Large earth-based astronomical telescopes are used for deep space observation. These telescopes are situated in observatories which are high on mountains and in remote locations to reduce the effects of atmospheric pollution and stray artificial light from towns and cities.
Such telescopes have mirrors several metres in diameter which are coated with a reflective thin film, usually of aluminium or silver. Over time, this film gets damaged by oxidation, dust and impact of charged particles from space. The old coated film must be removed and a new film applied. The delicate and hugely expensive mirrors cannot be brought down from the observatory and so the mirror coater with is built into the observatory.
HHV has supplied a number of telescope mirror coaters to observatories in India in recent years, and has also recently completed an export order from a customer in Russia for a large sputter coater to deposit single or multi-layer coatings on an astronomical telescope mirror of 2.55 metre diameter and a weight of 3 tonnes.
Telescope mirror coating
This sputter coater has a process chamber comprising two torispherical halves size of 3.1m diameter and 1.5m height. During opening of the chamber the upper half is lifted and moved away from the lower part of chamber horizontally using a system of rails and a high-output motor drive. The inner chamber surfaces are suitably polished for low out gassing rates
Three HHV 20kW water cooled rectangular magnetron sources hold Aluminum targets of size 180mm x 1000mm. The magnetrons are designed so the magnets are isolated from the cooling water. The magnetrons are supported in the upper chamber with mechanisms to adjust the distance and angle for downward deposition onto the telescope mirror. The SiO2 based protective layer for both Al and Ag coating is deposited using ion assisted reactive sputtering of a high purity silicon target using a pulsed DC power source.
The 3 tonne mirror is supported in the lower part of the chamber on a centrally-mounted rotary hub. Arms from the central hub lead to three 9-point kinematic supports with soft pads on which the mirror rests during the re-coating process. The location of these supports is critical since they have to take the weight of the mirror. Finite Element Analysis (FEA) is used to determine the optimal position of the mirror supports.
A cryo pump based vacuum system with roots and dry pumps facilitates to evacuate the process chamber to a vacuum level of 5x 10-6mbar. A mass flow control system admits process gas into the chamber with a flow rate of 1 to 500sccm.
Sensors are provided to monitor the coating thickness, reflectivity and uniformity and the entire system including vacuum and deposition process is controlled automatically by a SCADA-based control system.
The system was supplied with a separate pre-cleaning chamber fabricated out of corrosion free materials. The chamber has a whiffle tree mirror mount to hold the various sizes of telescope mirror during manual cleaning operations to remove the previous reflective films without damaging the substrate with HCl, HNO3 and distilled water.
Specification: Reflection coefficient value of coated mirrors
Reflection coefficient value (Al+SiO2)
Muti-layer Aluminum based coating
0, 32 – 0, 38 µm : > 80%
0, 38 – 0, 78 µm : 85%
1, 00 – 2, 50 µm : < 90%
Muti-layer Silver based coating (NiCr +Ag+SiO2)
0, 38 – 0, 78 µm : 94%
1, 00 – 2, 50 µm : < 94%
Total coating thickness on the reflectingsurface is < 350 nm
Total thickness variation of the coatings is < 5%.
Exporting a well engineered, complex sputter coater for precise multi layer metal coating with protective layer coating on a heavy weight, large surface telescope mirror with proven processes perfectly illustrates the capabilities of the HHV company.