[Poita (Peter)]

From the PHD FAQ:

Originally Posted by Poita

Q. When near the pole PHD doesn’t seem to guide or calibrate well.
A. When you guide near the pole there is less noticeable drift. and guiding becomes a lot easier. Calibration, particularly in Dec, becomes tougher. Think of it this way. If you were at the pole itself, you could turn your mount’s motors off and image all night long without any error. Polaris takes 24 hours to move in a small circle. In terms of physical, linear motion across the sky per second it’s darn near still. As you get closer to the equator you speed up and get to see your mount’s real errors. Right near the pole any EQ mount can track. This is why, when measuring your mount’s PE, you need to divide whatever PE you measured by the cosine of the star’s Dec. At the celestial equator, this is 1 and the readings are accurate. As you move nearer the pole, your PE measurements are smaller than truth and you need to scale them up. By the time you’re at Polaris with a Dec of 89.17, you need to scale your PE measurements by 68x to get them to really reflect what the mount was doing.
Or, if that doesn’t sit right with you, ever try moving your EQ mount manually to try to get to an object near the pole? As you get near the pole, even moving the scope manually to get to the right spot requires lots of effort - huge motions of the scope make little motions of the stars. Since the calibration process wants to see the stars move as a result of engaging the motors at guide rates, this gets to be a problem near the pole.
When imaging near the pole, you can increase the Calibration step and/or to turn off Dec guiding. You can also move away from the pole a bit more and calibrate there and then move back (don’t go too far). Remember, if you recalibrate on the one side of the meridian you must always recalibrate if you move to the other side of the meridian.