  In order to accept very forward angle scattering particles, Jefferson Lab HKS
experiment uses an on-target zero degree dipole magnet. The usual spectrometer
optics calibration procedure has to be modified due to this on-target field.
This paper describes a new method to calibrate HKS spectrometer system. The
simulation of the calibration procedure shows the required resolution can be
achieved from initially inaccurate optical description.
