The techniques for optical calibration of Jefferson Lab's large-acceptance
magnetic hadron spectrometer, BigBite, have been examined. The most consistent
and stable results were obtained by using a method based on singular value
decomposition. In spite of the complexity of the optics, the particles'
positions and momenta at the target have been precisely reconstructed from the
coordinates measured in the detectors by means of a single back-tracing matrix.
The technique is applicable to any similar magnetic spectrometer and any
particle type. For 0.55 GeV/c protons, we have established the vertex
resolution of 1.2 cm, angular resolutions of 7 mrad and 16 mrad (in-plane and
out-of-plane, respectively), and a relative momentum resolution of 1.6%.