Space telescope
Herschel (2009-2013) allowed fascinating insight into the
birth of stars.
(Photo: ESA)
(November 23, 2015) PTB
has measured thermal expansion at low temperatures for future space missions
Space holds numerous fascinating objects which we can only
investigate by observing their radiation – even beyond the visible range. For
space telescopes such as the European Space Agency's (ESA) infrared observatory
Herschel, whose mission is to observe radiation in the far-infrared, cooling
the instruments is of vital importance, since the instruments themselves must
not emit disturbing infrared radiation. The mirrors of these telescopes, which
are used at temperatures below -190 °C, are made of special, ultrastable
ceramics such as silicon carbide. In order to plan the exact dimensions
correctly, even at such low temperatures, the precise thermal expansion of the
materials used must be known. Within the scope of a recently completed ESA
project, the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig
measured the thermal expansion of these ceramics as well as that of
single-crystal silicon in the temperature range from -266 °C to +20 °C with
high accuracy. In vast parts of the temperature range investigated, the
accuracy attained corresponds to a relative change in length of approx. one
billionth per degree Celsius. The investigations have also shown that the
values used to date for the reference material of "single-crystal
silicon" must be corrected. The latest issue of the renowned scientific
journal "Physical Review B" contains a report dedicated to the latter
of these two subjects.
Space telescopes such as Herschel explore spectral ranges
that are not accessible from the Earth; they can therefore only be used in
space. How critical it is to know the exact thermal expansion of the materials
used when setting up such telescopes was clearly demonstrated during one of the
latest ESA missions, as it was revealed that the simulations performed
previously were not in agreement with the manufactured mirrors. The
discrepancies were fortunately not discovered in space, but still led to
unnecessary delays. To prevent such unpleasant surprises from recurring in the
future, in-depth investigations of the materials used were required. For their
investigations within the scope of the ESA project, René Schödel's research
group used PTB's ultra-precise interferometer to measure the length of the
samples across the whole temperature range with nanometer accuracy.