Hello Karen,
Peter is correct, and ideally you should have polar alignment accuracy at 1 arc minute of error or less. Of course, the lower the better. Even at this level, the measurements will still drift, but you can have Pempro adjust the graph so it reports your PE without the drift being reported.
You can see in the help section of Pempro, dift is normal:
http://www.siriusimaging.com/Help/PE.../grapharea.htm
To lock the drift to the one axis, in the "Acquire Tab", check the two boxes
Adjust X for X drift, and
Adjust Y for Y drift
Your RAW data will not be effected.
Steve