The large spectrometer is the MRS, or Medium Resolution Spectrometer.
It is moved around the track on air pads just like a hovercraft, and parked at a new angle to take measurements of the particles given off the target at each angle.
It's like nuclear billiards. If you can catch all the particles given off from a collision between the proton beam and the target material at location 4BT2, and you can measure the mass, the energy, the velocity, the charge and the spin of each particle at each angle, then you can mathematically put the original nucleus back together and see what shape it was in and how each nuclear particle was bound in the original nucleus before it was cracked apart by the beam.
