MAQ Magazine n. 14 / September 2019
THERMAL CONSTRAINTS
The mirror shells of the Mirror Modules have to be kept at an average temperature of 20°C, with spatial maximum temperature differences of ±2°C in order to limit thermo-elastic deformations. Therefore, the platform is maintained almost isothermal, with deviations of less than ±2°C. On the other hand, the Service Module equipment presents quite standard temperature ranges and attention is therefore mainly paid to simplicity and reliability.
The thermal design of XMM takes the full advantage of the stable environment provided by its high-altitude, long-period orbit and by the limited variation of solar attitude angles (±20° pitch combined with ±20° roll). In fact, the Earth albedo and infrared heat fluxes are negligible along the largest part of its high-altitude orbit. Only at perigee passes, when the altitude reduces to 7000 km, XMM's thermal stability is slightly affected by the influence of the Earth.
The largest thermal perturbations occur during the eclipse seasons, when the satellite does not receive the Sun's energy for a maximum period of 1.7 h (although, on average, the eclipses are much shorter). However, eclipses always occur below the minimum altitude that is required for observation (40 000 km), leaving time for the spacecraft to recover its temperature stability. Boost heating performed before and after the eclipses by means of heater helps to reduce the time needed for recovery of the temperature drop caused by eclipses.
In order to cope with all orbital perturbations and with changes of satellite attitude, the telescope tube is completely insulated from the external environment and the heater power that is dissipated inside it can be almost continuosly adjusted to compensate for changes.