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Acquisition and Guidance Unit

The Acquisition & Guiding (A & G) unit is located within the instrument support structure (ISS), which is mounted on the cassegrain rotator (CR) of each telescope. The CR may be rotated to any desired position angle. Because the telescopes are alt-az mounts, the CR normally moves during an observation to de-rotate the field.

To understand the limitations on use of the peripheral wavefront sensors (PWFSs) and Acquisition Camera, and how the telescope beam is redirected to the instruments or from the facility calibration unit, GCAL, it is useful to visualise the nested modules of the A & G system.

schematic of A & G modules

The A & G module contents are:

  • Module 4: (closest to the sky) first peripheral wavefront sensor.
  • Module 3: second peripheral wavefront sensor and facility AO pick-off mirror.
  • Module 2: science fold, directs telescope beam to a side-looking science instrument or retracts to allow passage of beam to the up-looking instrument. Alternatively can feed AO-corrected beam or calibration unit to science instruments. 
  • Module 1: acquisition camera/high resolution wavefront sensor.

Modules 2, 3 and 4 may rotate independently about the vertical axis.