HHV supplies a sputter coater to re-coat the telescopic mirror of approximately 4 meters dia x 1.7 meters height x 4.5 tonne weight.

Introduction

The mirrors of optical telescopes  are gets damaged  and tarnished over time due to oxidation,  dust , impact of charged particles from space   etc.  Therefore, the mirror has to be re-coated from time to time to maintain the accuracy of observations. Till recently, the ability to design and make such complex re-coating equipment for large mirrors was limited to a few specialized companies in the world. HHV has successfully designed, manufactured and supplied a re-coating machine for the 360 – cm telescope to be installed at Devasthal, 60 kms away from Nainitaland is  Situated at an altitude of 2500 meters above Mean Sea Level. The Aryabhatta Research Institute of Observational Sciences (ARIES) is setting up this facility.

Challenge

HHV’s re-coater will be positioning the 370 cm dia., 4.5 tonne weight primary mirror during the coating process on a special support called the Whittle tree arrangement. This rotary work holder is fabricated from stainless steel and basically comprises a central hub attached to a thick hollow tube.  Arms from this central hub lead to three 9-point kinematic supports with soft pads on which the mirror will rest during the re-coating process.  The location of these supports is critical since they have to take the weight of the mirror, which is 4.5 tonnes , while it is being rotated  at around 3 to 5 RPM during the coating process .  The thickness of the mirror is just 12 cm. Therefore, any small misplacement of the support will result in the mirror cracking.  The mathematical technique of Finite Analysis is used to determine the optimal position of the supports.

The accomplishment of HHV in making this re-coating equipment is all the more praise worthy since the optical image quality demanded of the telescope is:  50% energy in < 0.3 arc sec diameter, 80% energy in < 0.45 arc sec diameter and 90% energy in < 0.6 arc sec diameter (for 10 arc min field without corrector) .This in turn necessitates that after the re-coating process, the new mirror coating should have a surface accuracy of 1/16^th of 5500 nanometers.

Description

The coating of the mirror is done in a vacuum chamber of approximately 4 metres dia. and 1.7 metres height.  The chamber is made up of two torrispherical dishd ends.  The inner surface of the chamber is suitably polished to minimise entrapment of any gases.  The chamber and sub assemblies are welded using TIG Welding Technique for leak tight joints. The whole unit is leak tested using Helium Mass Spectrometer leak detector to an individual leak rate of 3 x 10^-8 m.bar lit/ sec. The chamber is provided with Viton ‘O’ ring seal made from Viton Chord to achieve vacuum sealing. 

Operations

For the purpose of recoating, the mirror is hoisted down by a special crane from its mount on the telescope on to the whittle tree located in the bottom half of the vacuum chamber.  Here the old coating is first completely removed using Caustic Soda and soap solution.  Then the top half of the vacuum chamber, which runs on rails, is brought over the bottom half and clamped down. After sealing the chamber, a combination series of Roots and Rotary vacuum pumps create a vacuum of 5 x 10 to the minus 6 m.bar.  The evacuation of the chamber takes around 4 hours.

Magnetron source

Then, the 20 KW magnetron in the chamber is brought into play to actually deposit the aluminium on to the mirror surface.   The needed thickness of the film is achieved in about half an hour. Sensors are provided to check the coating thickness, reflectivity and uniformity.

A water cooled Rectangular Magnetron Source to hold Aluminium target of size 89mm (W) x 2000mm (L),  6mm thick having purity of 99.999%.is provided and is supported on the inner side of the top lid with suitable supports and adjusting mechanisms to adjust the distance and angle for Aluminium Sputtering on 3.7m dia, Telescope Mirror.

Expertise

This is not the first time HHV has made and supplied coaters for telescope mirrors to Indian users.  It has supplied one to IUCAA, Pune for its 2.2 metre telescope which is installed at a height of 14,700 feet above MSL   and another unit to the Indian Institute of Astrophysics, Bangalore, for its 2 metre telescope.