Completion of manufacturing of coating chamber for the TAO 6.5m telescope

Mirror Coating of Telescopes

For infrared telescopes it needs to increase the reflectance and to reduce emissivity as much as possible, because infrared radiation emitted from the telescope itself interrupts the infrared radiation from universe and declines its detection limit. For this reason, telescopes need regular maintenance to remove surface contamination and to refresh the reflection film. Easily, washing the mirror can restore the optical performance temporarily, but in the long term, the reflective layer needs to be regenerated. That is (re)coating of the mirror.

Generally, mirrors are recoated in a cycle of one or several years. This is to improve observation sensitivity, i.e., to increase the reflectivity of the mirror and to reduce emissivity caused by contamination or deterioration of the metal layer. The coating material of telescopes specializing in infrared observation may use gold or silver, which have slightly higher reflectivity than other metal in the infrared wavelength. But the TAO telescope, which observes from visible to infrared wavelength, uses aluminum with high reflectivity over a wide range of wavelength.

▲Primary Mirror of TAO 6.5m before coating (at the Mirror Lab in Arizona University)

Coating plant of the TAO Telescope

The primary mirror of the TAO telescope has a large diameter of 6.5m, we need a big chamber that can hold the whole mirror. The main development item of the coating system is the chamber. The upper part of the chamber is the largest part of the TAO telescope facility, having the weight of almost 40 tons for one parts. The primary mirror cell, including the primary mirror and actuators (active control unit of the mirror), are also the parts of the coating chamber. That is the feature of the coating system of the TAO.
The primary mirror should be treated carefully, because it is very delicate component. Then we do not only carry it down from the summit, but also not take it out or put into the cell, after it is once set into the cell. The equipment consists of the upper part of the deposition chamber, primary mirror cell, and the lower part of the coating chamber. In addition, there is a specialized cart for removing the mirror from the telescope and to move it to the cleaning or coating areas. All items of this coating system will be installed in the operation building beside of the telescope enclosure at the summit.

▲Conceptual design of the coating system for the TAO telescope, including the primary mirror cell and the jack-up cart.

▲Layout of coating facility in the operation building at the summit. Various items are arranged orderly for efficiently usage.

Fabrication of the Deposition Chamber

The conceptual design of the coating system was started from 2015, continuously performed basic design such as material selection, strength calculations, and performance evaluation of attached equipment. Afterwards, we proceeded to consider of machining tools and processing methods. Fabrication of the chamber started in 2018, when the detailed design was almost finalized. In addition to the main chamber, we made various flanges, scaffolding of deposition equipment in the chamber, storage boxes of filaments, and weight shelves for the attached equipment.

▲(left) Inner floor to be installed in the chamber. The filaments will be set as same number of the holes. (right) As the chamber is large, special step is required for inspection of top and inside of chamber.

Coating to the primary mirror is performed in the chamber which in vacuum drawn inside. Due to the property of the evaporation vacuum vessel, which must not damage the primary mirror, welding defect must be absolutely avoided. Therefore, each part of the chamber, made entirely of stainless steel, was carefully checked and welded during the manufacturing process. After about a year of manufacture, during which we repeatedly considered the processing and holding methods, we were finally able to see the completed coating chamber. Despite maximum diameter of the chamber is nearly 7m, its accuracy was less than 0.5 mm, that is fully satisfying the specifications.

▲(left) Processing the chamber, (right) Inspection of the welding inside the upper chamber

Completed coating chamber with attached equipment, will be combined with the ancillary equipment (control equipment, power supply equipment, vacuum pumps, compressors, etc.) that had been separately manufactured or purchased domestically or internationally. And comprehensive performance evaluation test of vacuum and deposition will be carried out as the coating system.

▲Completed coating chamber: you can see its size when you compare it to the person beside the chamber.

Operation test of coating

The coating equipment will be used at the summit of Co. Chajnantor. Before its transportation and installation, it is necessary to make sure that the targeted performance is achieved. After combining vacuum pumps which are to achieve a vacuum inside the huge chamber, compressor, chiller for cooling the equipment, control unit for coating, power supply, etc. as the coating unit, performance evaluation tests were carried out under the same conditions as those for coating procedure at the summit. The first step in the test was to check the vacuum conditions (target vacuum level and the time required for coating). The higher the degree of vacuum, the better the film adhesion during deposition, it is necessary to reach a vacuum level higher than the specified level. It was confirmed that the vacuum level could be reached in about half a day.

The mirror surface must be clean for coating (metal film generation). If there is dirt or uneven cleaning, the film will not adhere properly, which may cause film peeling or partial reduction in reflectivity. In the process of coating operations cleaning is very important. After the mirrors are washed by water, an operation called "ion bombardment (ion discharger)" is performed to remove oxide film and molecular-level dirt that cannot be removed by washing. In this process, a small amount of gas is put into the chamber, ionized by a high-voltage power supply, and hit it to the mirror to remove contaminations from the mirror surface. A tape test was performed for each deposition on sample glass placed in the chamber (27 pieces at 108 locations) to determine the optimum parameters such as voltage and gas pressure for ion bombardment.

▲(left) Inside the chamber during the ion bombardment: the enclosed gas is ionized by the high voltage and appears to glow. (right) Deposition: you can see filaments placed above the round window at the top, which shines and illuminates the inside of the chamber. At the bottom side, you can see the base on which the sample glass is placed.

As the coating process the most important film deposition test is also conducted, thickness of coating film on sample glasses should be measured to determine the optimum conditions for the parameters. The deposition system for TAO telescope controls 147 filaments with four power supplies. In order to find the optimum values for film deposition, many tests were repeated; the voltage settings, voltage application time, timing, and other conditions of these power supplies etc. The weight of the filament before and after deposition were also measured to estimate how much aluminum had evaporated. The deposition tests were repeated multiple times in March and April 2020, and finally we could pass the tests in terms of reflectivity, film thickness, uniformity of reflectivity and film thickness, and tape test (film adhesion). Finally we succeeded in producing a coating film that satisfied the specifications for the mirror of the TAO telescope. Since coating is performed in the vacuum chamber (high vacuum ~10-6 torr), the same aluminum film would be produced at the summit (even if in different environment) under the same coating parameters as in these test.

After a series of tests, disassembly and export packing is underway for transportation to Chile. The equipment for coating system has many electrical components, and the huge chamber which is largest and heaviest as a single item of TAO. Sufficient measures will be taken to prepare for the long journey to Chile.

▲Total performance test of coating: all equipment is placed and connected to the coating chamber.