VISTA Coating Plant
It is necessary for the optimum performance of the overall VISTA system that a facility to periodically apply the reflective coating to both the primary (M1) and secondary (M2) mirrors is provided on site. This is the objective of the Mirror Coating Work Package. The plant which will be delivered with VISTA will utilize sputtering process technology with a minimum of three magnetrons to enable the automatic thin film deposition of the multiple layers required for a durable protected silver coating to minimise thermal emissivity. The plant will be situated in a building adjacent to the main telescope enclosure. The aim of the work package is to deliver to site a turn key system which has been fully tested and optimised at the facility of the contractor in the UK.
Specification
Vacuum
The vacuum system consists of a floor mounted rough pumping station and a cryopump system utilizing two cryopumps mounted on the vessel lid. The system is required to reach its working pressure within 2 hours.
- Rough Vacuum 2.7 x10-2 mbar
- Ultimate working pressure 3.0 x 10-7 mbar
- Partial pressure of water vapour 2.0 x 10-7 mbar
Magnetrons
The coating of the mirrors is done by using shielded DC planar magnetrons fitted with high purity targets. A shutter mechanism and appropriate mask is provided to ensure consistently repeatable gas distribution, film purity and uniformity of coating. The magnetrons are mounted inside the vessel in a radial orientation. Each layer is deposited during a single pass of the mirrors.
- Maximum deposition rate 800 ?/min
- Deposition rate reproducibility +/-1%
Control System
The coating plant is operated from a PLC based control system. This is capable of automatically controlling the complete coating process.
- M1 rotation between 1rpm and 0.016rpm
- M1 positional accuracy +/-2mm on the M1 circumference
- Shutter accuracy +/-0.2mm on the M1 circumference