So, the basic components of the system are now working independently:

• Servo and stepper control, including acceleration/deceleration ramp (no video yet!)
  
• Reading magnetometer (for compass heading)
  This seems to be heavily affected by the nearby stepper, even when it's turned off! The compass is necessary to know what direction the cat is pointing at, to accurately trace the orbit of the object being tracked.
  
• Set up a PythonAnywhere server to fetch ISS orbit data and compute the local azimuth and elevation using PyEphem
  

The next step will be to make all these components work together and somehow filter/clean up the compass data :-)