Telescopes Doppler Effect The frequency of electromagnet radiation

Telescopes

Doppler Effect The frequency of electromagnet radiation depends on the relative motion of the observer and the source.

Water Waves

Discussion If the leaf is moving toward the source of the water waves, will the frequency be greater or less than when the leaf was still?

Discussion Which has higher frequency red light or blue light?

Doppler Shift with Light If you are moving toward the source, light is shifted to higher frequencies, or toward the blue end of the spectrum. Astronomers call is a blueshift. Likewise if you are moving away from a source, the light is shifted toward the red end of the spectrum and is called a redshift.

Important points 1. A Doppler shift is not a change in color. You can have a red spectral line which is blueshifted, or a blue line which is redshifted. 2. The Doppler effect does not depend on distance, does depend on your relative velocity toward or away from an object.

Rest frame spectrum Doppler shifted spectrum

Doppler shifted spectra

Important points 3. The Doppler effect can only measure the radial velocity, the velocity toward or away from a source. It does not depend on the velocity of the object across the plane of the sky.

Radar measurement of Mercury’s rotation

Discussion If you had a Coke bottle and you wished to fill it with rainwater falling from the sky, how would you do it?

Telescopes Serve 3 Functions 1. To collect light 2. To resolve fine detail 3. To magnify the image

Discussion Of the three telescope functions which do you think is the most important in astronomy and why?

Refractor

Refraction (Marching Band Analogy) What happens to the marching band if it hits a muddy part which causes the individual band members to slow?

Refraction (Marching Band Analogy)

Refraction (Marching Band Analogy)

Refraction (Marching Band Analogy)

Speed of light in glass Light moves slower in glass than in a vacuum Speed of light through glass is different for different frequencies and each frequency has a slightly different focus.

Chromatic aberration

Mirrors

Reflectors

Mt Palomar observing cage

Resolution The ability to separate two closely spaced objects, such as a double star. The bigger your telescope objective the better your resolution will be.

Resolution

Alcor and Mizar

Resolution and the atmosphere Atmospheric turbulence limits the resolution of even the largest telescopes on Earth to about that of a 6 inch telescope.

Points of light Stars are so far away that they appear as points of light no matter how big a telescope you observe them with. Bigger stars in photographs are simply brighter.

“Seeing” The apparent size of the stars as viewed through the telescope. Good “seeing” is typically 1 arcsec.

Atmospheric distortion

Seeing

Atmospheric distortion

Discussion What can astronomers do to try and improve resolution of their observations?

Hubble Space Telescope

Mauna Kea

Keck primary mirror

Adaptive optics Actuators deform the primary mirror hundreds of times a second to try and remove atmospheric distortion.

Discussion Adaptive optic telescopes work best in the infrared region of the spectrum and not in the visible. Why do you think that is?

Discussion What do you think happens when very high energy gamma rays from space hit the Earth’s atmosphere?

Discussion Why can’t the very long wavelength radio waves make it to Earth’s surface?

Atmospheric absorption