casperround64:
Worked out it takes 3 Rotations for the large pulley to make 10 Deg
That seems to mean that the stepper motor would make 9 turns for 10°. If you are using full steps that would be 1800 steps. You can figure out the number of microsteps.
When you know how many degrees per minute (or hour) your telescope needs to move then you can work out the necessary interval between steps.
I don't know enough about astronomical observation to comment on @MarkT's observation. If poor Arduino time keeping is a problem maybe you could get a sufficient time correction by using a Real Time Clock (RTC) module.
...R