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.
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.