Hi Mark,
Welcome to the Forum.
As to re-aiming your camera, consider the field of view of your lenses and the size of your target object.
The sky rotates 1 degree in 4 minutes. You can check the lens reviews here in the Forum (
Pentax F Prime Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database) for the field of view size. For a 135mm lens, the FoV is only 10 degree wide. You will probably want to re-aim every 10-20 minutes.
Your stacking software will have to be able to deal with aligning exposures that do not cover exactly the same area (and may need to be rotated as well). For Az/El mounts, like a tripod, the sky will generally appear to rotate
around the center of the field as well as across the FoV during an exposure.