Hong Kong Observatory reported a temperature of 16.4°C this morning, the lowest record in May since 1917. Weather suddenly turned cold in the past two days reminds us about the cooling of the James Webb Space Telescope (Webb).

Around a month after its launch on last December, Webb's scientific instruments have been cooled down gradually under the sunshield. Yet, the Mid-Infrared Instrument (MIRI) of Webb requires an extremely low operating temperature that cannot be achieved solely by the sunshield. Therefore, an electrically powered cryocooler was installed for this purpose. The cryocooler was turned on last month and the MIRI was cooled down to its operating temperature, -266°C.

Why does Webb have to be extremely cold? Because a little rise in temperature will blur its "Golden Eye".

To glimpse the celestial objects obscured by dust and gas and to detect the redshifted starlight caused by the Universe's expansion, Webb is designed primarily for infrared light region. However, Webb's own electronic and optics emit infrared light. The dark current created by the vibration of atoms in the detectors also induce thermal noise. Those noise signals will drown out the real data that is weak and pale from afar.

Cooling the instruments effectively suppresses those unwanted signals. The MIRI detects longer infrared wavelengths than others and is more sensitive to dark current. MIRI has to be the "coldest".

Wanna know about this "very cool" space telescope and the infrared science behind it? Come visit the Space Museum's (already reopened) special exhibition "Golden Eye on the Cosmos"!