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QandA With Paul Geithner to Explain the MIRI
May 31, 2013

The James Webb Space Telescope's Mid-Infrared Instrument, or MIRI, recently arrived at NASA's Goddard Space Flight Center in Greenbelt, Md., and Paul Geithner, the deputy technical project manager for the Webb telescope, answered questions about the instrument.

The flight Mid-Inrared Instrument (MIRI) at the Rutherford Appleton Laboratory in England. › View larger
The flight Mid-Inrared Instrument (MIRI) at the Rutherford Appleton Laboratory in England. Credit: RAL
Q: What is the MIRI?

A: MIRI, or the Mid-Infrared Instrument, is an infrared camera and spectrometer that will operate as part of the Webb telescope to observe the universe at wavelengths that are difficult or impossible to observe from the ground.

Q: What will the MIRI do?

A: Along with the three other instruments that will fly aboard the Webb telescope, MIRI will be able to examine the first light in the universe and investigate the evolution of galaxies and the process of star and planet formation – helping to answer some of the fundamental questions about the origin of our universe.

Q: Who built the MIRI instrument?

A: MIRI was built by a European consortium together with NASA's Jet Propulsion Laboratory. It took eight years to design, build and test. MIRI was the first of Webb's four instruments to be delivered to NASA for integration into the observatory.

Q: What are the other three instruments that will fly with the MIRI on the Webb Telescope?

A: The Near-Infrared Camera, or NIRCam, provided by the University of Arizona; the Near-Infrared Spectrograph, or NIRSpec, provided by the European Space Agency, with components provided by NASA Goddard; and the Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph, or FGS/NIRISS, provided by the Canadian Space Agency.

Q: Can you provide a basic explanation of what the MIRI and the other three instruments are?

A: The instruments are essentially the eyes of the observatory. The instrument detectors are like the detector chips in your cellphone camera or electronic camera and function like the retina in your eye, and the instrument optics are like the cornea and lens of your eye. The big telescope optics (mirrors) gather huge amounts of light for the instruments and enable fine resolution and high sensitivity.

Q: How is MIRI different from the other instruments aboard the Webb telescope?

A: MIRI is special on the Webb telescope because it is the only instrument that sees mid-infrared light-the other three see near-infrared.

Q: What is infrared light?

A: Infrared light is electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves. Heat radiation is infrared light. Observing in the infrared is important for a few reasons. One is because the ultraviolet and visible light emitted by the very first luminous objects that formed in the universe when it was young has been stretched by the expansion of the universe so that it reaches us today, over 13 billion years later, as infrared light. Another is because stars and planets form in clouds of gas and dust, and these clouds obscure our view. Infrared light penetrates these clouds and allows us to see inside.

Q: What's the difference between mid-infrared and near-infrared light?

A: Both near-infrared and mid-infrared are invisible to the human eye, which can see light at wavelengths at 0.4 - 0.7 micrometers. Near-infrared is close to visible, but still not detectable with our eyes. These wavelength ranges are, however, sensitive to heat radiation coming from different temperatures. Roughly near-infrared for the Webb telescope is defined as light in the wavelength range 0.7 to 5 micrometers that is sensitive to objects with temperatures around 2,240 F (1,227 C, or 1,500 kelvins). Mid-infrared for the Webb telescope is light with wavelength between 5 and 28 micrometers and (at 10 micrometers) this means optimal sensitivity to objects around 80 F (26.8 C, or 300 kelvins), which is near the average human body temperature of 98.6 F (37.0 C, or 310 kelvins). As a side note, the peak wavelength where the human body radiates most of its energy (at rest) is around 9 micrometers.

The MIRI is both a spectrometer and an imager. MIRI contains two apertures that can be pointed at an object in space to record both its image and spectrum.

The MIRI is both a spectrometer and an imager. MIRI contains two apertures that can be pointed at an object in space to record both its image and spectrum. An aperture is an opening through which light travels. The MIRI is basically two instruments in one, so it has "two faces." MIRI records light with wavelength in the range of 5 to 28 microns. Credit: NASA

Q: The MIRI looks at spectra. Why?

A: Part of the MIRI is a spectrometer, which is a tool that disperses light into its constituent wavelengths or "colors" to produce a spectrum, like how raindrops disperse sunlight into a rainbow, and then takes a "picture" of it. Spectra are like "chemical fingerprints" and provide a wealth of information. For example, different elements and molecules absorb and emit unique and specific wavelengths of light, so by examining spectra from distant objects, scientists can determine the chemicals that make up the atmosphere of a distant planet.

Q: What are the two types of data that MIRI collects?

A: MIRI has an imaging element that takes familiar-looking pictures, and a spectroscopy element that disperses light and records spectra. To keep it really simple, the imaging part is like a very high-tech electronic camera that sees mid-infrared wavelengths and takes mid-infrared pictures, similar to how your cellphone camera takes pictures in visible wavelengths of light. The spectroscopy part of MIRI breaks up the infrared light into a spectrum, as described above.

Q: Why will MIRI be so revolutionary for astronomy?

A: There are three key reasons why MIRI will provide enormous increases in sensitivity as well as spatial and spectral resolution. First, the Webb telescope's location in space puts it above the effects of the atmosphere, which glows brightly in the infrared and absorbs much of the starlight coming through it, and which limits the use of ground-based telescopes. Second, the Webb telescope is cooled to a very low temperature, reducing the emission from the telescope and greatly improving its sensitivity. Third, the Webb will have a far larger mirror than any other infrared space telescope, giving improved angular resolution and sensitivity. This combination makes the Webb a very powerful space observatory that promises to revolutionize our view of the cosmos yet again - just as Hubble did.

Q: The Webb telescope is also a joint effort. Who are the project partners?

A: Webb is an international project led by NASA with its partners the European Space Agency and the Canadian Space Agency. One of the largest science developments in the world today, the Webb telescope project combines the talents of scientists, engineers, technicians, managers and administrative specialists from around the world.

For more information about the MIRI instrument, visit:


To see a short video about the MIRI, visit:


For more information on the James Webb Space Telescope, visit:

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The James Webb Space Telescope has multiple science instruments, but only one of them, the Mid-Infrared Instrument (MIRI), sees light in the mid-infrared region of the electromagnetic spectrum. Credit: STScI
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