Single dish observations are explicitly differential: the signal from the On position (target+sky) is compared with the signal from the Off position (sky only). Beam switching is (usually) used for observations of compact sources. The wobbling (chopping) Secondary mirror moves the secondary mirror to deflect the beam back and forth between the target and a nearby blank sky position. Beam switching is combined with Position Switching to symmetrize the response.
UIP> secondary throw frequency
uip> chop N