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Coating Technology on the Summit

Mirror Layers for the Large Snynoptic Telescope in Chile

This article focuses on mirror coating for the Large Synoptic Survey Telescope, which is being installed in Chile. The LSST coating system consists of a coating chamber for the deposition of highly reflective optical coatings and a cleaning and stripping station for the M1M3 and M2 mirrors. With the help of the sputtering process, bare and protected silver/aluminum coating recipes can be deposited. The cleaning and stripping station consists of a rotating washing/drying boom, perimeter platforms, and an effluent handling system within the M1M3 mirror cell. The article describes the status of the assembly work on the coating system. Furthermore, it provides information about the progress of factory testing, a about the basic design features and reflective/coating requirements and the coating results.

By Norman Müller and Ulf Seyfert

02.12.2019, originally published in German the magazine Vakuum in Forschung und Praxis

It could be mistaken for a UFO: the chamber of the vacuum coating system on its way to Cerro Pachón in Chile.

 Mission of the Large Synoptic Survey Telescope

At present, a special ground-supported mirror telescope is being built on the summit of the Cerro Pachón in Chile, which is 2682 meters high. Even though its largest primary mirror is only 8.4 meters in diameter, and, therefore, not as large as those of the biggest observatories in the world, its wide-field viewing angle of 3.5 degrees is one of a kind. For comparison, the full moon, when looked at from Earth, covers a viewing angle of 0.5 degrees. This telescope will scan the entire visible sky of the southern hemisphere in just three days using the largest digital camera to date, with a diameter of 64 m and a resolution of 3.2 billion pixels. Therefore, the specialty of this telescope is the mapping of large areas of the sky in short time. It is, in other words, the super wide-angle version among the mirror telescopes.

This is achieved by using an optical system with three mirrors (fig. 1). The primary mirror (M1) is ring-shaped, has a diameter of 8.4 meters and is made from a single piece. The tertiary mirror (M3) is integrated into the opening of the M1, has a diameter of 5 meters and a different curvature radius than the M1. Both mirrors form a monolithic structure that is referred to as M1M3 mirror. Above it, the secondary mirror is located (M2) with a diameter of 3.4 meters. This optical design enables a continuous overlapping image of 20,000 square meters of sky in six wavelength ranges between 32 and 1,062 nanometers.

With this observatory, at least ten billion stars and galaxies shall be catalogued. With a frequency of two complete images per week, even short-term events such as supernovae could be observed and asteroids that could get potentially dangerously close to the Earth could be identified.

Mirror Coating

In order to maintain the optical characteristics that are necessary to operate the observatory, the mirrors, which are made of borosilicate glass, will be coated. This coating is based on silver and/or aluminum and will be made by means of magnetron sputtering (fig. 2). The layer system of the M1M2 mirror consists of an approx. 100 nanometers thin aluminum layer, which is protected by an approx. 8.5 nanometers thin silicon nitride layer.

For the smaller M2 mirror, a multilayer coating consisting of an adhesion layer of 6.5 nanometers thin nickel-chromium nitride, the actual reflection layer of 110 nanometers thin silver and a final protection layer of 8.5 nanometers thin silicon nitride. This silver layer has exceptionally good reflection properties in the visible and near infrared range. As the M2 mirror is directed downward and, therefore, better protected against the influences of the climate, its silver coating might last for several years. Thus, the light will be reflected, in sequence, by the aluminum layer of the M1 mirror, the silver layer of the M2 mirror and the aluminum layer of the M3 layer.

For these coatings, the proven recipes of the Gemini and SOAR telescopes will be used, which are located in direct vicinity on the Cerro Pachón under identical climatic conditions. The experience gathered there have shown that there are no major losses in reflectivity are to be expected for the silver-coated M2 mirror of the LSST over the years. The reflectivity of the secondary mirror of the Gimini South observatory, which is coated similarly, has remained the same since its initial coating in 2004. It has been cleaned on a regular basis with CO2. For the M3 mirror, it is estimated that the reflectivity will decrease by 1.5 % per year in the range of 450 nanometers and by only 0.025 % at 650 nanometers.

Figure 1: LSST ray optics with M1M3 combination (diameter of 8.4 meters or 5 meters, respectively, focal length 1.18 or 0.83, respectively), M2 mirror (diameter 3.4 meters, focal length 1.00) and lens system of the camera (on the right, amplified view of lenses L1, L2 and L3). The field of view of the setup with a flat sensor array of 64 cm in diameter is 3.5°. The small rectangles show the point spread function (PSF) of the light sources.

Figure 2: Expected reflectivity of the coated main mirrors (M1, M2, M3) for the usable wavelength range (filters in u, g, r, I, z and y4 band). The minimum reflectivity values necessary for the operation of the telescope (LSST requirement) are given for every filter element.

Figure 3: Layout of the observatory with the actual telescope, the service floor with the coating system and the connecting platform lift.

Status

After a successful acceptance procedure including coating tests on dummy substrates that were made with the coating system, which had been fully assembled at the chamber manufacturer MAN in Deggendorf, Germany, at the time, the machine was dismantled and prepared for shipment. From Deggendorf, the entire machine was transported by ship to Antwerp in the Netherlands. From there, it was shipped to Chile where it arrived in October 2018.

The biggest challenge however was the overland shipment of the huge chamber halves from the harbor to the summit of the Cerro Pachón. As the cargo was nine meters wide, street lamps, road signs and overhead wires had to removed. Furthermore, the several linked towing vehicles had to move up a slope of 15 degrees.

The most complicated bottleneck for all the components of the observatory was the passage through the Puclaro tunnel, not far away from the summit (fig. 7). Moving the cargo through it was precision work, especially for the two halves of the chamber, which are the largest components. The assembly of the coating system began in January 2019 in the assembly halls of the observatory (fig. 8). In July 2019, the coating system mastered its first major challenge with the initial coating of the M2 mirror on site. The resulting reflectivity values surpassed those which had been specified (fig. 9).

Now, the machine is ready for the M1M3 mirror, which has arrived on the summit in the meantime. Thus, the way should be free for the completion of the telescope. In 2022, the first images of the stars shall be captured by this unique mirror telescope.