The answer to "is this field rotation?"

[themos]

I've looked for a method to relate polar alignement error to field rotation in images. Somebody's done the number crunching and presented it all here:

On Polar Axis Alignment

Unfortunately, some coding errors have crept in so please verify the calculations.

field rotation in microns =

(polar alignment error in arcmins)*

(exposure time in mins)*

(largest distance of guide star from edge of image frame in degs) *

(focal length in mm) /

45000 /

(cosine of declination of image centre)

So, for a guide star 3 degrees away, polar alignment error of 10 arcmins, exposure of 5 mins, focal length 300mm and target of M31 at 41 degrees declination, we get

1.3 microns field rotation.

[themos]

I'd love to have a formula relating field rotation as an actual angle, though. Anyone know of one?