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How unbalanced is balanced?

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#1 SFGagnon

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Posted 20 February 2010 - 03:22 PM

Hey everyone,

I'm reading that it's best to have your mount slightly loaded to the east in order to improve tracking. My question is, how unbalanced should things be? Currently, I balance my CGEM so that there is no movement when the clutches are released. It's slightly weighted on the east side, but it's balanced enough that the torque produced doesn't overcome the mount's own internal friction. Is this too balanced? Should I be adjusting things so that the scope slowly rotates on its own when the clutches are released?

Thanks!

#2 jay52

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Posted 20 February 2010 - 03:31 PM

Push the E and W "move" buttons on the Autoguider screen and take images with the autoguider to test response. If you push it once, it should move a predictable amount. If you push it twice, it should also move a predictable amount. This is the way to judge backlash in the system. It's also why we say to weight the eastern side a bit, to keep the gear seated in the worm.

The other way, quite obviously, is to look at the calibration results after calibrating the guider (do it with shorter iterations and larger movements for the best test). It should trace a good positive X move followed by a good negative X move and end up back in the same place. If not, then you might need to make some adjustments.

So, there is no real answer other than how your mount responds. For some, we don't have to unbalance the mount...others may need more. Even so, I'd say that if you release the clutches and your scope acts like it's going to take a suicidal nose dive into the ground, then you might have some big problems.

#3 jay52

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Posted 20 February 2010 - 03:34 PM

Oh, BTW, I'm assuming you are imaging. For visual only, just balance the mount.

#4 Eddgie

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Posted 20 February 2010 - 03:38 PM

I don't personally think it makes any difference whether you pre-;load the mount in East or West. My own preference it to always load the mount so that the west side is heavier, but that is just me.

The important things to remember ar that if you have any meaningful amount of worm gear lash, you should always ensure that you are pre-loading the mount in the same direction.

How much? Enought to overcome the stiction present in the bearings. The idea is that the pre-load always keeps the face of the worm wheel against the face of the worm gear. Sometimes if balance is "Perfect", the gear faces can momentarily loose contact, or at the end of a slew, the scope might have some period of time before the face of the worm gear catches up to the face of the wheel.

If your mount is tight though (no play in RA), then it usually doesn't matter so much. I found on my LXD-55 though that it was always necessary to preload a bit becuase the mount had a lot of stiction. If I use the slow motion control in RA, I could see where sometimes the scope would not move, then suddenly lurch ahead as the worm gear pulled away, then the weight would cause the mount to break free and catch up.. We are only talking about a half dozen arc minutes of play here, but it was enough to be bothersome. Pre-loading the mount to the west with just enough weight that it would slightly move to the west on it's own when the clutch was unlatched seemed to work perfectly.

Regards.

#5 blueman

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Posted 20 February 2010 - 04:15 PM

The reason for weighting one side, East side preferably, is so that the worm is always running with a slight preload on one side of the teeth and not rocking back and forth due to perfect balance.
This said, it is not so critical for visual use.
How much? Well, each mount will vary a little on that, but enough so that it wants to fall to the East when released with the clutches loose.
East vs. West? Well I will not say anything is absolute, but East is the right way according to most. Why? Because then gravity does not tend to force the teeth together, but rather makes the motor pull the weight along.
Blueman

#6 mike174

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Posted 20 February 2010 - 04:16 PM

I'm working with my first GEM and noticed some delay in one of the axis's. What is pre-loading mean?

#7 Eddgie

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Posted 20 February 2010 - 05:00 PM

Preloading means running the scope unbalanced by sliding the counterweight a little closer or further from the perfect balance point.

This ensures that the worm wheel is always being held in contact with the worm gear. Again, I personally don't think it makes any difference if you pre-load east or west. If you load to the west, the worm gear becomese a "Regulator". Rather than PUSH the weight of the off balance into position (as would occur if you preloaded to the east), the worm gear now has the worm wheel riding on the back face, so the motor of the worm is simply regulating the speed at which it "Falls" to the west.

I have done it both ways, but on my LXD750, it seems to prefer west loading because the motor is so stinking small it will stall if I pre-load to the east.

Again, for the most part it doesn't matter. If there is slack in the RA though, I would always advise loading in the same direction on both sides, which means that if you pre-load to the west, the counterweight will be higher on the shaft when you are viewing to the east, but you have to remember to lower it on the shaft when you are viewing to the west (or vice versa if you pre-load to the East).

#8 David Pavlich

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Posted 20 February 2010 - 05:18 PM

I balance so that it's neutral, then add a 3lb weight about half way up the CW shaft. Works for me!

David






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