Try this (full line) instead:

Code:
camera.pan += ang(target_ent.pan+180 - camera.pan) * minv(1, 4 * time_step);


The minv call is there so that if your frame rate dips below 16fps (time_step = 1), it won't overshoot and cause a violent shaking effect. Tweak the 4 if the rotation is not smooth enough for your liking (smaller number = more smoothing).