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CGEM: how big of an impact is a longer counterweight shaft?

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#1 FlorinAndrei

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Posted 23 September 2020 - 03:42 AM

The theory sounds simple: to achieve the same effect, either use bigger counterweights, or a longer counterweight bar. I am thinking to replace the default bar with a new one that's 20" long, and avoid using very heavy iron hockey pucks.

 

The total weight of the telescope + bar + counterweights will be less. That means less friction in the bearings. So that's good, no?

 

But the total moment of inertia (rotational inertia), if the telescope weight does not change, will remain the same. It will just be accomplished with less dead weight, spread further away from the polar axis. So that sounds about the same.

 

Bottom line is - will I see a more relaxed tracking, less struggling autoguiding, compared with using a shorter bar and heavier counterweights? What's your experience?

 

Oh, and my back will benefit from carrying smaller steel hockey pucks across the field. smile.gif



#2 xtrmfit

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Posted 23 September 2020 - 03:55 AM

The moment of inertia will increase as you move the weights farther from the RA axis.



#3 macdonjh

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Posted 23 September 2020 - 07:58 AM

Here is a good thread with a fairly comprehensive discussion of the differences between "more weight on a short shaft" versus "less weight on a long shaft".  

 

https://www.cloudyni...ent-of-inertia/

 

Let me try that again:

https://www.cloudyni...ent-of-inertia/


Edited by macdonjh, 23 September 2020 - 08:01 AM.

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#4 aa6ww

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Posted 23 September 2020 - 01:50 PM

I notice some mounts are now sold with very short retractable counterweight shafts but have an extension shaft if needed. The shorter shaft supresses vibrations more, which to me is the benifit of more weight on a shorter shaft, keeping the center of gravity closer to the center of the mount.

I think if your mount is right at the edge of stability, the shorter shaft and more weight could help tame the vibrations. If you still have margin, anything that balances should be ok. 

I have a CGEMII and I just feel better using two smaller 12 pound counterweights slid higher up the shaft with my APM-152 vs using the single 17 pound counteright further down the counterweight shaft. Either way in my case, I haven't noticed any differences, but I havent been able to use higher power magnifications on my 6" refractor since I bought my CGEMII because of bad seeing and smoky conditions. 

I am using a Losmandy G11 tripod with 4" legs vs the standard CGEM tripod, so the stability of my entire system seems to be improved with the G11 tripod.

 

 ...Ralph


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