Date:19/09/2002 URL: http://www.thehindu.com/thehindu/seta/2002/09/19/stories/2002091900080200.htm

Machine for coating telescope mirror

The unit comprises a vertically mounted, single walled cylindrical vacuum chamberand is fabricated out of non-magnetic stainless steel.

THE BANGALORE based, Hind High Vacuum (HHV) Ltd. has pioneered the commercialisation of high vacuum technology. When the Indian Institute of Astrophysics (IIA) wanted a sophisticated vacuum coating machine to service its telescope located at Hanle (elevation 4300 m) in Northeast Ladakh, it turned to HHV.

According to A. K. Saxena of IIA, who has developed the coating process, the performance of the telescope depends on high reflectance from the mirror .

The aluminium surface of the primary mirror, which is exposed to atmosphere, will corrode and result in a gradual decline in the reflectivity due to the atmospheric interactions. Thereforethe mirror has to be periodically coated and recoated. The large area vacuum coating unit is capable of aluminising the 2100 mm diameter primary mirror of the Hanle observatory telescope. The coating thickness for this application is 1000 Angstroms +5 per cent. The peak reflectance achieved is 93 per cent with average reflectance of 89 per cent.

The unit comprises a vertically mounted, single walled cylindrical vacuum chamber. .

The top dished end of the chamber is lifted and lowered with a hydraulic lifting arrangement. A vacuum seal between the top and the chamber is achieved by an `O' ring made of a special silicon based elastomer.

The giant mirror is accommodated in a specially designed work holder which can be rotated at speeds up to 10 RPM during the coating process by means of an AC drive and gearing.

After the mirror is placed in position, the chamber is evacuated in two stages to achieve a vacuum of 10 to the power minus 3 m.bar and 10 to the power of minus 6 m. bar. .

A sequential type ion bombardment gadget is provided for glow discharge cleaning of the substrate. The coating itself is achieved by evaporating aluminium and other substances from a low voltage, high current (10 volt/1500 amps) evaporation source. The thickness of deposit is monitored and controlled by a microprocessor-based quartz crystal deposition controller. Thus, the entire time for the process is 3 hours.

A provision has been made for the low ambient temperature that the unit can encounter when not in use. In case the temperature drops below freezing point of water, all the water in the pipelines connected to the diffusion pump, rotary pump and other electrical feed throughs will be drained out automatically by purging it with pneumatic air including liquid nitrogen. When the temperature reaches the required room temperature, the system automatically comes to the working mode.

N. N. Sachitanand