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I did a lot of testing on this. Although the OAGer (or SBIG internal guide) is ultimately the best way, it is possible to get acceptable results with external guide up to about 15min at a FL of 1500mm SCT). Unfortunately, in my tests, the Losmandy side by side set up gave the poorest results.
One problem is, the guideing refractor is heavy at the front end and an SCT is heavy at the back end, so they flexure is opposite.
In the end, I found the guide scope mounted on the main OTA eliminated flexure in the side by side plates. I hard bolted the guide scope to the main OTA with clamps (no adjustable rings) and bolted the guide cam to the guide scope with a steel bar, so I couldnt focus it. The wide field of the ST80 guide scope assured there was always a guide star without pointing adjustment, and focus stayed good enough over the long term without the need for refocus.
In short, 1st mount the Guide scope on the main OTA if you can, and if you find the above dodgy or too hard, get an OAGer ;-).
If you have a shorter FL and use DSLR (wider field of view), then the problem maybe solvable by just making things more ridged.
Fred
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