Keeping Cool: SRON Develops Thermal Calibration System for Deep-Space Telescope

The calibration source for SAFARI contains a blackbody cavity or radiation source that provides radiation with a spectrum depending only on the source temperature, making it a very reliable and accurate calibrator. However, SAFARI’s detectors are so sensitive that the power produced by the source is approximately a million times too high and must be optically diluted using apertures and an integrating sphere. After passing through the integrating sphere, radiation with the correct power and spectral distribution is then reimaged onto SAFARI’s detector arrays for calibration. Between the radiation source and integrating sphere are a mechanical shutter and iris mechanism. The shutter opens and closes the aperture to the radiation source, while the iris fine-tunes and modulates the output power. Thermal management is vital: the system is held in a “super-dark” environment at 4.5 kelvins (K) to decrease the background radiation from the equipment itself. Variation in the base temperature of the detectors, background radiation (affected by the orientation of the spacecraft), and power dissipated by the iris and shutter mechanisms can all disrupt calibration. The radiation source temperature can be set between 95 and 300 K to generate radiation — this creates a large temperature differential between the source and the 4.5 K environment, while available cooling power at these temperatures is limited to just tens of milliwatts. To account for this, the team needed to design a thermally insulating suspension system. The SRON team needed a stiff suspension with a high resonance frequency that would prevent heat transfer from the source to the rest of the device while also protecting it from unwanted vibrations.