Mirror Segments

Introduction and Layout
Alignment of the 91 mirrors

The spherical primary mirror has a master radius of curvature of 26 165 mm. It consists of 91 interchangeable hexagonal mirror segments, each of 1m inscribed diameter, forming a hexagon of ~11.1 x ~9.8 m.

Primary mirror Lay-out

Each mirror segment is manufactured of Sitall material (provided by Russian company LZOS) and designed to be identical to every other segment. The segments are interchangeable within the array. Segment thickness is 50 mm at the corner, and segment weight is approximately 100 kilograms. Probably the most important property of the Sitall material is its low Thermal Coefficient of Expansion, which is below 150 parts per billion per degree Celsius. The picture shows Arek Swat (SALT Optical Engineer), Dr. Magomed Abdulkadyrov (LZOS Chief Technologist), Dr. David Buckley (SALT Project Scientist) and Jian Swiegers (SALT Primary Mirror System Manager) behind the first blank produced for SALT.

Eastman Kodak Company in the USA will figure the optical surface to a surface quality of 1/15 a wavelength of light, or 33nm RMS. This is equivalent to 1/3000 the thickness of a sheet of paper. All segments have radii matched to the nominal radius to within 0.5 mm. Testing for final optical figuring of the segments will be performed at the average angle of 37° from the vertical, supported at nine points by a mirror mount identical to those in the telescope. Print-through of the nine support points will be removed to the specified level during this operation. Springback will occur when the segments are installed at angles different from 37°. The amount of springback is determined by the gravity vector direction and segment stiffness properties.

Mounts and Actuators

Each mirror mount spreads the mirror segment load from 9 contact points at the bottom of the segment surface through a lever to three actuators within the support frame. The actuators drive the segment up and down during operation, effecting tip, tilt and piston movement. The maximum error in the commanded movement that the actuator-lever system is allowed to make is 30nm.

Radial and in-plane rotational support is accomplished using a centre hub embedded in the hole in the bottom of the segment. The design of the hub has to be such that forces transferred to the segment cause optical surface distortions within allocated limits.

Once the segment is installed on the support, the entire assembly is attached to a special handling tool and hoisted into the telescope by means of the dome crane. The assembly is lowered onto jacking devise, released from the crane, and lowered carefully into its space in the array.

EOS Technologies in Tucson, Arizona will supply the mounts as well as the mirror handling tool.

Mirror Truss

The main structural element of the Primary Mirror System is a space frame structure called the mirror truss, constructed out of nodes and struts. It contacts the telescope structure on just three kinematic points. Mirror mounts are bolted to the top truss nodes and provide 9-point support to the mirror segments, as well as motion control in tip/tilt and piston. BKS Advantech in Pretoria, South Africa, has been contracted to deliver this sub-system.