Acquisition and Guidance Unit Layout

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

The A & G is located within the instrument support structure (ISS) mounted on the cassegrain rotator which may be rotated to any desired position angle (and normally moves to de-rotate the field of the alt-az telescope).

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 and future location of polarisation modulator.

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

Last update September 22, 1999; Phil Puxley