What motor to move a small telescope?
I wouldn't use a servo for this. You don't have very fine control over the angle with a servo. Also with a typical telescope the focus is more on rather slow and precise movements, instead of fast ones.
Instead I would use a stepper motor. Depending on your exact requirements, these can be rather cheap to rather expensive.
Here also the mechanics come into play. The motor should not be attached directly to the axis, but instead through some gears (the axis of the telescope having the larger gear). That will allow for smaller, more precise movements and also lower the needed torque for the motor. Of course this depends on your actual telescope hardware, meaning if you can put a gear right to the axis. If not, you could limit the motor driven movement a bit and install a level, which then could be pushed or pulled by a nut on a threaded rod, which gets driven by the servo motor. This would be rather easy to implement, though you may need to correct for the different rotation behavior, when you get to the outer sides of the movement (for the actuator being linear, while the resulting movement is circular), by some mathematics in your code.
Since you want to keep your costs very low, you might wanna use the 28BYJ-48 stepper motor (including the fitting driver). In terms of stepper motors they are rather bad, but they are really really cheap and - depending on the gear ratio/level length/thread pitch - it might not matter that much for you. You could always update to a better stepper motor in the future.