What happens during calibration? How does it work?
A. During calibration, PHD engages the mount’s motors and watches how the star moves in response to the motors. This happens in “steps” and begins with RA. A guide pulse will be sent (the duration is controlled by the “Calibration step” parameter in the Advanced panel) and the star located. Another pulse is sent and the process is repeated until the star moves enough for PHD to have a good estimate of how far to expect the star to move for each millisecond of guide pulse. This criterion is 5% of the width of your guide camera’s sensor. If, after 60 tries it can’t move the star enough, it gives up. Once it has this estimate, it sends the same number of pulses to return the star to the original position (or near it, as backlash will keep it from getting exactly there).If Dec guiding is turned on, it then starts the Dec calibration routine. It tries to clear out any backlash by engaging the Dec motor on one direction and waiting until the star starts to move by a few pixels.
