### BEAM SWITCHING

BEAM SWITCHING is realized through a rotating chopper wheel in the receiver cabin, which during each rotatation (= switching cycle) moves 2 reflecting (!) blades into the beam path in front of the 4th mirror, for a total of 4 phases: direct beam path to the source (same as in TOTAL POWER!), beam path offset by one blade of the chopper wheel, direct beam path to the source (same as in TOTAL POWER!), beam path offset by the other blade of the chopper wheel. The offset, rotation period, and blanking times are fixed. BEAM SWITCHING is normally only used for POINTING and FOCUS.

The source signal is calculated as difference between the direct and offset phases.

(NOTES. This “beam-switching” chopper wheel, should not be confused with a “calibration chopper wheel” as it is used at some other mm-wave observatories. Calibration at the 30-M Telescope is done with different hardware.)