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Why 2 c/weights rather than 1?

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Hey all,

I see some people using two counter weights on their mounts when one would suffice right at the bottom of the counterweight bar. Why do people do this?

My understanding is that you're now loading up twice the weight on the mount and that eats into your "imaging weight" allowance. Unless I'm missing something here, that seems purely detrimental?

Can anyone cast light on this for me?

Many thanks.

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In my case the answer is simple, one doesn't suffice even at the bottom of the bar, so I have to use two.

Eric.

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The counterweights don't count towards the load on the mount so that doesn't affect the decision. I believe it is to do with wanting to reduce the moment of inertia. A smaller weight further from the mount is more likely to suffer flex and vibration in the counterweight shaft, and take longer to settle down. General advice would be to have more weight closer to the axis of rotation. Some discussion on it here:

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11 minutes ago, Stu said:

The counterweights don't count towards the load on the mount so that doesn't affect the decision. I believe it is to do with wanting to reduce the moment of inertia. A smaller weight further from the mount is more likely to suffer flex and vibration in the counterweight shaft, and take longer to settle down. General advice would be to have more weight closer to the axis of rotation. Some discussion on it here:

I like this quote from that page: "Guiding with a shaft extension is like drawing on a post-it with a ten foot pole. It can be done. With a lot of beer."

I share the view that it's better to have more weight closer to the axis (within reason) because the moment of inertia varies with the square of the distance from the axis.  The greater the moment of inertia, the greater the resistance to changes in motion such as guiding inputs.

With the HEQ5/NEQ6 I usually put the counterweight(s) at the end of the bar and then push the bar back up inside the mount until everything balances.  That's ok for me because the only time I use the polar scope is when I've taken the mount off the pier and I need to get it roughly aligned when I replace it.

James

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Right I see, thank you. Also, thanks for clearing up whether the couterweights were considered with the mount's payload capacity. I always wanted to doubt it did.

So in a sense then it's better to use two smaller weights (or possibly both 5kg weights) higher up on my c/w shaft? Hm. I might try that then and see if that further improves my mount's performance!

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I thought it was more in line with keeping the weight as far inboard as possible, I tend to use as few as possible though.

Alan

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Having two counterweights instead of one heavier one also makes the mount more versatile. I use just one counterweight on my Vixen Great Polaris when using the APM 80mm F/6, but two with the C8 and planetary camera set-up, and even three if I add the big finder scope for visual DSO hunting.

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18 hours ago, Stu said:

The counterweights don't count towards the load on the mount so that doesn't affect the decision. I believe it is to do with wanting to reduce the moment of inertia. A smaller weight further from the mount is more likely to suffer flex and vibration in the counterweight shaft, and take longer to settle down. General advice would be to have more weight closer to the axis of rotation. Some discussion on it here:

Stu, that is correct and the only reason is Moment of Inertia. Some builders DO restrict the amounf ot Counterweights and Urban Myth more or less developed to the believing that the amount of counterweights is equal to the carrying capacity but that will never make you happy ...

The key word here is moment of inertia and let us make a simple calculation.

You balanced your mount with a 10 kg weight at 40 cm away from the RA rotation center.  This gives you then a factor of 400

Ok, you can also balance your mount using 2x 10kg weights at 20 cm away from the RA rotation center which gives you a factor of 400

That is for balance.

Now Moment of inertia is calculated Mass x Radius² and that means for

case 1 = 10 x 40² = 10 x 1600 = 16,000 Moment of Inertia

case 2 = 20 x 20² = 20 x 400 = 8,000 Moment of Inertia

¿ Which case is easier to " change direction of movement " ? Like we do when autoguiding ...

Edited by Rainer
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Consider when you balance a scope.....when horizontal it is perfectly balanced....if you change the angle, it wants to come back to the horizontal position.

Therefore when it is in use and not horizontal it is not balanced.....this would be pretty much all the time...unless you were observing the Horizon.

Are we doing it wrong !

In my opinion the only way to get it to be balanced at any angle is for counter wieghts to be used that exactly match the wieght of the equipment and for these weights to be placed at the same distance as the centre of mass of the equipment.

Actually the Horizontal method above would work fine.....the important bit is the counter weights matching the equipment.

In an ideal world you would have a selection of counter weights to exactly match the equipment.

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19 minutes ago, Kev M said:

Consider when you balance a scope.....when horizontal it is perfectly balanced....if you change the angle, it wants to come back to the horizontal position.

Therefore when it is in use and not horizontal it is not balanced.....this would be pretty much all the time...unless you were observing the Horizon.

Are we doing it wrong !

In my opinion the only way to get it to be balanced at any angle is for counter wieghts to be used that exactly match the wieght of the equipment and for these weights to be placed at the same distance as the centre of mass of the equipment.

Actually the Horizontal method above would work fine.....the important bit is the counter weights matching the equipment.

In an ideal world you would have a selection of counter weights to exactly match the equipment.

Since I moved to a dual rig, I now have my scopes balanced in 3 axis.  This means that it balanced no matter where it's pointing.  I think this is how we imagers do it anyway?

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29 minutes ago, Kev M said:

In my opinion the only way to get it to be balanced at any angle is for counter wieghts to be used that exactly match the wieght of the equipment and for these weights to be placed at the same distance as the centre of mass of the equipment.

No, that is not correct as the center of gravity has to be the same ...

I have 54 kg of counterweights on my mounts but know that my equipment just weighs maybe 42 or 43 or 44 or 45 kg and it is perfectly balance out with 54 kg of counterweights ...

¿ why ? Very easy

54 kg counterweights at let me say at a Center of Gravity  of 35 cm will perfectly balance out 43 kg at ~ 44 cm Center of Gravity.

Lever arm law ... as I posted a link above

Edited by Rainer

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6 minutes ago, Rainer said:

No, that is not correct as the center of gravity has to be the same ...

I have 54 kg of counterweights on my mounts but know that my equipment just weighs maybe 42 or 43 or 44 or 45 kg and it is perfectly balance out with 54 kg of counterweights ...

¿ why ? Very easy

54 kg counterweights at let me say at a Center of Gravity  of 35 cm will perfectly balance out 43 kg at ~ 44 cm Center of Gravity.

Lever arm law ... as I posted a link above

Look here at one of my mounts and yes that is most upper position of my counterweights as if the RA runs the most upper CW passes with a clearance of 1 mm from the mounts body. If I take off one of the 12 kg upper counterweights everything would be at the lower end.

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18 minutes ago, Rainer said:

No, that is not correct as the center of gravity has to be the same ...

I have 54 kg of counterweights on my mounts but know that my equipment just weighs maybe 42 or 43 or 44 or 45 kg and it is perfectly balance out with 54 kg of counterweights ...

¿ why ? Very easy

54 kg counterweights at let me say at a Center of Gravity  of 35 cm will perfectly balance out 43 kg at ~ 44 cm Center of Gravity.

Lever arm law ... as I posted a link above

So at an angle of 45 degrees if you let it go it stays there and doesnt return to the horizontal ?

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1 minute ago, Kev M said:

So at an angle of 45 degrees if you let it go it stays there and doesnt return to the horizontal ?

I know this question isnt addressed to me, but yes I can move my scope literally anywhere, unlock the clutches, and it wont move from that position.

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That sounds ideal, I want to get mine balanced the same

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50 minutes ago, Kev M said:

So at an angle of 45 degrees if you let it go it stays there and doesnt return to the horizontal ?

Yes that is correct. Once you have balanced in horizontal the Center of Gravity will remain the same for both sides ...

The length of the lever arm is equal to the position of the center of gravity and so in any position it will remain without moving. In the moment you have done it in real life then you will understand it.

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49 minutes ago, Kev M said:

That sounds ideal, I want to get mine balanced the same

This sounds like you do not get it balanced. Can you post some pics ?

And now that I am writing this yes it is possible that when you have balanced in horizontal and you put it in another angle that you setup could move to some side.

You can check this after having it balanced in horizontal and then put the whole set up in vertical position, If it stays there then you have it well balanced but if it starts to move to one side that means that you have an asymmetric weight distribution on the telescope side, which could be a finder scope, or a motor focuser, ot the filter wheel of the camera, etc. or whatever. if you see this then you have to add some weight on the opposite side to which it moves or rotate the scope in its rings, if possible, to get a symmetric weight ratio through the vertical line. Once you made that you will need to check balance in horizontal position again.

You can even have an asymmetric balan on the counterweight side just by putting the knobs for tightening not directly over the shaft but to one side or the other ... of course it depends on how well or easy the RA shaft rotates. If the mounts has ball nearings then this is easily measurable. If there ar no ball bearing then this can be a source of stiction and balancing gets difficult as the stiction can hold up some wight distribution.

Edited by Rainer

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Thanks for the wealth of advise, I will try this again, I might have missed something obvious.

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2 hours ago, Rainer said:

No, that is not correct as the center of gravity has to be the same ...

I have 54 kg of counterweights on my mounts but know that my equipment just weighs maybe 42 or 43 or 44 or 45 kg and it is perfectly balance out with 54 kg of counterweights ...

¿ why ? Very easy

54 kg counterweights at let me say at a Center of Gravity  of 35 cm will perfectly balance out 43 kg at ~ 44 cm Center of Gravity.

Lever arm law ... as I posted a link above

I missed the first point you raised above, I was looking at the balance between the equipment & the counterweights, but forgot the equipment itself....

I am assuming that your equipment is balanced above and below the Blue line I have added.....

Mine has the guidescope off to one side and it is this that is dragging it down.

It will balance horizontal but when moved up or down will return there as the centre of gravity is no longer at the pivot point ( yes i googled that bit ).

With the guidescope mounted directly above the Main scope it stays at what ever angle it is placed at ( RA only ....still need to balance it for Dec, may need a longer dovetail )

I had the guidescope there so I could fit a widefield camera the other side...when I do that I may have to rebalance it again.

Thanks again...for helping a thicko

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2 hours ago, Kev M said:

I missed the first point you raised above, I was looking at the balance between the equipment & the counterweights, but forgot the equipment itself....

I am assuming that your equipment is balanced above and below the Blue line I have added.....

Mine has the guidescope off to one side and it is this that is dragging it down.

It will balance horizontal but when moved up or down will return there as the centre of gravity is no longer at the pivot point ( yes i googled that bit ).

With the guidescope mounted directly above the Main scope it stays at what ever angle it is placed at ( RA only ....still need to balance it for Dec, may need a longer dovetail )

I had the guidescope there so I could fit a widefield camera the other side...when I do that I may have to rebalance it again.

Thanks again...for helping a thicko

No Problem.

Quote

I am assuming that your equipment is balanced above and below the Blue line I have added.....

That is correct. And the balance there you achieve with the method below.

Sometimes it is easier to balance a side by side set up as you have the freedom to move the assembly left or right and so find the correct balance when the CW shaft is vertical.

Somewhere I recommended to make a simple side by side set up e.g.  turn the DEC saddle 90° and even if you only have on scope then you have the freedom of sliding the scope left or right and so get vertical balance ... That even gives you the freedom to have you accessories in the orientation you like best  ...

So you balance first with CW shaft horizontal. Then you put it vertical and move the scope left or right and then, I do it that way I balance the DEC axis.

Simple as that.

Many people say balancing side by side is a 3 axes balancing. Well yes, but a normal scope set up also needs that either by adding weights ( 3rd axis) or by rotating until the whole scope weight is 50/50 over the vertical axis e.g. CW shaft ...

Balancing means I have always 50/50 of the weight of each side of the rotation axis. RA has a rotation axis and DEC also has a rotation axis.

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3 hours ago, Kev M said:

I missed the first point you raised above, I was looking at the balance between the equipment & the counterweights, but forgot the equipment itself....

I am assuming that your equipment is balanced above and below the Blue line I have added.....

Mine has the guidescope off to one side and it is this that is dragging it down.

It will balance horizontal but when moved up or down will return there as the centre of gravity is no longer at the pivot point ( yes i googled that bit ).

With the guidescope mounted directly above the Main scope it stays at what ever angle it is placed at ( RA only ....still need to balance it for Dec, may need a longer dovetail )

I had the guidescope there so I could fit a widefield camera the other side...when I do that I may have to rebalance it again.

Thanks again...for helping a thicko

I took some images of my other set up but it is the same game.

As you can see the heavier side e.g. the two Takahashi scopes are nearer to the center line then the AstroTech 8" RC

Edited by Rainer

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Lots of people claim that the counterweights don't count to a mounts payload for imaging.  Is that really true?

The manufacturers are very careful in what they quote in their advertising blurb and I have yet to find one that actually says that in plain English. i.e excluding counterweights.

I  struggle to understand that adding say 10Kg of weights to a mount doesn't impact is load / tracking capabilities.  What magical physics is going on?

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4 minutes ago, wornish said:

Lots of people claim that the counterweights don't count to a mounts payload for imaging.  Is that really true?

The manufacturers are very careful in what they quote in their advertising blurb and I have yet to find one that actually says that in plain English. i.e excluding counterweights.

I  struggle to understand that adding say 10Kg of weights to a mount doesn't impact is load / tracking capabilities.  What magical physics is going on?

This is from Celestron's knowledgebase:

"Celestron's payload capacity is the weight the mount and dovetail clamp can hold when adding on the optical tube and the accessories you wish to use with your scope (Finderscope, StarSense, Camera, etc).  Celestron's payload capacity does not include the weight of the counterweights."

And in the specs section of the manual for Skywatcher's EQ8, it says:

"Payload (Rated for astrophotography; counterweights excluded)   50kg"

Same descriptive text for the EQ6-R, but the payload is 20kg.

James

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well this isn't what u asked for but my 2 cents is the opposite I rather have 1 CW and it be lighter verses 2 and be heavier.

for visual and using a manual mount with slow motion controls its not a big deal if your few smidgen from being 100% true balanced. its when u doing imaging or have goto that if its not balanced properly you can trip the gears and motors.

joejaguar

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19 hours ago, wornish said:

What magical physics is going on?

None. The motors do not have much to do when it is very well balanced. I do not say perfect as perfect balancing is nearly impossible.

It gets critical when you do auto guiding and need to move it fast for- and backward and there is where the Magic of the " Moment of Inertia " gets into our way.

When setting guide commands very often then the mount can get into a resonance state and you will never get it out of it due to the pendulum effect ...

And so we get now into the discussion of many more variables but this is not the main topic of this thread ...

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