Best Autoguiders and Guide Scopes

Guide scopes are easier to set up and work well with focal lengths under about 1000 mm because differential flexure between the guide scope and the main tube is small enough to ignore. Off-axis guiders pick off light from inside the main optical path, eliminating flexure at the cost of a smaller guide-star field and a more complex setup. At focal lengths above 1000 mm or with smaller sensors, an OAG is usually the better choice.

A guide camera only needs to find a magnitude 8 to 10 star reliably in its field of view, which most dedicated astro guide cameras handle easily. The key factor is the combination of guide scope aperture and camera pixel scale. A 50 mm guide scope gives a wide field to find guide stars; if your sky is poor, moving to a 60 or 80 mm guide scope gives more stars to choose from.

Only if you add an off-axis guider. You cannot use the main camera alone because it is capturing the long exposure you are trying to guide. An OAG inserts a pick-off prism in the light path that diverts a small fraction of the field to a dedicated guide camera, so both cameras run simultaneously from the same optical axis.

PHD2's built-in Guiding Assistant is the right starting point. Run it for five to ten minutes while the mount tracks, and it will measure your mount's drift and periodic error and recommend a minimum move value, plus Dec backlash compensation if it measures significant backlash. Avoid manually tuning guiding settings until you understand what the Guiding Assistant results tell you about your mount's periodic error profile.