1. Apparatus for calibration of a probe that includes a magnetic position sensor and an acoustic imaging device, the apparatus comprising: a rigid mechanical framework; one or more field generators, fixed to the framework, and operative to generate a magnetic field of known spatial characteristics; an acoustic target assembly, which is in a known spatial relation to the framework and comprises a phantom coupled to a motion mechanism, which is arranged to move the phantom in a known orbit relative to the framework; a jig, fixed to the framework, for holding the probe within the magnetic field of the one or more field generators, in an orientation suitable for the imaging device to image the phantom; and a processor, which is coupled to receive, while the probe is in the jig, position signals from the position sensor responsively to the magnetic field and image signals from the imaging device responsively to reflection of acoustic waves from the phantom, and to process the position and image signals in order to calibrate coordinates of the imaging device relative to the position sensor.