We would need to derive two transformation matrices:
SKY_TO_FP
FP_TO_CCD
These matrices would need data which would be:
Imaging of astrometric data at different drive angles
Imaging data of the focus pinhole
METHOD:
Derive a transformation from FP_TO_CCD based on the focus images. Shui has done some of that using the routines to focus DEIMOS.
Observe an astrometric field with DEIMOS (close to senith) at 5-10 drive values to derive a SKY_TO_CCD transformation.
Use the FP_TO_CCD and the SKY_TO_CCD transformation to derive a SKY_TO_FP transformation per drive angle.
Determine what the best transformation to use. One option is to combine all the SKY_TO_FP per angle into one.