[JohnH]

As has already been mentioned this should be about relative image scale in "/pixel and not just fl there is dependency on the chip and pixel sizes.

As we are talking autoguiding there is no need to be at the same fl as the main scope if you are autoguiding. Typical autoguiding software will do sub-pixel guiding to +-0.2 or less pixels.

In my case the FL of my guidescope is 388mm, my main scope is 1800mm, my guidecam is an Orion DSI (one shot colour) and the imager is a Canon20D.

So my guide resolution is 3.42" per pixel and imaging is 0.73"". This is in fact marginal at 0.2 pixel accuracy.

One discovery I have made recently is that one shot colour cams really should be avoided for guiding - they are less sensitive, and the debayering process requires processing and can cause loss of precision.

The second issue is the type of guiding setup you intend to use, some of the software out there is MUCH better at guiding than others (thus you may be able to get to 0.1 pixel precision or better).

Finally it will depend on your mount and its accuracy.

I have a long boring thread on this elsewhere.