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Tension springs vs counterweights for Dobs

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#1 MMICKELS

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Posted 20 January 2004 - 01:16 AM

I have built a 12.5" F6 truss tube Dob. This is my first telescope. I've been having some problems with the balance. It is O.K. indoors but when it is being used outside it tends to drift in the altitude. The bearings are aluminum with velcro attached. My question is, is it better to use a tension spring or counterweights.

Thanks,

Mark Mickels

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#2 Peter Argenziano

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Posted 20 January 2004 - 01:40 AM

Hey Mark,

Nice design! It looks like a nice, light rig.
Tension springs are not normally used to achieve balance, but rather to increase tension on the altitude bearings.

I don't understnad why the balance on your scope changes between indoors and outdoors?

Peter

#3 Trever

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Posted 20 January 2004 - 02:40 AM

Maybe there is swelling between the surfaces of the axis when you enter the heated indoors and they contract when you go out in the cold outdoors? I would use counterweights so that any movement you use is smooth and balanced.

Otherwise, if you have a tight tension, it will be stiff and will overshoot the objects you are viewing.

#4 Mike Hosea

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Posted 20 January 2004 - 03:34 AM

Seems like you could design a sliding counterweight for the top strut that would make it easy to adjust for any eyepiece. Springs are used sometimes in lieu of counterweights, not like Orion has done but like several ATMs have done to provide variable tension as the scope is moved in altitude. It's your call. Both can work, but I think counterweights are easier to get right.

#5 MMICKELS

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Posted 20 January 2004 - 09:48 AM

Peter,
I'm thinking that the cold makes the teflon and velcro less sticky.


Thanks,

Mark

#6 Scott Hamilton

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Posted 20 January 2004 - 09:57 AM

It seems to me that tension springs defeat the purpose
of light, buttery smooth movement by increasing the
friction.

Try Tom Kracji's virtual counterweight scheme in which a
spring on the rocker box is rigged through a pulley to a
fixed mounting point on the altitude bearing. The beauty
of this idea is that it works at all tube elvations from
0 to 90 degrees.

This is due to the fact that the pulley changes the tension
developed by the spring from linear to sinusoidal which
exactly mimics the dynamics of the shift in balance as the
scope swings from a vertical to a horizontal position.

I have used this construction principle on a paralellogram
bino mount and am adapting it for my 8" Dob.

It works like a charm.

See:

http://overton.tamu.edu/aset/krajci/

a link which I hope is still valid. Otherwise check on
Mel Bartels ATM site.

Scott

#7 MMICKELS

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Posted 21 January 2004 - 02:01 AM

Mike,
After using the telescope again tonight I think that the sliding counterweight Idea is the most feasable. It would be infinitely adjustable and very simple to make. Thanks to everyone for their ideas!

Mark Mickels

#8 Peter Argenziano

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Posted 21 January 2004 - 11:12 AM

After using the telescope again tonight I think that the sliding counterweight Idea is the most feasable.



Mark,

Good luck with your project, and let us know how it turns out.
I'm sure you're already aware of this... a virtual counterweight 'moves' the center of gravity, which could introduce some instability - especially if you are using an equatorial platform.

Peter

#9 rboe

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Posted 21 January 2004 - 06:45 PM

You didn't say if the scope seemed to be nose or tail heavy. If tail heavy maybe you just need to drill a hole or two to store extra eye pieces up top.

If nose heavy, see above. But it does appear that you can move the bearings to change your balance point and avoid all that rigarmarole.

#10 MMICKELS

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Posted 22 January 2004 - 12:18 AM

The scope is nose heavy. I can't move the bearings any farther up on the truss tubes without running into the bottom of the rocker box. I'm going to make a brass counterweight for the top tube when I have the time. Unfortunately work comes first.

Mark

#11 MMICKELS

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Posted 22 January 2004 - 08:17 PM

I've made the counterweight. It is probably heavier than needed, but I don't have my 8x50 optical finder mounted yet. I gave myself an extra .005 on the internal diameter to provide a good slip fit. I also made a dowell pin from delrin. It goes inside the tapped hole so that the screw does not ding the truss tube,

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#12 MMICKELS

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Posted 22 January 2004 - 11:31 PM

Sorry the picture didn't load on the last attempt

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#13 Stacy

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Posted 22 January 2004 - 11:44 PM

That is an awesome looking telescope!

I think the counterweight should be red as well..

#14 imjeffp

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Posted 23 January 2004 - 10:27 AM

Why did I waste my time in academics when I should have been taking metal shop?

#15 Tom L

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Posted 23 January 2004 - 01:42 PM

Out of curiosity, what is the wieght of the entire scope? Beautifully done!

#16 MMICKELS

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Posted 23 January 2004 - 02:00 PM

The scope weighs 65 lbs minus batteries and counterweight. I cut off 250 lbs of aluminum during the project. It has an Rfroyce 12.5 conical mirror, protostar quartz secondary, and a HC2 focuser.

#17 MMICKELS

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Posted 25 January 2004 - 09:34 PM

I used the scope last night with the new counterweight. It worked great! Thanks to everyone for their advice.


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