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GEM Balancing -- DEC horizontal and vertical?

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#1 ks__observer

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Posted 12 January 2018 - 02:39 PM

Most instructions I see for balancing a GEM show the DEC balanced horizontally.

But the DEC can be unbalanced along the vertical axis.  The focuser adds weight to one side. 

For me to balance the OTA vertically in DEC I need to add counterweights opposite the focuser side.

I see a lot of pictures AP set-ups that don't have counterweights opposite the focuser side.

 

I am curious if people balance in DEC in both the horizontal and the vertical position.

 

Also, do people balance the RA first or the DEC first?  I have seen it recommended both ways.



#2 555aaa

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Posted 12 January 2018 - 03:06 PM

These are the steps as I recall and it applies for most mount designs.

 

1) with the dec axis horizontal and locked, and RA axis free, move the RA counterweight to get initial balance in RA.

 

2) with the dec axis still horizontal, lock RA and loosen dec. Put the OTA in a horizontal position, and then adjust weight (or position) along the length of the OTA to get the dec axis in balance.

 

3) With the dec axis still horizontal, put the OTA in the vertical position. Now, balance the OTA about the vertical axis.

 

I usually then swap to the other side of the mount (in RA) and re-check step #1. People often skip step #3 but it covers the other plane in which the OTA can be out of balance. This is a common issue in Newtonian telescopes because there is a lot of weight off to one side.  As long as the OTA weight always lies in the plane that contains the dec axis and long axis of the OTA, then the asymmetry shouldn't matter, but if there is any weight out of that plane, then it will become unbalanced in some sky positions so it needs a counterweight on the opposite side of the OTA.


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#3 AstroPics

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Posted 12 January 2018 - 04:42 PM

These are the steps as I recall and it applies for most mount designs.

 

1) with the dec axis horizontal and locked, and RA axis free, move the RA counterweight to get initial balance in RA.

 

2) with the dec axis still horizontal, lock RA and loosen dec. Put the OTA in a horizontal position, and then adjust weight (or position) along the length of the OTA to get the dec axis in balance.

 

3) With the dec axis still horizontal, put the OTA in the vertical position. Now, balance the OTA about the vertical axis.

 

I usually then swap to the other side of the mount (in RA) and re-check step #1. People often skip step #3 but it covers the other plane in which the OTA can be out of balance. This is a common issue in Newtonian telescopes because there is a lot of weight off to one side.  As long as the OTA weight always lies in the plane that contains the dec axis and long axis of the OTA, then the asymmetry shouldn't matter, but if there is any weight out of that plane, then it will become unbalanced in some sky positions so it needs a counterweight on the opposite side of the OTA.

That's a little different from what I have done but I think the end results are similar:

 

1) Loosen RA. Move the RA axis to horizontal, roughly adjust the counterweight and lock. Loosen DEC. Put the OTA in a horizontal position and then adjust the OTA (with all gear and cabling) to balance in the DEC axis. If I need to slide the OTA however, I normally bring the whole rig back to the zero position for safety reasons. I'll loosen the mount's dovetail clamp, slide the OTA and tighten the mount's dovetail clamps at the zero position then restart step 1.

 

2) Move the OTA to vertical on the DEC axis and ensure balancing in the vertical direction. Normally if there isn't balancing here, it requires some counterweight needs to be applied to the scope. If modifications made to weighting here, repeat step 1.

 

3) Return scope to horizontal on the DEC axis and lock.

 

4) Loosen RA and finalize counterweight adjustment to balance in RA. Lock.


Edited by AstroPics, 12 January 2018 - 04:43 PM.


#4 ks__observer

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Posted 12 January 2018 - 04:54 PM

I have one of each type of scope -- SCT, Newt, and APO.  They are all off balance in vertical.  I use a jerry-rigged system of clamps and strong-ties plates from Home Depot to balance the DEC in vertical -- as best I can.  But I got impression, as noted above, that a lot of people "skip" the vertical balance.

Was not sure if I was missing something.

 

Thanks :)



#5 AstroPics

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Posted 12 January 2018 - 07:06 PM

I found that skipping the vertical balancing could cause some changes in auto guiding performance for long integration times or if there was a meridian flip. So, I think it is normally a good idea. More for the Newtonian than than my refractor or SCT. -- I am not sure everyone does the vertical balancing though.



#6 ks__observer

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Posted 12 January 2018 - 07:35 PM

For Newt, i minimize vertical imbalance, and minimize RA balance load, by postioning the focuser and camera straight down in the home position.
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