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Hi Matty
If you imagine your mount guiding on a star in the centre of the field of view, then the mount should faithfully follow that star and produce a nice, round stellar disc.
However, stars around this central star will show levels of trailling the further they are away from that central star; the whole field will appear to revolve around the central star. This applies to long duration images by the way.
For short duration images, where the exposure is insufficient to record star trails, you will have a series of overlapping frames that gradually rotate around the centre star.
Just imagine starting an exposure with your scope pointing at the SCP. If you take a series of photos, you will note that the Southern Cross may be on your left (East) when you start and with time it will cross the Meridian and then begin to drop into the West to your right.
So, on your starting frame you will have your guidestar in the middle with Crux to the left. Then, you will have the guidestar in the middle with Crux above it and finally, the guide star in the middle and Crux to the right. This is field rotation.
Cheers
Dennis
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