A question of balance...counterweights
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The question was asked: Is it better for imaging performance to put fewer counterweights at the far end of the bar, or use more weights close to the RA axis of the mount ? I am not doing deep sky at the moment, and if I bump my scope during planetary video imaging, I can see the video image jump briefly before it damps out.?? Therefore, you can also experiment: if you use your autoguider camera, and put it on your main imaging scope prime focus location, and point to a bright star, and give a bump to your counterweight, you can see or record on video how much the star jumps around.? The video could quantify the frequency and amplitude of the vibration. Then try to adjust your weights position and give a similar bump.? Can you see a difference?? If yes...tell us what you discover!? Can you say whether there is an improvement or no real difference? In terms of academic theory, this would need "finite element" modeling.? And you'd need to know something about the "bump" or driving force.? ? If it's a constant wind, it could drive oscillations of the system (think the famous Tacoma Narrows bridge).? But in the absence of any bump or wind, the only movement is the constant tracking rotation.? There are sporadic autoguiding corrections, and these if large might cause an oscillation.? I don't see any oscillations in my planetary imaging and that is getting frequent autocorrections from FireCapture through ASCOM to the Gemini-1 to the G11. The trade-off is on counterweight bar flex, and image vibration. Also comes up on how heavy a scope and counterweight load can you put on a G11 before it breaks down. For the vibration question...it's a good mechanics/physics question.?? The problem looks like this: [Very Heavy Scope] ----(W1)--------(W2)---- ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ^ ? ? ? ? ? ? ? ? ? ? ?^ is the RA axis pivot What are the frequencies of this case above, versus an alternative putting more weights close to the RA pivot: [Very Heavy Scope] --(W1)(W2)(W3)------ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ^ ? ? ? ? ? ? ? ? ? ? ?^ is the RA axis pivot In general, the bottom case will have higher frequencies than the upper case.? The energy of the higher frequencies might be the same as the energy of the lower frequencies, but the higher frequency vibrations might look just like "Seeing" fluctuations.? The lower frequencies might make larger blurs if your stats. This is a different puzzle for each of us, as we all have different weight scopes, cameras, and weights can vary too.? The heavier the scope the less it will move.?? Think of a flute or better a rope with weights on it: the holes in the flute are either open, providing a point where a sound wave can have no pressure.? On a rope with weights, the weight positions act as a null point. The rope can't vibrate there but if can oscillate between the weights.?? I'll say that both cases can oscillate, but if you space out the weights, you may be able to reduce the oscillation amplitude. ----- Here in the top photo is my overloaded system (in the dark), showing the hollow 3 foot long counterweight shaft.? I put 3 counterweights spaced out to act as nodes to kill off some vibration modes.? The idea was to forces oscillations to go to higher frequency, which are harder for bumps or wind to create - at least that's my thinking.?? ( By the way, finite element analysis is used in bridge design to ensure there are no harmonic frequencies like caused the Tacoma Narrows bridge collapse.? ) Here is a photo of FireCapture set up to evaluate the ZWO ADC atmospheric dispersion corrector.? Seeing usually appears here very severely ad the colors jump all over the place.? A bump here would make the star or planet jump all over too.? The 3rd photo is how Jupiter looked the time of the photo this week.? Hope you can try some experiments to see what is optimal for your system! And let us know what you discover! Best regards and stay healthy, Michael |
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Hi Bob, Thanks for your encouraging?words.? Every time I think I know the answer to a topic ...I need reminding that I don't.? Life and imaging are just big experiments.?? I do not know the answers but think we all just try out ideas and see?what happens.? The nice part of these groups is the sharing of new ideas.?? I'm evaluating the images from my run with the ASI178MC two nights ago, vs the earlier good results with the ASI224MC.? ?I thought the 178 results should be sharper than the 224 results because it has smaller pixels...but at this moment I'm not yet sure.?? There are a few more factors to consider that can explain better 224 results... Same scope, same ADC, same PC, etc. Different night though!? I cannot say the seeing was identical...even in the same night the seeing and air moisture haze varies.?? I think the exposures were longer per frame on the 178, and that makes sense since the 224 has 3.75 um pixels, and the 178 has 2.4 um pixels, each 224 pixel gets 2.4X as much light as a smaller 178 pixel. [ 2.4x = (3.75um/2.4um)^2? ] So I was shooting frames that were 2.4x faster on the 224.? Faster frames are better to freeze out the seeing fluctuations.? That is in favor of using the 224 over the 178.? Faster frames makes it more possible to shoot better images in mediocre seeing conditions.? I also had not found my UVIR filter the night of the 224 camera.? I said the 224 was getting extra IR light from Jupiter, as Andy mentioned this is not recommended.? So I put on the UVIR filter on the 178, but didn't have it for the 224 camera images a night earlier.?? But...? what do we know about IR light?? Much less affected by seeing than B, G or R.? The IR light is less scattered by the atmosphere too (why the sky is blue of course...extra scattering of blue light by molecules of air...also why Halpha deep sky images can appear sharper than the RGB version).?? Is it possible that letting in the IR light on the 224 images made the Red channel brighter and less noisy without the UVIR filter than with the filter?? This needs another experiment to see.?? ?I recall hearing Jupiter expert Christopher Go saying to try a Methane filter....also deeper in the IR...to see more details.? I need to get such a filter and see...?? Anyway, I'm trying more things...the program PIPP might do a better job picking the best frames than AS3...so I'm trying that step too.? I'll report on that...? Gotta get some sleep though... This staying up all night to have fun can really boomerang. Also here is a photo of the ZWO ADF optic.? See the long levers in the side? Their "home" position is at the average position of 3 o'clock, if you are looking from the back.? ?You then put the little bubble level at the 12 o'clock position.? You use the FireCapture ADC tool to adjust the levers and get the Red and Blue dots coincident.? You do have to be on your toes during the night as the target travels across the sky, to re-balance the ADC optic.?? Best to all,...stay well! On Mon, Jul 20, 2020, 4:20 PM Robert Runyan <runrob@...> wrote:
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Michael Herman