6.3 Telescopes and the Atmosphere Our goals for learning How does Earth’s atmosphere affect ground-based observations? Why do we put telescopes into space? How can we observe non-visible light? How can multiple telescopes work together?

How does Earth’s atmosphere affect ground-based observations? The best ground-based sites for astronomical observing are Calm (not too windy) High (less atmosphere to see through) Dark (far from city lights) Dry (few cloudy nights)

Calm, High, Dark, Dry The best observing sites are atop remote mountains Summit of Mauna Kea, Hawaii

Effects of the atmosphere Light pollution, or the scattering of human-made light in the atmosphere is a growing problem for astronomy

Twinkling and Turbulence Star viewed with ground-based telescope Same star viewed with Hubble Space Telescope Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle

Adaptive Optics Without adaptive optics With adaptive optics Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence

Adaptive Optics In order make the corrections, astronomers have to monitor the a bright star in their field of view. If no bright star is visible, they have to make an artificial star using a laser. The laser excites atoms high in the atmosphere which give off light creating a “star”. A computer monitors the distortions of the star’s image and calculates the changes that have to be made to the shape of the mirror to compensate.

Another example of adaptive optics.

Why do we put telescopes into space?

Transmission in Atmosphere Only radio and visible light pass easily through Earth’s atmosphere We need telescopes in space to observe other forms of light

Radio telescopes A standard satellite dish is essentially a telescope for observing radio waves

Radio telescopes A radio telescope is like a giant mirror that reflects radio waves to a focus. Shown here is the Arecibo Observatory in Puerto Rico.

Observing in Space: IR & UV Telescopes SOFIA – visible, IR and UV Spitzer - IR Infrared and ultraviolet-light telescopes operate like visible-light telescopes but need to be above atmosphere to see all IR and UV wavelengths

What you can see with infrared A Comet Warm dust in a young spiral galaxy Newborn stars that would be hidden from our view in visible light

X-Ray Telescopes Focusing of X-rays requires special mirrors Mirrors are arranged to focus X-ray photons through grazing bounces off the surface

What you can see in X-ray NASA launched the Chandra X-ray Telescope in 1999 X-rays are emitted by very hot gas in the universe. Colliding galaxies… A cluster of galaxies Saturn

Gamma Ray Telescopes Focusing gamma rays is extremely difficult Unlike optical light and X-rays, gamma rays cannot be captured and reflected in mirrors. The high-energy photons would pass right through such a device. Compton Observatory

Gamma Ray Telescopes Gamma-ray telescopes use a process called Compton scattering, where a gamma-ray strikes an electron and loses energy, similar to a cue ball striking an eight ball. Compton Observatory the second of NASA’s Great Observatories was launched in 1991.

What you can see at Gamma ray wavelengths The hearts of solar flares, supernovae, neutron stars, black holes, and active galaxies would be very bright. These two pulsars in the crab nebula would be among the brightest thing in the sky. These two pulsars in the crab nebula would be among the brightest thing in the sky.

How can multiple telescopes work together?

Interferometry Interferometry is a technique for linking two or more telescopes so that they have the angular resolution of a single large one

Example of an Interferometer Example of an Interferometer The Very Large Array (VLA): 27 dishes are combined, to simulate a dish miles across

Future of Astronomy in Space? The Moon would be an ideal observing site It’s cool, dark, dry (no atmosphere means no light pollution or absorption).

What have learned? How does Earth’s atmosphere affect ground-based observations? Telescope sites are chosen to minimize the problems of light pollution, atmospheric turbulence, and bad weather. Why do we put telescopes into space? Forms of light other than radio and visible do not pass through Earth’s atmosphere. Also, much sharper images are possible because there is no turbulence.