior) does the mirror box need to be? I'm attaching the poles to the sides.

You can use the existing rocker and ground board if you follow the tip I gave you previously.

 

However...if you want simple and effective, then you want a hexapod. Don’t get confused by all of us on the hexapod thread making life hard for ourselves working out the ultimate purist engineering refinements; a basic hexapod is dead simple to build, and simpler than what you are doing now. Trust me here, it is easier to build truss collimation than it is to build mirror cell collimation, and far far easier to use.

 

Suggest you use cheap 1/4” Heim joints and simply bolt them to the sides of your UTA and LTA with wingnuts pretty much exactly as you are doing now. Your only trick is to pot some RH and LH nuts in the tubes, but this not hard and is described in detail the the hexapod thread. Once you learn how to build and use a hexapod you will never go bother with anything else.


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#29 SeaBee1

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Posted 11 January 2019 - 08:30 AM

CB, when you built that scope I discussed these same formulas with you about the stiffness of thin walled tubes varying with the fourth power of the diameter.  Your tubes are adequately large. What are they  1 3/4" diameter?  Zane's, unfortunately as we now know, are a mere 1/2".  Even if the ends are clamped as well as you did with widely spaced wooden clamps, I'm afraid it will suffer serious collimation shift with altitude. I was trying to put it delicately....

 

Those 1/2" poles would do just fine as a truss in tension and compression on a small scope. In bending, probably not well enough on a strut scope. 

 

Zane, I encourage you to plug in some numbers into that calculator I sent you to get a feel for this.

 

Good morning J.T.! I do remember that discussion. I couldn't remember what size poles I used, however, so I just checked... 1.25", and steel, not aluminum.

 

You are quite correct, 1/2" poles for a three pole strut scope seems terribly light. Based on my experience with the 1.25" poles, I think he MIGHT get by with 1.25" using the design he is considering, it is an 8 inch f/5 after all. If his UTA is fairly light, even better. I would suggest he use either an offset spider or wire spider to reduce flexure there as well.

 

We all live and learn... you taught me a bunch... and many thanks!

 

CB


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#30 Augustus

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Posted 11 January 2019 - 12:09 PM

So.... if I want to use the current poles, does doing 6 in a traditional truss arrangement suffice? 


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#31 SeaBee1

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Posted 11 January 2019 - 12:25 PM

So.... if I want to use the current poles, does doing 6 in a traditional truss arrangement suffice? 

 

I think the short answer is... probably not satisfactorily. HOWEVER... Albert Highe in his book does describe ways to do it... as long as certain rules are followed... I can't remember if the pole size you have fits into his rules... he does describe a method of testing to find out if what you want to do can be done...



#32 Augustus

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Posted 11 January 2019 - 12:38 PM

I think the short answer is... probably not satisfactorily. HOWEVER... Albert Highe in his book does describe ways to do it... as long as certain rules are followed... I can't remember if the pole size you have fits into his rules... he does describe a method of testing to find out if what you want to do can be done...

Kriege's math says I could get away with 0.5-inch diameter poles, but that's assuming 8 poles and a full-sized UTA weighing several pounds. My UTA is three, maybe three and a half pounds with the focuser, finder, secondary, and an eyepiece installed. 

 

I'm going to try it because I already have the parts, and if it really doesn't work I'll buy some new poles.


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#33 jtsenghas

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Posted 11 January 2019 - 04:09 PM

It takes only a modest increase in diameter to make a six pole arrangement as stiff as an 8 pole arrangement with the same materials.  I say give it a shot.  I generally build with what I have rather than what I haven't.  At worst I learn something and have parts left over when I replace them for use in another project.


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#34 Oberon

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Posted 11 January 2019 - 06:02 PM

Thirty years ago I built my first 8” dob with 1/2” plastic coated steel curtain rods, flattened on the ends. Solid as a rock. Heavy steel UTA, heavy timber box. 

 

Go for it. But be prepared for plan B.

 

The trick with 6 poles is that flattening the pole ends is not likely to work due to compound angles. Hence the Heim joints, but balls will do too. By which point you may as well go hexapod.


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#35 jtsenghas

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Posted 11 January 2019 - 06:19 PM

... Hence the Heim joints, but balls will do too. By which point you may as well go hexapod.

Just note, you'll have to make or buy some hardware if you go this route  Half of your ends will need to be left handed. Even if you use heim joints, which I agree is probably the simplest and still reasonably cheap way to go on hexapods, you'll have to either get left handed Nyloc nuts to match your heim joints, or left handed taps and be willing to do a fair amount of busy work. I've noticed that more left handed hardware is available in metric rather than inch dimensions, so 6 mm rather than 1/4" heim joints may be a better choice. 

 

As for me on my current build, I don't consider my time to be valuable, although progress will slow to about one day per week. I've got some left handed 1/4"-20 taps and I'm making the mating hardware.

 

Whatever you do... fixed truss or adjustable, keep your forces in line with your poles by having your attach points in line with their axes.  This is more important with these 1/2" tubes than on your previous bigger builds. 


Edited by jtsenghas, 11 January 2019 - 06:20 PM.


#36 SeaBee1

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Posted 12 January 2019 - 11:07 AM

Kriege's math says I could get away with 0.5-inch diameter poles, but that's assuming 8 poles and a full-sized UTA weighing several pounds. My UTA is three, maybe three and a half pounds with the focuser, finder, secondary, and an eyepiece installed. 

 

I'm going to try it because I already have the parts, and if it really doesn't work I'll buy some new poles.

 

As you should... It wasn't my intent to discourage you from trying, only to make you aware there could be some adjustments needed to make it work. I would pay particular attention to J.T. he was very instrumental in the success of my 10 inch build!

 

CB


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#37 Augustus

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Posted 17 January 2019 - 08:10 PM

Started work on the lower tube assembly ("mirror box") and ordered another two poles. You guys were right about the difficulties of mounting six!

 

 


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#38 Augustus

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Posted 18 January 2019 - 08:21 PM

More progress today. I shrunk the mirror cell in height so that the entire LTA is only 2.25" in height.



#39 Augustus

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Posted Yesterday, 04:21 PM

Only a little left to do! Just need to cut and attach the trusses, add the floatation points to the mirror cell, do some more finishing, and install the optics, and I'm done!

 

utacompletes.jpg

 

UTA is 100% complete, lower assembly needs more varnish/wax.

 

IMG_1080s.jpg

 

IMG_1081s.jpg


Edited by Augustus, Yesterday, 04:22 PM.


#40 ccaissie

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Posted Today, 12:35 AM

 

 

Does anyone know of a cheap source of aluminum tubing?

 

Our club still has a bunch of titanium tubing...it's about 7/8" dia x about .020 wall...like a feather but strong.




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