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How much benefit to more weight closer to RA axis?

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#1 AnthonyQ

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Posted 03 March 2021 - 03:48 PM

I am trying to understand whether or not I would see a substantial improvement in RA guiding if I added an 11 lb counterweight so that I could slide my weights closer to the RA axis on my counterweight shaft. (Another $90...)

Currently RA tends to trail Dec +/-2/1 in quality, and I have done lots of other improvements to PHD2 settings, mount enhancements etc. over the past couple of years. Guiding is typically pretty good when seeing permits, (+/- .6"-.8", occasionally better/worse), but as I mentioned RA is worse than Dec (typically would be something like .32" Dec, .68" RA).

I am currently imaging at .78"/px so not necessarily an issue at the moment, but I might want to change that at some point, and on lesser-quality seeing nights, maybe my guide graph could stay below that with a better counterweight arrangement?

I know the best practice is more weight, further up the shaft, but how much does this matter and would I see improvement in my guide graph?

Image for reference in next post


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#2 AnthonyQ

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Posted 03 March 2021 - 03:50 PM

See current CW config:

Attached Thumbnails

  • D4C158F2-D4BE-480F-B210-935106BAD304.jpeg

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#3 Jim Waters

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Posted 03 March 2021 - 04:10 PM

This is generally considered bad.  Move the weights up.  If needed get more weights.

 

https://www.youtube....h?v=qHJYkJLBZDc


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#4 PirateMike

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Posted 03 March 2021 - 04:13 PM

I can't say how much it would help (number wise) to add more weight and moving them higher up on the shaft, but most likely it wouldn't hurt. It is good practice to do things in the most advantageous way.

 

Having more error on the Ra as compared to the Dec could be caused by more that one issue.

 

Have you tried balancing a little east heavy?

 

 

Miguel   8-)

 

.


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#5 RSJ

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Posted 03 March 2021 - 04:19 PM

FWIW, I recently went down this path and purchased a 22lb counterweight for my CGX, adding to the two existing 11lb weights. I moved the 22lb closest to RA, then an 11lb next to it, and used the remaining 11lb for fine tuning the balance. I can't share any specific improvement numbers because I've made other changes as well and couldn't attribute them specifically to the new weight. That being said, I believe it did improve the guiding and am thinking about adding an additional 11lb weight in the future.


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#6 AnthonyQ

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Posted 03 March 2021 - 04:33 PM

I can't say how much it would help (number wise) to add more weight and moving them higher up on the shaft, but most likely it wouldn't hurt. It is good practice to do things in the most advantageous way.

 

Having more error on the Ra as compared to the Dec could be caused by more that one issue.

 

Have you tried balancing a little east heavy?

 

 

Miguel   8-)

 

.

This mount uses a spring loaded worm that supposedly negates the need to weight east-heavy, but I have and not any difference.

 

I have had pretty equal guiding numbers, but with a lighter scope/less weight. 
 


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#7 AnthonyQ

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Posted 03 March 2021 - 05:52 PM

FWIW, I recently went down this path and purchased a 22lb counterweight for my CGX, adding to the two existing 11lb weights. I moved the 22lb closest to RA, then an 11lb next to it, and used the remaining 11lb for fine tuning the balance. I can't share any specific improvement numbers because I've made other changes as well and couldn't attribute them specifically to the new weight. That being said, I believe it did improve the guiding and am thinking about adding an additional 11lb weight in the future.

This is the sort of reply I am hoping for, either way.

 

I’ll count this in the “for” column.

 

Thanks!



#8 dhaval

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Posted 03 March 2021 - 07:58 PM

Moment of arm = Force * Distance (squared). The point here being, the farther you go away from the center of gravity, the moment arm increases exponentially (squared). So, even if your rig is perfectly balanced, the RA axis has to work that much harder to counteract the moment arm. You are much better off keeping the weights closer to the RA axis and reducing the moment arm.

 

CS!


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#9 speedster

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Posted 03 March 2021 - 09:59 PM

It's counterintuitive but adding weight to get it closer to the axis is less strain on the drive when changing speed or direction because it reduces moment of inertia.  For static balance, 20# 1' from the axis is the same as 10# 2' from the axis or 5# 4' from the axis.  But, since inertia is the square of the length, the moments of inertia of this example are 20, 40, and 80.  Weights farther out have to move faster for a given rpm.  More angular velocity = more torque.  Closer weight is better but for a telescope mount within its weight limit, it's splitting hairs.  No need to rush out to buy more weight.


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#10 Der_Pit

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Posted 04 March 2021 - 03:45 PM

The moment arm length will also influence resonance frequencies.  Shorter arm means higher frequency and faster damping.  For mounts, IMO that aspect is more important than inertia on speed change...


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#11 Cliff Halliwell

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Posted 07 March 2021 - 08:19 AM

It's counterintuitive but adding weight to get it closer to the axis is less strain on the drive when changing speed or direction because it reduces moment of inertia.  For static balance, 20# 1' from the axis is the same as 10# 2' from the axis or 5# 4' from the axis.  But, since inertia is the square of the length, the moments of inertia of this example are 20, 40, and 80.  Weights farther out have to move faster for a given rpm.  More angular velocity = more torque.  Closer weight is better but for a telescope mount within its weight limit, it's splitting hairs.  No need to rush out to buy more weight.

Which suggests it is important for better guiding, with better response to guiding corrections.  Thanks. 



#12 rockstarbill

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Posted 07 March 2021 - 09:20 AM

Good write up on this topic.

https://www.astro-ph...ize-guiding.pdf

#13 KNak

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Posted 07 March 2021 - 11:08 AM

I've read a lot of these threads about balancing and of course the physics is the physics and the recommendation is sound, but the question in the original subject is "how much".  Can someone link to some actual measured data/results of someone doing the experiment and quantifying the difference for their setup?  I'm sure its out there, just haven't come across it yet.



#14 rockstarbill

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Posted 07 March 2021 - 11:27 AM

Good luck with that.
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#15 dx_ron

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Posted 07 March 2021 - 01:08 PM

If you have the right combinations of counterweights, it wouldn't be difficult to just do the experiment. You'd have to figure out how to account for changes in seeing between changes in the setup, but that's not an insurmountable barrier. Changing the counterweight configuration should only take a few minutes for each change (note: do it with a helper you trust to hold on to the OTA!)




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