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Hi Lineout,
It's all due to field rotation, as the ALT-AZ scope tracks an object, the rotation of that object changes in the eyepiece and therefore affects the Astrophoto.
Not so with the EQ mount as the TUBE rotates with the earth and therefore the view in the Eyepiece stays the same, hence the Astrophoto is not affected.
Computer controlled ALT-AZ mounts (like Meade and the huge observatory scopes) use field De-rotators on the eyepiece / focuser to compensate for the field rotation and therefore allowing long exposures.
Depending on how long you go with your exposures, you can get away with some field rotation by fixing it in processing and stacking software, but really only for short exposures.
I hope this answers your question.
Cheers
Chris
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