James Webb's MIRI instrument has both a heater and a cooler

James Webb’s MIRI instrument has both a heater and a cooler

The long process of getting the James Webb Space Telescope ready for science operations continues, with the ongoing alignment of three of its instruments.

Webb recently reached the major milestone of aligning its mirrors with its NIRCam instrument, in a successful step that promises great results to come. “Webb’s alignment at the NIRCam field showed some spectacular diffraction-limited images, producing a tantalizing glimpse of the capabilities this observatory will carry for its science program,” wrote two Webb researchers, Michael McElwain, Webb observatory project scientist, and Charles Bowers, Webb deputy observatory project scientist, both at NASA Goddard, in a recent blog post. “This was a major milestone because it required nearly all of the observatory systems to be functioning as designed. It all worked as well as we dared to hope, and it was certainly a moment to celebrate.”

Now, the Webb team is working on aligning two more of the instruments — the Near-Infrared Slitless Spectrograph (NIRISS) and Near-Infrared Spectrometer (NIRSpec) — as well as the guider, called the Fine Guidance Sensor (FGS). This process is expected to take around six weeks and will ensure that all of the instruments can work together. Along with NIRCam, these comprise Webb’s near-infrared instruments.

While the three near-infrared instruments are passively cooled — meaning that heat is dispersed from the telescope and into space using design elements like heat sinks which require no power — the fourth instrument, MIRI, works in the mid-infrared wavelength and requires active cooling. Because MIRI uses a different type of detector than the other instruments, and these detectors need to be at an extremely low temperature of less than 7 kelvin to work properly, the instrument needs to be fitted with a cryocooler. This refrigeration system uses helium gas and includes pumps that require power but must produce very little vibration to avoid interfering with instrument readings.

In addition to this cooling system, MIRI is also fitted with heaters so that the cooldown process can be carefully managed to prevent ice from forming on the components. The heaters will shortly be turned off, allowing the cooling system to bring the instrument down to its operating temperature.

With the cooling of MIRI underway, it will take a few weeks until the final instrument gets cool enough to be ready for alignment. Then, with all four of the instruments aligned, the Webb team can move onto the next phase of commissioning — optical stability tests and instrument performance measurement — to get the telescope ready for science operations this summer.

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