How important is it to match the CofG in multiple scope setups ??

[Skipper Billy]

I am rebuilding my rig of an ED80 for imaging and an ST80 for guiding and I have been looking at the balancing of the scopes and the mount.

In the past I have just bolted the ST80 onto the ED80's rings in any place where the holes lined up then slid the whole rig up and down on the mount until it balanced in a horizontal position. I did notice that as it is rotated the balance point shifts.

This time I measured the centre of gravity of the ED80 and mounted it so that the CofG was exactly over the centre of the mount - then measured the CofG of the ST80 and cut holes in a much longer dovetail so I could mount it in a position where the CofG of the ST80 is directly above the CofG of the ED80 and the centre of the mount.

Now when I balance it in a horizontal position and then rotate it the balance point doesn't move.

I tried it last night albeit in far from ideal conditions - no real darkness and breezy but my guiding graph was much improved over anything I have achieved in the past.

Do you think this is coincidental or is it something experienced people already know/do ??

[ollypenrice]

Dynamic balance is the thing, for me. You need to be balanced, or slightly imbalanced east heavy, across the whole range of sky that you intend to photograph. This is often more tricky with side by side than piggyback rigs but the principles are simple enough. The dual rig balancing tutorials are there to be seen. I think Steve Richards did a good one somewhere. Olly

[laser_jock99]

Balancing your scopes and cameras requires some thought as they need to be balanced in more than one axis. As you suggest the scopes may well end up off-set to each other.

The proceedure I use is to pre-mark the balance point of each scope (with any cameras attached) with coloured insulating tape.

You can then easily place the marked centre of balance directly over the saddle centre every time without too much faff.

Any secondary scope/camera should also have it's balance point marked with tape. The dovetail bar then needs to be placed so it sits direct above the balance point of the first scope- this may involve drilling new holes or getting  longer bars to get things exactly right.

Now that's one axis sorted- you may need to think about other 'offsets'. For instance with the small Newtonian rig below I had to shift the refractor tube over to one side to counter balance the side mounted DSLR (note also the 'balance point tape' on both tubes for quick setup).