A question of balance

[AlistairW]

Hello,

My scope is fairly well balanced, however I have to put probably nearly 2kg of counterweight on the front end of the scope in order to counter the weight of the camera at the rear. This has always seemed an unnecessary burden on the mount.

My question is if I were to get a "long" Losmandy style plate, and fling the scope "forward", would this then allow me to possibly remove this excess weight ? - Indeed is this a better solution at all ?

Thanks

Alistair

[AlistairW]

.... or does it just make more sense to move the scope forward anyway on the current plate ?

I think my question would then be would if having the scope "so" far forward would affect balance\turning momentum etc ? - or does that matter - as simply if it's balanced then it is balanced ?

... thanks

[LightBucket]

Yes the idea would be to move the scope forward, then if you still can’t balance add more weight, so you only add weight if you have reached the limit of the dovetail lengths.

Looking at your set up I would guess you could slide that forward enough to balance quite easily, it won’t need that much as the centre of gravity will be shifted and so a small amount of weight pushed out further will act as a larger weight added...

P.S forgot to say, yes balanced is balanced, one more thing, the consensus is the weights on your mount counterweight bar are best nearer the mount, so add a bit more weight there and keep near as you can to the mount head, apparently that helps the motors....in saying this you seem to have done that already...

[widotje]

Hi. I'm not an expert on this.

However, i recently moved the dovetail of my scope backwards (or rather...the scope forward) because i've added a electronic filterwheel to the mix. 

It worked out quite nicely, scope was well balanced without adding extra weights. 

Only thing now is that my dovetail is now very close to the focusser, making it a bit harder to work with  the focusser.

Cheers!

 

[AlistairW]

Thanks for the replies. 

I have moved the scope forward on the plate, and that has allowed me to remove 1.5kg of front loaded counterweight. I am guessing as the rig is balances, that having the scope more forward than before is not an issue ? - that was my remaining concern . I expect he answer to be that it does not matter, as it is to do with moments about a centre point.

One thing I have noticed is that the remaining weights I do have placed a the front of the scope, need to be placed slightly off to one side to attain perfect balance. But I think that makes sense too as the distribution of weight on the scope due to focusers\wheels can not be even, - and having the weights slightly offset help balance this out.

 

Thanks

Alistair 

[LightBucket]

3 hours ago, AlistairW said:

Thanks for the replies. 

I have moved the scope forward on the plate, and that has allowed me to remove 1.5kg of front loaded counterweight. I am guessing as the rig is balances, that having the scope more forward than before is not an issue ? - that was my remaining concern . I expect he answer to be that it does not matter, as it is to do with moments about a centre point.

One thing I have noticed is that the remaining weights I do have placed a the front of the scope, need to be placed slightly off to one side to attain perfect balance. But I think that makes sense too as the distribution of weight on the scope due to focusers\wheels can not be even, - and having the weights slightly offset help balance this out.

 

Thanks

Alistair 

It doesnt matter that the scope is more forward, as it is balanced and that is what is important...

its all about balance around the centre point, so it makes no difference what you have one side or the other as long as balance is achieved..

[ollypenrice]

Balanced is indeed balanced but the moment of inertia varies as the square of the distance to the counterweights so if you balance with half as much weight twice as far down the shaft you'll double the moment of inertia working against the drives. It is best to have more weight closer to the mount. Take it from Astro Physics: http://www.astro-physics.com/products/accessories/mounting_acc/Balance_to_Optimize_Guiding.pdf

Olly

[Adam J]

Exactly this I recently added weight to my rig to reduce the moment of inertia....also I could walk past the scope with greater ease in the obsy without having to have the counter weight bar fully extended.

The main question is:

Is there anything actually wrong with your guiding?

If its not broken dont fix it!

I can guide down to 0.7 with a HEQ5 with much more kit on it....cant imagine your having too many problems with that tiny refractor.

Adam

[LightBucket]

8 hours ago, ollypenrice said:

Balanced is indeed balanced but the moment of inertia varies as the square of the distance to the counterweights so if you balance with half as much weight twice as far down the shaft you'll double the moment of inertia working against the drives. It is best to have more weight closer to the mount. Take it from Astro Physics: http://www.astro-physics.com/products/accessories/mounting_acc/Balance_to_Optimize_Guiding.pdf

Olly

Hmmmm, yes exactly what I said above.. but thanks for the endorsement...

[ollypenrice]

5 hours ago, LightBucket said:

Hmmmm, yes exactly what I said above.. but thanks for the endorsement...

Sorry, I just added the maths because what's surprising, to me at least (not being very mathematical), is just how much the moment against the drives is increased by using a lighter weight at a longer distance. 

Olly

[LightBucket]

3 hours ago, ollypenrice said:

Sorry, I just added the maths because what's surprising, to me at least (not being very mathematical), is just how much the moment against the drives is increased by using a lighter weight at a longer distance. 

Olly

Absolutely, I was amazed at the difference too, in fact I didn’t believe it at first, so had to get it checked out, my dad has a degree in maths and one in physics so got him to do the math and calculations for me....

[glowingturnip]

I think the moments of inertia calculation are for the effort needed to accelerate the angular velocity of the mount around one of the axes. 

When the mount is stable and guiding, there is no change in angular velocity, so it doesn't matter then whether you have a small weight far away or a big weight closer, or for that matter how far away your scope and camera are away from their joint c.o.g.

However, when the mount is slewing, that's when it's accelerating, and that's when it could put strain on the motors depending how far away your weights are.  Mind you, I do most of my big slewing with the eyepiece in and balanced for that, so no biggy.

Same principal how iceskaters accelerate when they pull their arms in btw.

Wikipedia - When a body is free to rotate around an axis, torque must be applied to change its angular momentum. The amount of torque needed to cause any given angular acceleration (the rate of change in angular velocity) is proportional to the moment of inertia of the body. Moment of inertia may be expressed in units of kilogram meter squared (kg·m2)

[ollypenrice]

1 hour ago, glowingturnip said:

 

When the mount is stable and guiding, there is no change in angular velocity, so it doesn't matter then whether you have a small weight far away or a big weight closer, or for that matter how far away your scope and camera are away from their joint c.o.g.

 

I don't think that's true. The guider is there to induce changes in angular velocity because it has found the velocity be be incorrect. Of course they are insanely tiny changes, far below the level demanded in most engineering applications, but they are changes none the less. And the precision we are looking for is comparably insane. A deviation limited to one third of an arcsecond (which is what I ask of my mounts) equates to about 3.9 millionths of a circle. (Please check my sums!!) At this level of precision I suspect that giving the mount/guider all the help it can get really does matter. 

Olly

[LightBucket]

1 minute ago, ollypenrice said:

I don't think that's true. The guider is there to induce changes in angular velocity because it has found the velocity be be incorrect. Of course they are insanely tiny changes, far below the level demanded in most engineering applications, but they are changes none the less. And the precision we are looking for is comparably insane. A deviation limited to one third of an arcsecond (which is what I ask of my mounts) equates to about 3.9 millionths of a circle. (Please check my sums!!) At this level of precision I suspect that giving the mount/guider all the help it can get really does matter. 

Olly

Yes, I tend to agree with you here, like you say “although they are tiny, they are changes nonetheless”.... anything you can do to help the mount has got to be good..

[ollypenrice]

4 minutes ago, LightBucket said:

Yes, I tend to agree with you here, like you say “although they are tiny, they are changes nonetheless”.... anything you can do to help the mount has got to be good..

Yup, and the debate around backlash-free mounts bears witness to this. Many owners feel that that such mounts deliver a tracking accuracy even better than their guide traces suggests. The guide trace only knows where the mount was pointing at the moment when each guide sub was taken, but where was the mount pointing between these guide subs? From the point of view of the final image this is very important indeed. Did it get back on target quickly after the guide input or more slowly? If you're using 4 second guide subs to average out the seeing you have 4 second dwell during which the mount could be doing anything at all.

Olly

[Alien 13]

Inertia is not the only thing to be considered, the counterweight is not directly connected to the mount head but through a long spring and low frequency resonance with the weight far from the pivot point is far more destructive to guiding than a heavier weight closer to it.

Alan

[mikeonnet]

Very interesting... I have been using 1 counterweight right near the end of the shaft to try and keep the whole rig as light as possible, taking into account the Max payload wieght should be halfed (at least) for AP etc.

Now after reading this, I have just added the extra counterweight with both weights near the top of the shaft to obtain balance and with the clutch released, it does feel much easier to turn with my hands, even though more weight has been added. Angular velocity (or I would have said - Centre of gravity ?) does make sense.

I just need to give it a go now and hopefully my PHD graph will be much more stable with less subs ruined. One downside though is it's going to be even heaver to carry back indoors now....

Thanks all.

[LightBucket]

59 minutes ago, mikeonnet said:

Very interesting... I have been using 1 counterweight right near the end of the shaft to try and keep the whole rig as light as possible, taking into account the Max payload wieght should be halfed (at least) for AP etc.

Now after reading this, I have just added the extra counterweight with both weights near the top of the shaft to obtain balance and with the clutch released, it does feel much easier to turn with my hands, even though more weight has been added. Angular velocity (or I would have said - Centre of gravity ?) does make sense.

I just need to give it a go now and hopefully my PHD graph will be much more stable with less subs ruined. One downside though is it's going to be even heaver to carry back indoors now....

Thanks all.

Yes I do exactly the same, I have two 5kg weights up close to the head, where as I could probably manage with just one much lower down the shaft... 

Of course you still need to keep within the mount weight limits...

[glowingturnip]

14 hours ago, ollypenrice said:

I don't think that's true. The guider is there to induce changes in angular velocity because it has found the velocity be be incorrect. Of course they are insanely tiny changes, far below the level demanded in most engineering applications, but they are changes none the less. And the precision we are looking for is comparably insane. A deviation limited to one third of an arcsecond (which is what I ask of my mounts) equates to about 3.9 millionths of a circle. (Please check my sums!!) At this level of precision I suspect that giving the mount/guider all the help it can get really does matter. 

Olly

I suspect that's not an argument that I'm going to be able to win without calculating the torque required to accelerate the rig's moment of inertia by at worst case 2x guide speed for one guide pulse (for a fast correction from one side of the mount to the other, full aggressiveness and no hysteresis in the guiding program) compared to the torque that the stepper motors are capable of delivering, and those motors are no slouches - lower rated motors are used in some very 'jerky' precision applications like 3D printing, laser etching, etc.  I think the maths is going to be beyond me though, so we'll have to agree to differ  ?

However, isn't there a case to say that you would prefer a smooth acceleration for a guiding correction, that you would get from a motor that is under load, rather than a jerky acceleration followed by a jerky deceleration and possible overshoot that you would get from a motor that is under very little load ?

Actually, I think the answer is going to be in the aggressiveness and hysteresis settings that we all tune in PHD - different torques and inertia loads will result in different tunings, and I think you'd be really struggling to spot the difference between the results from separately well-tuned guiding sessions for different inertia load-outs.

[ollypenrice]

4 hours ago, glowingturnip said:

I suspect that's not an argument that I'm going to be able to win without calculating the torque required to accelerate the rig's moment of inertia by at worst case 2x guide speed for one guide pulse (for a fast correction from one side of the mount to the other, full aggressiveness and no hysteresis in the guiding program) compared to the torque that the stepper motors are capable of delivering, and those motors are no slouches - lower rated motors are used in some very 'jerky' precision applications like 3D printing, laser etching, etc.  I think the maths is going to be beyond me though, so we'll have to agree to differ  ?

However, isn't there a case to say that you would prefer a smooth acceleration for a guiding correction, that you would get from a motor that is under load, rather than a jerky acceleration followed by a jerky deceleration and possible overshoot that you would get from a motor that is under very little load ?

Actually, I think the answer is going to be in the aggressiveness and hysteresis settings that we all tune in PHD - different torques and inertia loads will result in different tunings, and I think you'd be really struggling to spot the difference between the results from separately well-tuned guiding sessions for different inertia load-outs.

There are probably even more variables, come to think of it. Maybe a backlash free mount behaves differently from one with backlash. However, Astro Physics do come down clearly in favour of minimizing the moment of inertia by using more weights closer to the axis.

Olly

[Dragon_Astro]

After follow this thread and looking at the Astro Physics link, I tried moving my heq5s' weights further up towards the mount last night whilst imaging.

To me, the mount seemed to cope better and the guiding was a lot smoother and less erratic. In the past I've always extended the counter weight bar out fully, and I think especially for "beginners" it's assumed that's what you're "meant" to do, it even implies so in the manual. From now on I will definately have the weights further up