Orthogonality and Polar alignment

In various threads, I have read discussion that if the mount's axes are not at right angles, you cannot get a good polar alignment. I have also read that you can overcome this by shimming. Well. Here is my question.

Is the requirement that the mount (where the dec meets the right ascension) be orthogonal, or that the scope be orthogonal to ??? (that the optical axis of the scope be at right angles to ????)? (and then, of course, just what does the optical axis of the scope have to be orthogonal to?)

I ask this because I have seen setups where an astrophoto guide scope rides on top of an imaging tube. It has it's own rings and can be set at a different angle than the main scope (pointed slightly elsewhere to find a good guide star). By definition, unless the imaging scope and guidescope are exactly collimated, one or the other would be non-orthogonal.

I understood that even if two scopes were pointed to different places, on a given mount, that neither would experience drift (as long as the mount itself were properly polar aligned).

Just wondering.

Alex

Orhogonality isn't a big issue vis-a-vis polar alignment these days, since most folks are autoguiding. In my experience autoguiding usually--surprisingly--works better if you're not dead on polar aligned. Orthogonality can affect go-to accuracy. The usual cure is shimming the scope in the dovetail/saddle.

Orthogonality of the RA and Dec axes does not
affect tracking and guiding, but may affect
go-to accuracy.

So, it seems that orthogonality has to be in the mount (where the Dec meets RA) rather than in the scope. At least as far as tracking goes.

As for goto and DSC, one may need to shim the scope, depending on whether the computer can figure out the degree of misalignment and compensate for it.

Alex