Thanks Kurt, I had a look at the Covington applet which I had not seen. I guess the Dec drift and field rotation calculation here assumes that any RA drift is being corrected ie for photography, which is not quite what I'm after. I'll downloaded your spreadsheet and will see if I can get a hold of that S&T article - sounds interesting. I don't have that issue so will need to visit the library I guess. Thanks for the info! Cheers David