| More or farther counterweights ? | |
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2.11
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I recently stumbled onto this question while setting up my rig a few days ago. Would you put more counter weights but closer to the mount or less but farther on the shaft ? The resulting balance would be the same but you would have more weight on the mount. On the other hand, I would think that closer counterweight would reduce sensitivity to vibration and wind. What do you do ? |
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2.81
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| Closer weights are better. The moment of inertia (resistance to rotation) will be smaller. |
10.16
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The torque is smaller when the weight is denser. Thus, the stress on the mount is less. But with a weight farther away, the telescope can be balanced more finely. My suggestion: A heavy weight as close to the mount as possible and a light one for fine adjustment with more distance. |
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7.46
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| The less weight you put on the system the better it is for the bearings to operate smoothly. On the other hand the harmonic response of the cantilevered weight is larger, for any given rod x-section, when the weight is further from the centre of action. If your counterweight bar is quite stiff that doesn't really change that much so you are better off moving the counterweight out than adding counterweights. Moment = Force x Length, so in each case the moment is the same. At high slewing speeds the centrifugal force would increase in the same way as far as the two options go (i.e., force=-angular speed^2 *Mass*Length), i.e., irrelevant which way you go. |
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| Agreed- I faced this a year ago. Mathematically, the moment of inertia is proportional to the square of the distance. Hence, for the same balance (mass x distance), you are better off going more weight closer to the mount to lessen the inertial response. It requires more force to accelerate less mass further away than more mass closer in(depending on the mass, of course). The mass X distance is linear, but the inertial moment is quadratic. |
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| Has anyone observed actual guiding or tracking error improvements for the "more" configuration relative to the "farther" configuration? For example what is the ROI of spending $$ to buy another counterweight? Does it really make any difference for a mid-range or better mount? |
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Adam Landefeld: Yes I have noticed. My refractor has better RA guiding because the weights are close to the top in that configuration. When I have the RC on, they are further down the shaft and the RA suffers a bit. However, the refractor being a refractor the DEC is worse than on the stubby RC. Now is it worth it to buy more counterweights? I would say it depends on the pixel scale you are imaging at. Anything above 1 arc second probably not. However once you start getting below 1 arc second guiding becomes super duper critical and you might want to pony up for a better counterweight system. As for the farther vs more debate: So long as the mount is rated to handle the weight, 10 times out of 10 it is better to have more weight but higher up the shaft because physics says so. Think of it like this: Let's pretend you are a human equatorial mount rated for 40 kg. Now, which is easier? Picking up a 30 kg box keeping it close to your body, or picking up a 20 kg box while trying to hold it out at arms length? Now try moving those boxes up and down in those scenarios. |
11.24
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| Many aspects of astro photography can be manipulated and adjusted simply based on personal or financial circumstances, environmental influences, or just individual style or flare. This is not one of them. This is not a topic for debate based on personal opinion and a survey result should reflect 100% support of the physical reality. No offense to the OP and thank you to all the folks that have have issued posts reflecting the science on which the topic is based |
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Yes I have noticed. My refractor has better RA guiding because the weights are close to the top in that configuration. When I have the RC on, they are further down the shaft and the RA suffers a bit. However, the refractor being a refractor the DEC is worse than on the stubby RC. Thanks! I have always wondered if this was purely an academic exercise or had visible impact. |
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You always want more weight to get the counter weights as close to the RA axis as possible. As many have pointed out, that's important to reduce the rotational moment of inertia, which helps with motor torque and mount responsiveness during guiding. Having a large rotational moment of inertia can cause motor stalls during slews because it makes the motor work harder to accelerate the scope. However, the bigger reason to keep the weight "up high" is to raise the resonant mechanical frequency of the whole system, which reduces its sensitivity to vibration. The added weight on the mount bearings is generally an insignificant effect. Most common bearing assemblies used in small mounts can handle very high radial loads in the range of 100's and even 1,000's of pounds so adding another (say) 10 pounds is inconsequential. The important thing is to maximize the stability of the mount--not to minimize its weight. Here is a slide from one of my talks at NEAIC 2019 that should help clarify the point. I'll have a bit more information on this topic at the next AIC meeting so I hope to see you there! John  |
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10.16
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Hi John, that's an interesting aspect that I hadn't thought of. Is there any way to see your lectures online? From Germany it is a bit far to the AIC meeting.  Olaf |
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Well, in my case (EQ35M, unguided tracking with a 5kG Newt which is very close to the limit of the mount for astrophotography) there are exactly two ways to achieve balance:
The first setup is very prone to vibrations. It takes 4-5 seconds for the system to stop vibating after a slew, or even if I so much as touch the scope e.g. to move a cable. But does perform very well if left alone. The main reason a sub must be dropped is because I walked in or out of the balcony, e.g. to grab a beer and my footsteps resonated with the whole system. I doubt it could handle guiding though and it absolutely sucks when there is the slightest wind. Which is why I tried the second setup. So the second setup is practically vibration free, but the mount is very obviously struggling to maintain tracking in RA. More than half of the subs have elongated stars. So I am using the "1 counterweight - farther" alternative and basically try to tiptoe withing 2m of the scope  I understand I am pushing it with the 5kg scope but based on what I am reading here the extra 3kg of counterweight shouldn't make such a dramatic difference, right? Perhaps I need to disassemble and see if there's anything physically wrong? The mount does seem to "dislike" it when the shaft is close to horizontal (e.g. telescope pointing at Zenith), even with lighter scopes. Cheers, D. |
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Olaf Fritsche: Hi Olaf, Yeah, I can completely understand that problem! NEAIC was supposed to put everything on line but they never did it. AIC makes all of their presentations available after the meeting but it's not free. I'm not sure how it works so you'll have to check their site to see how it works. I put a lot of work into the presentations that I do for those meetings and I occasionally share a few slides but I reserve the whole thing for attendees per my agreements with the organizers. Maybe someday I'll finish that book I'm working on so I'll have one place to point to when I get questions like yours! John |
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More weights up close is far less strain on the drives. So long as you don’t use more weights then the mount your using can handle. Astro-Physics for instance gives you a total equipment load not including counter-weights. For an example, I have a AP Mach-1 with a image train weight of about 25lb and I use 3 14lb weights (42lb) and it tracks perfectly. It will balance with one 14lb and one 6lb weight, but the tracking is much worse.See the Astro-Physics link attached, [b]*[/b]balance-to-optimize[b]*[/b] https://www.astro-physics.info/tech_support/accessories/mounting_acc/balance-to-optimize-guiding.pdf |
