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Old 15-09-2019, 09:37 AM
RyanJones
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Counterweight Position

Hi all,

Based on a recent thread that brought up the discussion I thought Iíd ask the question to see if anyone knows what is the best position for counterweights.

Is it better to have less weight further down the shaft or more weight closer to the top ? My theory was that less overall mass being acted on by gravity would create less friction on the bearing surfaces. Others have suggested the valid point that the further out the weight is the worse the moment of inertia presumably making minor adjustments to the tracking harder for the mount to perform ?

Looking at images of mounts setup for imaging, I see a variety of different options. Some with weights split between top and bottom, some with more weight close to the top, then others with less weight on extension bars.

What is the answer and the justification? Obviously itís a balancing act ( pardon the pun ) but surely thereís a justified answer to where to put them and how much to put on ?

Thanks in advance.

Ryan
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Old 15-09-2019, 10:30 AM
AnakChan (Sean)
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Hmmm..for me, if I can, I'd rather have the weights closer upwards toward the scope (maintaining balance the whole time naturally). And if it's a retractable shaft, I shorten that too. My thinking is to reduce the duration of the vibration where possible - may it be someone accidentally brushing the counterweights/shaft whilst walking by, or wind, etc.


So if I can, I actually rather have extra counterweights with a shorter shaft, than to have fewer counterweights with longer shaft.
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Old 15-09-2019, 12:23 PM
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Slawomir (Suavi)
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Hi Ryan,

I think there are three reasons why people put counter-weight down the shaft, one being unaware of how it can impact guiding, second some may not have enough counterweights and lastly some may not care as it does not significantly impact their mount for what it is being used for.

Here are a few words from a premium mount maker about counterweight positioning: https://astro-physics.info/tech_supp...ze-guiding.pdf

Last edited by Slawomir; 15-09-2019 at 03:04 PM. Reason: Clarity
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Old 15-09-2019, 02:42 PM
LewisM
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Having always done it as AP suggests and never having a guiding issue since (many years) I suggest there is a lot of truth in AP’s advice.

My EM200 currently sits with an FSQ on top with 2 cws half way up the shaft, with a 1 kg trim balance CW
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Old 15-09-2019, 03:36 PM
Startrek (Martin)
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Suavi , excellent technical paper from AP
I suspect quite a few members will reassess their counterweight strategy after looking at this ( me included )
Thanks Ryan for raising the post
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Old 15-09-2019, 07:05 PM
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doppler (Rick)
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I've tried it both ways and haven't noticed any difference even when slewing. My guiding is pretty well always sub arcsec and that's with a 10" newt on a HEQ5 pro. With the longer shaft I find it easier to fine tune the balance as well, but the HEQ5 does have a pretty short shaft as standard.
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Old 15-09-2019, 07:19 PM
Startrek (Martin)
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Rick,
I have a 6Ē f6 newt on a HEQ5 with a 50mm guide scope and ZWOASI120MM guide camera tuning PHD2 and can never get under 1 arc sec
Iím imaging under Bortle 8 Sydney skies

You have 25% more payload and longer focal length so tell me secret about sub arc sec guiding ?
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Old 15-09-2019, 07:38 PM
RyanJones
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Thank you all for your input and thank you Sauvi for your technical reference. Based on this Iím going to buy an extra weight and try mounting the weights higher. My Nexguide is not the most accurate guider going around so it highlights tracking faults. Iíll report back my findings once Iíve added the extra weight.

Cheers
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Old 15-09-2019, 09:00 PM
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doppler (Rick)
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"You have 25% more payload and longer focal length so tell me secret about sub arc sec guiding ?"
I got myself one of those Orion thin oags and noticed a big improvement over the 50mm finder guider that I was using before, and I got my finderscope back. I think that the belt mod helped a bit as well, but an oag's the go.
Rick
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Old 15-09-2019, 09:01 PM
RyanJones
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Quote:
Originally Posted by doppler View Post
"You have 25% more payload and longer focal length so tell me secret about sub arc sec guiding ?"
I got myself one of those Orion thin oags and noticed a big improvement over the 50mm finder guider that I was using before, and I got my finderscope back. I think that the belt mod helped a bit as well, but an oag's the go.
Rick
From my experience the belt mod makes a significant difference.
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Old 15-09-2019, 09:07 PM
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doppler (Rick)
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And a good tune of the worm gears helps as well.

http://www.astro-baby.com/heq5-rebuild/heq5-we1.htm
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Old 15-09-2019, 09:25 PM
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Quote:
Originally Posted by doppler View Post
And a good tune of the worm gears helps as well.

http://www.astro-baby.com/heq5-rebuild/heq5-we1.htm
Iím so glad I started this thread

Thank you Rick for that link too. Itís now on my to do list as well!
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Old 15-09-2019, 09:54 PM
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From the AP suggestions:
Quote:
We recommend a slightly heavy RA balance towards the counterweights and a slightly heavy Dec balance toward the
camera end of the scope when imaging.
Can someone explain what is the logic behind this? Balancing the RA so it is slightly "counterweight heavy" goes "half the times" against the usual "East heavy" rule (for the cheaper mounts like EQ6).
And why "camera heavy" for the DEC?
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Old 15-09-2019, 10:36 PM
RyanJones
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Quote:
Originally Posted by luka View Post
From the AP suggestions:
Can someone explain what is the logic behind this? Balancing the RA so it is slightly "counterweight heavy" goes "half the times" against the usual "East heavy" rule (for the cheaper mounts like EQ6).
And why "camera heavy" for the DEC?
I would presume ( just me talking out aloud so I could be wrong ) that if you go counterweight heavy then you donít need to change the position of the weights depending on which side of the meridian youíre on. I think either would be fine as long as youíre marginally heavy one side to constantly rest on one side of the worm gears. Extrapolating from that I guess itís the same reasoning on DEC. Again, happy to hear more solid logic than what my grey matter can come up with.
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Old 16-09-2019, 01:13 AM
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Quote:
I would presume ( just me talking out aloud so I could be wrong ) that if you go counterweight heavy then you donít need to change the position of the weights depending on which side of the meridian youíre on. I think either would be fine as long as youíre marginally heavy one side to constantly rest on one side of the worm gears.
Not quite. The RA axis always moves in the same direction. Unlike DEC any guiding corrections do not change the direction, they only slow down or speed up the movement of the axis. The mount needs to be east heavy to keep the gears engaged properly during tracking/guiding. When you do the meridian flip so that the mount is west-heavy (counter weights on west side) the tracking direction will still be the same. The gears will be engaged "the wrong way" and the backlash will become important.
(That's why east-heavy approach requires shifting of counter weights after a meridian flip.)

The DEC axis moves both ways during guiding so I am not sure any more.
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Old 16-09-2019, 06:29 AM
Startrek (Martin)
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Thanks for the explanation Rick
Sounds like an OAG and a belt mod is on the cards in the future
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Old 16-09-2019, 08:37 AM
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The_bluester (Paul)
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I have found pretty good results by making my setup counterweight heavy rather than east or west heavy and make it by enough that the worm stays in mesh no matter of it is lifting the counterweights while pointing to the east or lowering them when pointed west. Bascially the imbalance needs to comfortably overcome stiction when pointed west and the imbalance just needs to be moderate enough that the RA motor can lift the load when pointed east.

All things being equal I want whatever I am imaging to cross the meridian about the middle of the night to get the most time one it and I don't want to have to get out of bed for the flip.
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Old 16-09-2019, 08:51 AM
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troypiggo (Troy)
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The best solution I have found to overcome this was kind of by accident. I have RA balanced pretty evenly, but have most cables (particularly the Mount Hub Pro power and USB) that drop down to ground on the east side of mount. This keeps it a little east heavy before and after meridian flips.

Quote:
Originally Posted by luka View Post
Not quite. The RA axis always moves in the same direction. Unlike DEC any guiding corrections do not change the direction, they only slow down or speed up the movement of the axis. The mount needs to be east heavy to keep the gears engaged properly during tracking/guiding. When you do the meridian flip so that the mount is west-heavy (counter weights on west side) the tracking direction will still be the same. The gears will be engaged "the wrong way" and the backlash will become important.
(That's why east-heavy approach requires shifting of counter weights after a meridian flip.)

The DEC axis moves both ways during guiding so I am not sure any more.
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