High-precision measurements of thermal expansion at cryogenic temperature on stable materials

The European Space Agency ESA aspires the exploration of the universe using space-borne infrared
telescopes with large aperture operating at cryogenic temperature. The Herschel space telescope is
currently returning its first scientific observations orbiting from the Lagrange point L2 and the
foreseen infrared observatory Spica is currently under study. The large diameter of the primary mirrors
and the operation at cryogenic temperature require the use of ultra stable materials and wellcharacterised
dimensional change of the structure as function of temperature. Dimensional stability
and high precision data on the thermal expansion is also relevant for the cryogenic spectographs MIRI
and NIRspec on the James Webb Space Telescope JWST, for high-precision laser interferometer LISA
and for spacecraft sent to a hot environment such as BepiColombo and Solar Orbiter.
Although European industry has proven effective at designing and building thermally stable structures,
problems have been encountered in measuring the absolute CTE of stable materials. In one recent
mission, the design CTE had to be changed 6 times over a 2 year period and when the telescope was
tested in cryogenic conditions, an error was found in the back focal plane of over a centimetre. It is an
objective of ESA to determine the CTE of stable materials continuously as function of temperature
with an uncertainty of 3 ppb/K down to a temperature of 5 K.