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GEM Balancing

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#1 scodavis

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Posted 21 February 2013 - 11:56 PM

I was looking at some information on Powernewts today, and came across their directions for balancing scopes on a GEM.

Since purchasing my astrograph, I have always balanced my scope on the RA axis first, and then the DEC axis. I’ve had decent guiding and no major issues. The longest subs I've tried were 7 minutes long and they went without a hitch.

Then today, I read that one must always balance the DEC axis first.

Which is correct? And most importantly, why?

-Scott

#2 korborh

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Posted 22 February 2013 - 12:07 AM

After adjusting the DEC balance the RA balance might need to adjust since DEC adjustment could have shifted the center of gravity the OTA on the DEC axis. So RA adjustment will be the last.

#3 EFT

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Posted 22 February 2013 - 10:34 AM

Balance the DEC first. It will likely not balance perfectly and that is fine. Just center the DEC balance (e.g., if it is heavy in one direction, like the saddle knobs, then point that straight back over the RA housing), then balance the RA. If you don't balance and center the DEC first, then the RA balance will be off in one direction or the other.

#4 Cliff Hipsher

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Posted 22 February 2013 - 11:14 AM

It has to do with Gravity Vectors, moment arms, and Center of Gravity.

Move the mount in RA and then visualize which direction gravity is pulling things like the finder, diagonal, camera, ect., and then look at it relative to the RA axis, which is ideally the mount CofG.

If you have an off center load like the 9x60 finder on my Meade 2080 SCT, for example, as the mount moves in RA, the CofG will shift.

As the CofG shifts away from the RA axis, torque loads on the drive gear train change. If the CofG shifts one way the torque load may increase, and if it shifts the other way the load may decrease. By keeping the mount balanced in DEC, we make sure the CofG for the entire rotational mass stays centered around the RA axis. This keeps the torque loads constant, minimizing gear wear, gear noise, motor drive currents, and PE (Periodic Errors).






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