Azaqui 3)Any follow up attempts at calibrating are unsuccessful with the same settings and target - this is the last calibration step before a failure:
The clue of why calibration failed is right in that screenshot.
Notice that it says "West Step 60, dist 21.7."
The way PHD2 autoguiding works is to detect how many pixels the guide star has moved from its original location, in delta-X and delta-Y in the coordinate system of the guide image.
PHD2 then tries to send a slow slew to the hour angle and declination motors of your mount to move the guide star back.
But, unless your guide camera angle is precisely aligned to the Right Ascension and Declination axis, the delta-x and delta-y are not directly related to the HA and declination, but through a rotation matrix. Even after rotation from (RA, declination) to (delta-X , delta-Y), PHD2 also needs to know the plate scale of the guide camera (i.e., how many arc seconds in RA and declination corresponds to a pixel.
Once it knows how much to correct the hour angle and declination, PHD2 then sends a N millisecond slew to the RA motor, and M milliseconds to the declination motor. These are called guide pulses.
To get the angle and plate scales, PHD2 goes trough a claibration process by purposefully applying the known guide pulses. In ASIAIR, the duration of these "calibration pulses" are given in the "Calibration Step" value of the Guide Settings window.
In your case, PHD2 has applied 60 calibration steps and the guide star has only moved 21.7 pixels!
PHD2 therefore gives up.
If you have a short focal length guide scope, you need to increase the Calibration Step time, and/or increase the guide rate (in Telescope settings).
Typically, with high quality mounts that only needs gentle guiding, you want to set the guide rate to 0.5 sidereal rate (i.e., 7.5 arc seconds per 1000 milliseconds). If the slope of the periodic error of the mount can become large, you can typically increase the guide rate to 0.75, or even 1x sidereal rate.
You then adjust the "Calibration Step" until you get somewhere between 10 and 20 setps for that "dist" number to reach or exceed 25. Lower than 4 steps will give you inaccurate guiding, greater than 20 steps is just a waste of time, but no real harm otherwise. 60 steps is too many to complete 25 pixels worth of movement, since that is when PHD2 decides your mount is not moving enough.
Chen
