I know that this is a pretty old thread, but I wanted to offer some additional comments about guiding on a star that is a great distance from the center of the imaging field.
First, as far as the guiding rate itself, it really doesn't matter how far the guide star is offset from your imaging field because the rate will be exactly the same as long as you are using an equatorial mount with perfect polar alignment and with zero atmospheric refraction (however, "perfect" polar alignment is almost impossible and the only place you will have zero atmospheric refraction is in outer space -- or if you happen to be pointed directly at the zenith). So, under those highly improbably conditions any corrections to the guiding rate will work correctly regardless of where the guide scope is pointed. That said, the apparent movement of a star decreases as you get closer to one of the celestial poles and therefore to get accurate guiding you need to be able to detect far smaller offsets (errors) which will make guiding harder and that's one of the reasons why PhD prefers to be calibrated on a star that is closer to the celestial equator (and not nearer to one of the celestial poles).
Next, atmospheric refraction can be a pretty significant factor when trying to track a star and thus if the guide star is "suffering" a greater or lesser amount of atmospheric refraction than the main imaging camera that will change the apparent tracking rate over time (as the degree of refraction changes, and it will as a star rises or sets from its meridian).
Also, any field rotation that happens because of a non-perfect polar alignment will be magnified by the distance that the guide star is offset from the imaging field. I'm pretty sure this is a completely linear relationship, so the error would double if you offset the guide star by twice any given distance (going from one degree offset, to a two degree offset, to something that could be much greater if the guide scope is completely misaligned to the imaging field).
What all of the above means is that you really don't want to try to guide on a star that is a great distance from the center of the imaging field. Thus, in practice some of the earlier statements were wrong or at least incomplete.
Of course, if your image scale is very low (meaning a very short focal length lens) then your guiding or tracking doesn't need to be that good, so you may get acceptable results even when there is a large offset between the guide star and the imaging field.
As for any field rotation, you can use the following online calculator to see how much that can/will affect your image.
Edited by james7ca, 22 November 2018 - 06:35 AM.