Introduction and Layout
Mirror segments

A major driving factor of the telescope's optical performance is how close the segmented Primary Mirror array approximates a perfect monolithic spherical mirror. The process of manipulating the 91 segments in the Primary Mirror array in order to approach the master spherical surface as closely as possible is referred to as Primary Mirror Alignment.

Alignment takes place in two stages:

• Periodic calibration (also referred to as initial alignment) of SAMS will be performed with input from an optical alignment sensor situated at the centre of curvature of the Primary Mirror. The baseline system for the performance of the initial alignment is a Shack-Hartman sensor.
• A Segment Alignment Maintenance System (SAMS) will continuously measure, calculate corrections, and send commands to the actuators in the mirror mounts to achieve this. The baseline design for SAMS consists of a PC-based control system and edge sensors, two of which will be fitted in the gap formed by each pair of two segments. The sensors will measure the relative height of all segments with respect to its neighbours.
  

The goal is to keep the tip/tilt deviation of the segments from their ideal positions on the master sphere to less than 0.06 arcseconds. This is equivalent to keeping rays of light reflecting from the surface of all the segments focussed so that they form a spot on a screen 1 km away, within a circle with radius of 0.3mm on the screen!

Aluminium will be used as the coating material for the optical surface of each mirror. A coating plant will be built and installed in the telescope building by Special Products Technologies in Cape Town. Each segment will be coated at least once every two years.

In order to achieve the required quality of coating, the vacuum in the chamber will be of the order of 10-6 Torr and 99.99% pure aluminium will be used. The chamber will be able to receive two segments at a time and will include a glow discharge cycle to ensure the surface is free of any contaminants.

The mirrors will be cleaned every 2 to 3 weeks by means of  snow spray.