1. 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?

2. 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)

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

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

5. 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

6. 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

7. 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.

8. Another example of adaptive optics.

9. Why do we put telescopes into space?

10. 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

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

12. 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.

13. 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

14. 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

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

16. 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

17. 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

18. 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.

19. 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.

20. How can multiple telescopes work together?

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

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

23. 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).

24. 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.