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Calar Alto Mirrors in Space
14 May 2009
Calar Alto Observatory-CAHA
The space telescope Herschel is the largest space-borne observatory to date, carrying the biggest astronomical mirror ever launched into orbit. Calar Alto Observatory has participated in a way in this challenging project of the European Space Agency as the company that built the telescope, EADS Astrium (Toulouse, France), relied on Calar Alto facilities and staff to perform the critical operation of applying the reflective aluminum coating to the mirror.
Herschel space telescope is a spacecraft launched by the European Space Agency (ESA) to perform cutting-edge astronomical studies of the cold universe; in particular, the formation and evolution of stars and galaxies and the relationship between the two. A pioneering mission that will help understand how the Universe came to be what it is today.
The space mission Herschel carries the largest space telescope ever launched. With its 3.5-m primary mirror, it is four times larger than any previous infrared space telescope and almost one and a half times larger than the Hubble space telescope. The size of the primary mirror is the key to a telescope's sensitivity: the larger it is, the fainter the objects it can see. But the surface of the mirror has to be precisely shaped and show a perfect and uniform reflectivity since the slightest roughness would distort its image. Herschel mirror, an innovative device made from silicon carbide, got its top quality aluminum reflective surface at Calar Alto coating facility.
Calar Alto coating facility
Ground-based telescopes have mirrors whose reflective aluminium coating has to be renewed from time to time to keep them in perfect conditions for astronomical research. For this reason, the best observatories in the world are equipped with coating facilities to periodically re-aluminise their mirrors. Calar Alto Observatory has two of such devices: one is capable of coating mirrors up to 2.5 m diameter, and the other accepts mirrors as large as 3.5 m.
The reflecting quality of the mirrors of Calar Alto telescopes is periodically tested. When time comes to clean and re-coat the mirrors, the procedure follows several strictly pre-specified steps. First of all, the mirror has to be removed from the telescope, a task not as easy as it may seem, given the huge weight of these vitro-ceramic blanks: around one ton for the 1.23 m telescope, 2.5 tons in the case of the 2.2 m telescope and an incredible 12 tons of the 3.5 m primary.
The mirrors are carefully cleaned and dust, spots, and the old aluminium coating are totally removed. The mirror, deprived from its reflecting capability, is then introduced into a huge vacuum chamber. The vacuum chamber at the building of the 3.5 m telescope has an inner volume of 90 cubic metres. After closing the chamber, powerful pumps create a high vacuum inside the chamber: the inner pressure is reduced down to one millionth of one thousandth of the normal atmospheric pressure for standard coating procedures (1×10-6 mbar).
Once the intended vacuum is reached, an overwhelming electrical intensity of up to 1300 Ampères is driven through a set of resistances, into which small pure (99.999%) aluminium bars had been previously inserted. In less than 80 seconds, the aluminium is completely vaporised and it expands through the space inside the chamber, coating all the surfaces it finds in its way.
In the standard procedure, the aluminum layer deposited onto the mirror surface grows up to a thickness of one tenth of one thousandth of a millimetre (one tenth of the size of a bacterium). The system versatility allows to perform non-standard coatings, including nickel-chromium layers and plasil protective films. Also, aluminium layers of different thicknesses can be applied. In the case of Herschel mirror, a final thickness of almost one thousand of a millimetre was required by EADS Astrium.
Calar Alto offers this coating service to external entities, both public and private. Mirrors, reflectors, and antennae from many different observatories and companies around the world have come to Calar Alto to get their surfaces coated according to the maximum standards of quality: from Europe to Japan, from Earth to space, many mirrors carry Calar Alto coatings whose outstanding performance is guaranteed by the many years of experience of Calar Alto staff and the extreme high quality of the facilities available at our Observatory.
Inspection of Herschel primary mirror after aluminisation at Calar Alto. Copyright Patrick Dumas (Astrium).
Herschel primary mirror exiting from the vacuum chamber at Calar Alto Observatory after aluminisation. Copyright EADS Astrium.
Joint test of both primary and secondary mirrors of Herschel space telescope at Calar Alto, after aluminisation. Copyright Patrick Dumas (Astrium).
The primary mirror of the 3.5 m telescope exiting from the vacuum chamber after aluminisation.