Counterweight bar. Long or Short ?

[LaurasStar]

Hi, this may seem a silly question but i’m looking at buying a counter weight for a scope and i’m wondering if the length of the pole makes any difference? Some people seem to have short poles so weights are closer to the scope and some have long poles with the weights on the end so very far away from the scope. 

The scope weighs roughly 4kg and there is also an azgti tracker if that makes any difference? 

[John]

Hi and welcome to the forum.

Generally I think its best to keep the counterweight close to the body of the mount. On the end of a long counterweight shaft there is more likely hood of vibrations.

 

[MarkAR]

Remember that the weight of telescope times the distance to pivot point has to equal the weight times distance of the weights.

Say the pivot point is 0.4m from the 4kg of the telescope, then you would need 4kg of weights 0.4m the other side to counter balance.

Moving the weights closer to the pivot say 0.2m means that you would need 8kg of weight. Alternatively it'll be 2kg at 0.8m distance.

[Bukko]

2 hours ago, LaurasStar said:

Hi, this may seem a silly question but i’m looking at buying a counter weight for a scope and i’m wondering if the length of the pole makes any difference? Some people seem to have short poles so weights are closer to the scope and some have long poles with the weights on the end so very far away from the scope. 

The scope weighs roughly 4kg and there is also an azgti tracker if that makes any difference? 

Hi and welcome to the forum.

It is not a silly question, but the idea of counterweights is to balance the load on the mount - just like a see-saw. Big heavy person on one end needs the same weight on the other (2 lightweight ones?).

Do you have the az-gti? have never used one, but it looks like it does not need counterweights. It can cope with a load up to 5kg so you SHOULD be OK but this really depends on exactly what scope you are going to fit.

Hope this helps and good luck.

Gordon.

[michael8554]

As already said, you can balance with a little weight on a long bar, or a heavy weight on a short bar. 

The heavy weight on a short bar has a lower Moment of Inertia..

Some say this reduces the load on the motors.

Michael 

[MarkAR]

I'm not sure about reducing load on the motors as mass x distance is the same either way but it will reduce moment of inertia.

 

[wornish]

This has been discussed on an earlier thread.

 

[joe aguiar]

I prefer less weight even if its lower rather than carry double the weight.

joejaguar 

[900SL]

I would have thought that it is beneficial to have the lens or scope as close to the RA axis as possible.  The farther the lens is from the RA axis (ie the greater the radius), the greater the swept path from any vibration, backlash or periodic error etc.

 

S = Radius x theta where S = arc length and Theta is the angular movement therefore to minimise pixel movement keep R as low as possible.  

[pete_l]

On 10/03/2020 at 18:03, LaurasStar said:

the length of the pole makes any difference

Yes, it does.

The longer the length of the counterweight bar, the more "whippy" it is. And that leads to greater vibration whenever a tracking correction is made. A shorter bar, even if it the same diameter, will be much stiffer. The stiffness decreases much faster with increasing length than the need for less weigh improves the situation. So it is advantageous to have a short bar with more counterweights than a long bar with fewer.