Counter weights and balance question - Forums

Topic starter

Sep 15, 2021 · Edited: Sep 15, 2021

Hello community,

once again, your help is needed. This time, I am looking for your opinion about counter weights.

With my latest purchase of the TS 70mm Flatfield APO I do own a scope with a really great optical performance. My EQ6R Pro handles the scope easily, which I expected. Now, I also do a three point polar alignment at the beginning of each night, which increased the guiding performance a lot. But with the resulting quality of the images I get from this setup, I am a little dissapointed with my other scopes (I know it happens to the best of us :wink-1

. The problems I have with my Explore Scientific MN152 comet hunter will be part of another topic in the forum. Here, I want to adress the performance of my TS 8" f/4 Newton.

I know that the scope is not the most impressive one on the planet. This is ok and I can live with that. But there are some issues that I realized I may fix, that are not part of the scope itself or its optical design.

The three point polar alignment had a really strong effect on the small refraktor. My guiding graph is at least 50% better than before. So I expected the same for my Newton. But in reality, I only got at best 20% better performance. This is clearly visible in the star shapes of my images. So I thought about the reasons for that.

I am quite sure, the reasons are the weight of the setup and maybe the position of the counter weights. The scope is quite heavy (metal tube) and with guide scope, filter wheel, cameras etc. it may reach the limit of the mount. A solution could be to reduce the weight by replacing the guide scope with an off axis guider, maybe use a filter slot instead of my 2" filter wheel, etc. I can get creative here.

And after a long story, I finally reached the point of this question. I saw a lot of setups with additional counter weights to keep the center of mass closer to the mount itself. This makes sense so far. The question is, If I add another weight to reach this goal and so prevent the setup from oscillation, I am adding additional weight to the mount that may amplify the problems again. What is your experience here? Does this make sense and will it increase the performance or will it have no effect or is even worse? I do not own another weight (I have the two that came with the mount), and therefore cannot test it myself securely.

Thanks for your experiences!

CS

Christian

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Linwood Ferguson

Sep 15, 2021

Generally speaking for tracking performance on a balanced mount, rotational inertia is more relevant than weight, provided the system overall can handle the total weight (very few mounts seem to give a total weight limit, most talk of imaging load).

Rotational inertia involves the square of the moment arm, balance is linear, so a lot of weight up close is very, very different than less out far even if both balance. Think ice skater drawing in their arms to spin faster. It allows the motors to start, stop or change rotation with less energy, so it can respond faster. Here's a recommendation from Astro-Physics that includes a math example:

https://www.astro-physics.info/tech_support/accessories/mounting_acc/balance-to-optimize-guiding.pdf

A secondary effect that weight near the end of the counterweight shaft can have is ringing. It's like a pendulum, in a sense, and changes in speed (either large ones like settling after a slew, or even just small ones from guide impulses) can resonate easier in a counterweight shaft with weight at the end.

So absent indication the mount cannot carry the weight, I would definitely try more weight up close and see if it helps.

Incidentally, look at RA and DEC separately. PA issues are more likely to show up in DEC (though backlash can be confounding here as some PA error can actually help guiding on a mount with significant uncompensated backlash as it reduces reversals). Counterweight inertia issues in RA. Assuming a properly balanced mount to start with of course.

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andrea tasselli

Frequent contributor

Sep 15, 2021

In my experience, both with sub-optimal mounts and with higher end ones (better than SW for sure plus few others) is that changing position of the weights on the CW shaft makes not much a difference if any at all, especially if you consider meridian flips. One thing I cannot fathom of what you said is how a more accurate pointing model has any effect on the guiding, something I find quite puzzling to say the least. I'd understand better PA error but a pointing model?

Christian Großmann

Topic starter

Sep 15, 2021 · Edited: Sep 15, 2021

Thank you @Linwood Ferguson. That totally make sense to me. I could've guessed that

.

I think, that the effects that I see are due to the pendulum effect of the weights on the far end of the shaft. I usually see the stars be slightly blurrish only in one direction. Depending on the camera angle this movement differs in the subs of the different targets. But it's not a different direction, it is a different view angle.

As you mentioned, my Dec graph is much more stable than the RA one is. I tried different settings for the exposure/reaction time in PHD2 for RA but never get close to the DEC axis (with the heavier scope). I also know, that there is always some backlash, but checked my mount and it can't be that worse. I am sure, the reason for my problem is given above.

So I will order another counter weight and give it a try. They are quite cheap compared to other gear. But they may do more for image quality than even expensive equipment will do.

Thanks again and clear skies

Christian

John Hayes

Frequent contributor

Exemplary contributor

Sep 15, 2021

In general, it is always better to have more weight as high on the dec axis as you can get it. That will increase the resonant frequency of the system and reduce the amount of torque required to move the mount. Both of those things help to make the mount more stable. Having said that and judging from your description, your mount performance probably isn’t your biggest problem. Using a guide scope on Newtonian system is much more likely to be the limiting factor in your system. You don’t need very much mechanical flexure between your guide-scope and the optical axis of the imaging system to cause pretty serious problems. My recommendation is to figure out how to implement guiding through the main imaging system using OAG. I think that you’ll see a significant improvement in both star size and star shape once you start using OAG. And…while you are at it, you’ll need to rebalance the scope anyway so add some extra weight and move them up as high as you can on the DEC shaft.

John

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Christian Großmann

Topic starter

Sep 15, 2021

John Hayes:
In general, it is always better to have more weight as high on the dec axis as you can get it. That will increase the resonant frequency of the system and reduce the amount of torque required to move the mount. Both of those things help to make the mount more stable. Having said that and judging from your description, your mount performance probably isn’t your biggest problem. Using a guide scope on Newtonian system is much more likely to be the limiting factor in your system. You don’t need very much mechanical flexure between your guide-scope and the optical axis of the imaging system to cause pretty serious problems. My recommendation is to figure out how to implement guiding through the main imaging system using OAG. I think that you’ll see a significant improvement in both star size and star shape once you start using OAG. And…while you are at it, you’ll need to rebalance the scope anyway so add some extra weight and move them up as high as you can on the DEC shaft.

John

Thanks also to you @John Hayes. Basically, I bought an OAG to use it with a larger scope. I do own a 10" f/4 Newton and had the problems of the 8" magnified. I decided not to use it, because I thought it is too much for the EQ6R Pro. Maybe after the readings here, I have to give it another try. But thats another topic.

What I'm trying to say is, that I never really used the OAG, because dealing with a guide scope seemed to be much easier for the 8" Newton. I always noticed quite a huge difference in focus position when I switch from Luminance to Narrowband filters. That's why the OAG camera had to be adjusted every time I switch filters, doesn't it? But this may be less of a problem than I expect it is. Now, to reach a higher level of image quality, I had to give it another try. In theory I know about the pros and cons of an OAG. I just was too lazy to try it and maybe I also did not want to lose valuable image time. It's strange...

CS

Linwood Ferguson

Sep 15, 2021

Most (I am sure not all) broadband and narrowband filters are parfocal, if you bought them from the same vendor. That doesn't mean they are perfectly parfocal, but you should not need to adjust focus enough to affect an OAG guide camera. If you have wildly not-parfocal filters, which require really substantial change in focus, you will have a hard time using an OAG. OAG's do not need perfect focus, but they do need to be fairly close.

Now that said often you cannot guide through a focus adjustment, especially with a SCT due to mirror movement (i.e. target movement), but even on a non-SCT the AF routine's measurements in badly out of focus positions will break guiding, so generally assume you need to turn off guiding during autofocus with an OAG.

I shifted some time ago from using a guide scope on a small refractor (TV NP101is at 540mm) to using an OAG. I did it because it was simpler – I had the imaging train set up so I just disconnected in front (OTA side) of the OTA from my SCT, and moved the OAG, filter wheel and camera all together to the other OTA. Because the OAG stayed in the same relation to the camera, no need to even refocus the guide camera, saved a fair amount of time.

Now that said, some scopes will simply not provide enough back focus for an OAG.

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Christian Großmann

Topic starter

Sep 15, 2021

Linwood Ferguson:
Most (I am sure not all) broadband and narrowband filters are parfocal, if you bought them from the same vendor. That doesn't mean they are perfectly parfocal, but you should not need to adjust focus enough to affect an OAG guide camera. If you have wildly not-parfocal filters, which require really substantial change in focus, you will have a hard time using an OAG. OAG's do not need perfect focus, but they do need to be fairly close.

Now that said often you cannot guide through a focus adjustment, especially with a SCT due to mirror movement (i.e. target movement), but even on a non-SCT the AF routine's measurements in badly out of focus positions will break guiding, so generally assume you need to turn off guiding during autofocus with an OAG.

I shifted some time ago from using a guide scope on a small refractor (TV NP101is at 540mm) to using an OAG. I did it because it was simpler -- I had the imaging train set up so I just disconnected in front (OTA side) of the OTA from my SCT, and moved the OAG, filter wheel and camera all together to the other OTA. Because the OAG stayed in the same relation to the camera, no need to even refocus the guide camera, saved a fair amount of time.

Now that said, some scopes will simply not provide enough back focus for an OAG.

I am pretty sure, that backfocus may be no problem on my scopes. There is enough space left in each direction. I was able to buy an LRGB filter set as well as an Ha from astronomik. I think they are really worth the money I paid. The OIII and SII filters are made by Optolong. I simply could not afford the Astronomik ones yet but will replace them in the future. Then they all are parfocal. The focus shift between the Optolong and the Astronomik is sadly really drastic.

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