Telescope pan / tilt - stepper motors or DC motors with optical sensor?

Definitely use the stepper motors...they aren't as complicated as you would think once you start playing around with them. They are fantastic for open loop control. Yes - you would need to have a holding current supplied to them in order to hold position once you are no longer slewing; however, with a DC motor you would require a feedback control architecture (encoders) and then a brake of sorts to hold the axis in position once achieved.

Start with a simple stepper motor and integrated controller (you can get inexpensive solutions through Amazon) - you may have to tune your design based on torque requirements of the overall payload you are looking to position and how fast you need to move it.