Chapter 6: Telescopes

By using telescopes, we can see the incredible sights of the cosmos
Galileo first used a telescope for astronomy, saw Jupiter in detail
With modern telescopes, We can see this!

The abilities to focus and detect light are fundamental to eyes, cameras, and telescopes
Eyes (the most common example):
Retina: detects the light
Lens: focusses the light so that an image of the original object
appears on the retina
Detector: film or CCD
Focussing: lens
Detector: can be human eye, film, CCD
Focussing: can use lenses ("refracting telescopes") or
mirrors ("reflecting telescope")
Refracting telescope diagram and example (Yerkes obs. in Wi.)
Reflecting telescope diagram of simple design and
diagram of more complex design

Important Abilities of Telescopes:
Collect many photons
light collecting area = pi (diameter/2)2
Resolve objects that are separated by small angles
angular separation
calculating angular size: figure
"Angular resolution" = smallest angular separation
where you can still see that 2 objects
are actually 2 objects rather than 1 blobby object
The human eye's angular resolution = 1 arcminute
The Hubble Space Telescope's ang. resolution = 0.05 arcseconds
Hubble Deep Field
Diffraction limit
Light waves spread out when they go through holes (lenses)
"diffraction limit" = 2.5 x 105 arcsec * wavelen./diameter

Question: Suppose that two stars are separated on the sky by
0.1 arcseconds. What will you see if you look at them with a
telescope that has an angular resolution of 0.01 arcseconds?
What will you see if you look at them with a telescope that has
an angular resolution of 0.5 arcseconds?

How do we learn from telescopic observations of the heavens?
Imaging: Crab Nebula example
This is a combination of images in various colors and false colors
Spectroscopy: spectrum from Mars
Resolution: high spectral resolution example and
low spectral resolution example
Timing: Example of light curve from binary star

Atmospheric effects
Clouds, light pollution, distortion, absorption
Solutions: put telescopes on mountains, on satellites in space,
or quickly adjust optics ("adaptive optics")

Why we want to see other wavelengths of light:
Orion in the Visible, IR, and UV
Crab nebula in radio, IR, Optical, and X-ray
The Milky Way in Gamma-ray, X-ray, Visible, IR, and Radio
Tricks for detecting other wavelengths:
Infrared (SIRTF) --> cool the detector (ex of blackbody: bright human)
X-rays (Chandra) --> nested mirrors
Radio waves have very large wavelengths, so need
Huge telescopes (Arecibo, diameter = 305 meters
and Green Bank) or
"Link" several telescopes together to get better angular
resolution. (Very Large Array (VLA))
= 27 telescopes 25m over 36km

Supplemental Material:

Microwave Anisotropy Probe the Cosmic Microwave Background
Hubble Space Telescope: the Hubble Deep Field
Gamma rays: Compton (GRO)
If you can afford it get a close-up.
Voyager2 to the Giant Planets the Grand Tour
Cassini to Saturn
Mars Rovers