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One point. Mounting a guide scope on top of large diameter scope can cause serious balance problems.
I put a William Optics 80mm on top of an RCOS 12.5 inch once and it burnt out the motors of an MI250 mount. It was at balance when horizontal but once it went to an angle the balance point shifted a lot causing too much strain on the motors.
So if you do that I'd check the balance at various angles. It may be a matter of positioning the top scope at the right point so the weight is more equally balanced over the balance point of the main OTA. Not sure.
So either a very light top mounted scope or side mounted guide scope like with a tandem bar. (I currently have a 10 inch scope with an Astrotech 66ED mounted on top and it is a small and light scope and balance does not seem to be an issue and I am getting great gudiing from it).
You can also mount a little guide scope using a Losmandy dovetail adapter mounted to the guide scope and it can attach to the front of the dovetail plate of your main OTA if you have a bit sticking out the front.
Works great with a Tak FSQ (which is a small, compact scope).
Greg.
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