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A balance question

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#1 Ben the Ignorant

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Posted 08 April 2021 - 01:37 PM

I am 95% sure I will buy a 115mm (Teleskop Service) triplet in the upcoming weeks, it will ride on a manual equatorial or a manual altaz mount and it weighs 7 kilos with rings and Vixen rail. A 2" diagonal (450grams) plus a 30mm or 50mm finder (at least 300grams) will complete it, and my heaviest eyepiece weighs 850 grams. That's 8.6 kilos total but probaly a half-kilo more because my smaller apos need a sliding counterweight along the tube and this bigger one can't be an exception. We're talking 9 kilos in all likelihood.

 

So, how much counterweight will the mount need?



#2 havasman

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Posted 08 April 2021 - 02:21 PM

That's going to vary with the mount. Different designs have different requirements.


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#3 TimK

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Posted 08 April 2021 - 02:37 PM

Balance is Balance.

 

9 Kgs at X millimetres from axis centerline. = Y kgs of counterweight Z millimetres from axis centerline.  (Assuming the mount balances on it own axis.)

You will also need to account for the weight of the counterweight shaft.



#4 Ben the Ignorant

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Posted 08 April 2021 - 03:03 PM

That's going to vary with the mount. Different designs have different requirements.

Yes, I should have been more specific. The mounts in question are the classic Sky-Watcher EQ5 and the TS AZ-5.

 

https://www.teleskop...-bis-10-kg.html

 

https://www.teleskop...erstellung.html

 

I want to keep my rig as lightweight as possible, placing the weights at the very end of the arm, so for those who have experience with the 115mm triplet, what are the smallest counterweights I can use? I swapped the counterweight on my 80mm triplet + EQ3 mount and the saved kilo (or 0.8 kg?) makes a noticeable difference when I carry it.



#5 havasman

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Posted 08 April 2021 - 07:50 PM

Balance is Balance.

 

9 Kgs at X millimetres from axis centerline. = Y kgs of counterweight Z millimetres from axis centerline.  (Assuming the mount balances on it own axis.)

You will also need to account for the weight of the counterweight shaft.

A DiscMounts DM4 or DM6 takes NO counterweights. A Rowan AZ100 may or may not need counterweights for a load w/in its allowable range. Some designs incorporate measures to "absorb" imbalance and change the simple balance ratio. There are several other examples of mounts that, by design, may be said to offset the axis of balance including the Rainbow Astro. Sometimes this is specifically done to avoid the use of counterweights.

It is not so simple a matter as balancing on a fixed fulcrum.

Hence the requirement for balance weights for a given scope on an otherwise undefined manual alt/az or EQ mount does in fact depend on which specific mount is used.


Edited by havasman, 08 April 2021 - 09:34 PM.


#6 SeattleScott

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Posted 08 April 2021 - 08:49 PM

The Vixen mounts require less counterweight due to their design and where they put the motors, essentially using a motor as a counterweight.

Generally it seems like the required counterweight is a bit less than the scope but it depends on the length of the shaft, etc. Just have to try and see. I would guess around 7-8 kilos.

Scott

#7 Ben the Ignorant

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Posted 09 April 2021 - 12:39 AM

Some designs incorporate measures to "absorb" imbalance and change the simple balance ratio.

Interesting but they are probably outside of my budget. I will look them up, though, because I was not aware of these mechanisms.

 

 

Generally it seems like the required counterweight is a bit less than the scope but it depends on the length of the shaft, etc. Just have to try and see. I would guess around 7-8 kilos.

That seems like a lot relative to the 9 kilos of the scope load. The tripod legs will be filled with gravel for stability, making the total setup 3 or 3.5 kilos heavier. It's been proven effective on my other scopes but now the whole combo is bound to become rather heavy.

 

Experience tells me (balanced) weight is not negotiable as a stability asset but when it's time to carry the thing weight is annoying. The 80mm went from 20 to 19 kilos thanks to a lighter cylinder of steel and the small removed weight is already a nice relief.

 

Thinking of that makes me a bit worried of the total weight of the 115 when I get it.



#8 SeattleScott

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Posted 09 April 2021 - 12:43 PM

7-8 kilos could be high. A lot depends on the length of the shaft. My CG5 had a rather short shaft and I couldn’t balance a 26lb scope (technically maybe 27.5 with eyepiece) using 22lbs counterweight. Rather than up the counterweight, I got a shaft extension. The further out you attach the weights, the less weight you need. And the less stable your scope will be. So if you like to keep the weights pretty close in for optimal stability, probably looking at 7-8 kilos. If you attach them further out you can get by with less weight. Also my SXW doesn’t need as much counterweight due to the mount design. So it can vary with the mount.

Scott


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