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ASTRONOMY 161
Introduction to Solar System Astronomy
Class 11
Telescopes
Friday, February 2
Telescopes: Key Concepts
(1) Telescopes use either a lens or a mirror to gather
light.
(2) The main purposes of a telescope are to gather
light and resolve detail.
(3) Radio and microwave telescopes use a reflecting
dish to focus waves.
(4) Telescopes in orbit avoid the distorting effects of
the atmosphere.
(5) Ultraviolet, X-ray, gamma ray, and most infrared
light is visible only from orbit.
(1) Telescopes
use either a lens
or a mirror to
gather light.
Refracting telescopes
use a lens to bend (or
“refract”) the path of
light.
Light with a short wavelength is bent through a
larger angle than light with a long wavelength.
(This is why prisms spread light into a spectrum.)
A convex lens (thicker in the middle) focuses light
to a point:
Light from a large area is funneled into a small area.
Reflecting
telescopes use a
mirror to change
the path of light.
Angle of incidence
(I) equals angle of
reflection (R),
regardless of
wavelength.
A mirror shaped like a parabola focuses light to a
point:
Light from a large area is funneled into a small area.
Lenses and mirrors (if shaped correctly) produce an
accurate image of an object.
Reflecting telescopes use a number of designs:
(2) The main purposes of a telescope
to gather light and resolve detail.
A telescope is sometimes called a
“light bucket”.
Number of photons collected per
second is proportional to the area of
the lens/mirror:
Area = π/4 x D2
where D = diameter of the lens/mirror.
are
A bigger telescope is
able to resolve finer
detail.
Two stars are resolved
if they are seen as two
separate points.
Smallest angle resolved
is proportional to 1/D.
BIGGER
IS
BETTER
Larger diameter
for your lens
or mirror
means more
light, higher
resolution.
The world’s biggest telescopes are
reflectors (mirrors), not refractors (lenses).
The problem with lenses:
1) Lenses absorb light.
2) Lenses sag.
3) Lenses have chromatic aberration:
colors do not focus at same point.
The world’s largest refracting telescope:
Yerkes
Observatory,
Wisconsin
1 meter
diameter
Completed
1897
Soon to be the world’s
largest reflecting telescope (2x8.4 m):
(3) Radio and microwave telescopes
use a reflecting dish to focus waves.
Parabolic “dish” of a
radio telescope
acts as a mirror,
reflecting radio
waves to the focus.
Radio telescopes can be huge, because they do
not have to very smooth.
Mirrors must be smooth compared to the
wavelength of light observed:
Visible light: λ ∼ 0.0005 mm
Microwaves: λ > 1 mm
Radio waves: λ > 100 mm.
Resolution of a telescope: θ ∼ λ/D
(4) Telescopes in orbit avoid the
distorting effects of the atmosphere.
Turbulence in the air makes stars “twinkle”
and limits resolution (so called “seeing”).
City lights drown out faint stars.
Great idea: place a telescope in
orbit, above the atmosphere.
Earth at Night
Hubble Space Telescope (launched 1990)
Diameter of mirror = 2.4 meters
Angular resolution = 0.05 arcseconds
(5) UV, X-ray, γ ray, and most IR is
visible only from above the atmosphere.
Infrared (IR) light is
absorbed by water
vapor in the Earth’s
atmosphere.
The Spitzer Space
Telescope, launched
in 2003, uses a 0.85
meter mirror.
Ultraviolet (UV) light is absorbed by ozone in the
Earth’s ozone layer, 25 km up.
The Hubble Space Telescope could detect UV light with
λ = 100 nm.
X-rays are absorbed by most molecules in the Earth’s
atmosphere.
Chandra X-ray Observatory, launched in 1999, uses
“grazing incidence” mirrors to focus X-ray light.
Gamma rays are absorbed by just about everything.
Making gamma ray mirrors is beyond current technology.
Compton Gamma Ray Observatory, launched in 1991
and “deorbited” in 2000, had several instruments.
Sky: Optical
Sky: Radio
Sky: Microwaves
Sky: Infrared
Sky: X-ray
Telescopes: Few closing questions:
1) If you double the diameter of a telescope, how
much more light will it collect?
2) If you double the diameter of a telescope, how will
its resolution change?
3) Which has better resolution: 1 meter diameter
optical telescope, or 30 meter radio dish?
4) What is the largest X-ray telescope on Earth?
5) How much ($$$) is a 10 meter optical telescope?