Infrared TelescopesInfrared Telescopes

The Electromagnetic Spectrum

see different phenomena at different wavelengths

1800: Discovery of IR Light

Herschel used a prism to separate sunlight into colors.

He used a thermometer to determine the temperature in each color. Two were placed off to the side as controls.

The highest temperature was found beyond red light where no light was seen.

Infrared

Optical

IR penetrates dust more easily than optical

Far-infraredOptical

Dust glows at the longest IR (and radio) wavelengths

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Near-Infrared

Because the most distant galaxies are highly redshifted, IR telescopes are needed to detect them

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Challenges for IR telescopesThe atmosphere produces many absorption lines at IR wavelengths (called telluric absorption); some wavelength ranges are totally opaque

in space

on the ground

The telescope and atmosphere glow brightly at IR wavelengths, which hampers detection of faint IR photons from space. It’s like trying to observe stars at optical wavelengths during the day!

Challenges for IR telescopes

Designing an Infrared TelescopeThe same basic design and materials are used for collecting both optical and IR light. As a result, telescopes often can observe in both wavelength regimes. However, telescopes are usually optimized for specific wavelengths through the details of their design (mirror coatings, camera detectors, etc.).

Designing an Infrared TelescopeTo reduce the absorption of starlight by the atmosphere (mostly water vapor) and thermal emission from the atmosphere, place a telescope on a high mountain, on an airplane, or in space.

Designing an Infrared TelescopeTo reduce the thermal emission by the telescope and instrument, make them as cold as possible. For shorter IR wavelengths, it is sufficient to cool the instrument with liquid nitrogen (77 K); the telescope can be at room temperature. But for longer IR wavelengths, both the telescope and instrument need to be close to absolute zero, so they are placed in space.

IR telescopes became possible in the 1960's with the advances in IR detectors. At an elevation of 13,796 ft., the summit of Mauna Kea on the island of Hawaii is above much of the absorbing water vapor in the atmosphere. It was one of the first sites for IR telescopes.

Mauna Kea

In 1983, the Infrared Astronomical Satellite (IRAS) scanned most of the sky at far-IR wavelengths. IRAS doubled the number of cataloged astronomical sources by detecting about 500,000 IR sources. IRAS discoveries included a disk of dust grains around the star Vega, six new comets, and ultra-luminous IR galaxies, as well as wisps of warm dust called IR cirrus.

IRAS

Between 1997 and 2001, the Two Micron All-Sky Survey (2MASS) imaged the entire sky at near-IR wavelengths using two telescopes in Arizona and Chile. 2MASS has been valuable for many topics in astronomy, including the structure of the Milky Way and the discovery of brown dwarfs.

2MASS

2MASS movie of Milky Way

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Although it focuses primarily on optical wavelengths, Hubble does operate at near-IR wavelengths as well. As in the optical, Hubble provides very sharp images at IR wavelengths because it is above the atmosphere.

Hubble Space Telescope

2MASS Hubble

The Spitzer Space Telescope was full operational from 2003-2009, and continues to function in a more limited capacity. It operates from near- to far-IR wavelengths and is the last of NASA's "great observatories”. Like Hubble, Spitzer observes specific targets rather than the entire sky.

Spitzer Space Telescope

Spitzer

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optical

Finding brown dwarfs companions with Spitzer

infrared with Spitzer

Finding brown dwarfs companions with Spitzer

optical

Finding brown dwarfs companions with Spitzer

infrared with Spitzer

Finding brown dwarfs companions with Spitzer

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Herschel Space ObservatoryHerschel was launched in 2009 by the European Space Agency for a 3 year mission. It operates at far-IR wavelengths. Because it has a larger mirror than Spitzer (3.5 m vs. 0.85 m), it produces sharper images.

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Herschel’s Orbit

Wide-field Infrared Survey Explorer (WISE)

WISE was launched by NASA in late 2009 and completed a mid-IR imaging survey of the entire sky in July 2010. The survey is a new version of IRAS with much sharper and more sensitive images. WISE will be used to search for objects that are cold, dusty, or highly redshifted. IRAS WISE (predicted)

Orbit of Wise

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As Earth orbits the Sun, WISE’s orbit also rotates to maintain the spacecraft’s orientation to Earth and Sun.

Orbit of Wise

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Each orbit, a circular strip of the sky is imaged. As the orbit itself rotates, a slightly different strip is imaged.

Orbit of Wise

B) HubbleA) ChandraC) Spitzer

Compton

Which of these 3 space telescopes would be best for detecting light from a brown dwarf? (Brown dwarfs are very cold objects, sort of like planets free-floating in space.)

B) HubbleA) ChandraC) Spitzer

Compton

Which of these 3 space telescopes would be best for detecting light from very hot gas spiraling into a black hole?

B) HubbleA) ChandraC) Spitzer

Compton

Which of these 3 space telescopes would be best for obtaining a sharp image of Mars to study its surface features?