Balancing issue - third axis?

[Jonny_H]

Hi all,

I wanted to seek some advice on balancing since changing some of my equipment.

My set-up consists of an Esprit 100ED, a Evoguide 50ED (on top) a Mini PC and Pegasus PPBA just behind the 50ED (these are the latest addition). I also have a ZWO EAF. My mount is an AZ EQ6 GT.

Balancing in RA isn't an issue - balancing in DEC though is a little more interesting. With the RA at 90 degrees/horizontal and the scope horizontal, with a bit of toing and froing the scope 'balances'. However when slowly pressing the scope down at the back, it starts to swing to near vertical. However if moving the scope back to horizontal so it is 'balanced' again, and pressing the scope down the front, it does the same thing and swings to near vertical.

So.....what do I do? I have been reading articles about balancing the third axis but these seem to all be on the Ioptron mounts so assume that it is a common issue with these mounts.

Interested to hear what others do in this situation.

Thanks,

Jon

 

[Zermelo]

Hi Jon,

I've no personal experience, but I have seen a few articles and I got the impression it was general mechanics rather than anything specific to a brand of mount. As your title identifies, balancing in the two axes isn't always sufficient. If there is a significant net imbalance of extra fittings off the axis of the OTA, it won't be possible to balance it in all positions. If you get it right when it's horizontal, then it will be out of balance when it gets near vertical.

One solution people have used it to rig up a spring such that it has no effect when horizontal, but imparts a force that counters the imbalance as the angle changes. Another method is to affix a counterweight on a threaded shaft at right angles to the axis, to correct the imbalance, as in this video:

 

[Jonny_H]

22 hours ago, Zermelo said:

Hi Jon,

I've no personal experience, but I have seen a few articles and I got the impression it was general mechanics rather than anything specific to a brand of mount. As your title identifies, balancing in the two axes isn't always sufficient. If there is a significant net imbalance of extra fittings off the axis of the OTA, it won't be possible to balance it in all positions. If you get it right when it's horizontal, then it will be out of balance when it gets near vertical.

One solution people have used it to rig up a spring such that it has no effect when horizontal, but imparts a force that counters the imbalance as the angle changes. Another method is to affix a counterweight on a threaded shaft at right angles to the axis, to correct the imbalance, as in this video:

 

Thanks @Zermelo

I may have to go down a similar route.  I think I prefer the idea of a counterweight.

I notice ADM do two variants - one where the counterweight is attached to a thread at the side of the dovetail (like in the video) and the other type where the counterweight is attached to a thread underneath the dovetail. I think i will need the former as per the video!

[Louis D]

You want the the CG (center of gravity) of your setup to pass directly through each rotational axis.  I'm guessing your DEC axis has bits and bobs hanging asymmetrically off of your OTA forcing the CG in that axis of rotation above or below the actual rotational axis.  It's probably directly above or below it with the OTA horizontal, so it balances nicely in that orientation.

I have a similar issue visually with my alt-az mount at high altitudes when using heavy eyepieces.  The weight protruding up and away from the diagonal has to be counterbalanced by a similar amount of weight protruding down and away from the OTA on the other side of the altitude axis to prevent my rig from turning turtle.

[DaveS]

I mounted a short solid dovetail bar on an ADM Vixen saddle and mounted a Baader dovetail counterweight in it so I would have fine adjustment in two axes. ASA mounts are *very* sensitive to balance around three axes.

If it ever stops raining I'll go out to the obsy and get a photo.

[tomato]

Here is the very straightforward method I use for balancing a heavy side by side dual rig. In my case the scopes are in a fixed position on the plate bolted to the mount so I have to fix counter weights in the appropriate position, OK if your mount has the load capacity.

http://www.wilmslowastro.com/tips/g11gemini.htm#balancing

[Same old newbie alert]

3rd axis is usually mounts with fluid axis, as you say with the ioptron mounts in particular, small offset counterweights can be used, I personally use some big washers screwed into the counterweight itself but my imbalance is on the Ra.. and I always balance Dec first