telescopes and Doppler Effect Flashcards
refracting telescopes
use a lens to focus light
Light is ref racted when
when it mov es from air to
glass or f rom glass to air
A curv ed lens will focus
A curv ed lens will focus parallel ray s of
light to a focal point
problem with lenses
(1) Lenses absorb light.
(2) Lenses sag.
(3) Good lenses expensiv e
and dif f icult to make.
(4) Chromatic aberration
Largest refracting telescope
Yerkes Observatory, 102 cm in diameter
Reflecting telescopes use a mirror to focus light
For a mirror, equal angles of incidence and
ref lection (independent of wav elength!)
A mirror shaped like a parabola
f ocuses parallel rays of light to a point
The primary purpose of a
telescope
is to gather light.
Telescope is a
“light bucket.”
of photons collected
each second is
proportional to the area
of lens (or mirror)
Area = pi/2 D2
where D is the diameter of
the lens or mirro
Angular measures
360 degrees (360°) in a circle
60 arcminutes (60’) in a degree
60 arcseconds (60”) in an arcminute
The secondary purpose of a
telescope is to resolve fine detail.
two stars are resolved if they are seen as
two separate points.
Smallest angle resolv ed is proportional to
o stars are resolved if they are seen as
D = 1 inch 4.6 arcseconds
2.3 arcsecondsD = 2 inch
resolved
Larger diameter mean
more light, higher
resolution
Charge-coupled devices (CCDs)
capture images electronically
Atmospheric blurring
Due to movements of
warmer and colder ‘cells’ in the atmosphere
Modern telescopes use mirrors rather than lenses for all
of these reasons EXCEPT
Reflecting telescopes aren’t affected by the atmosphere
as much.
Telescope A has a diameter of 4 meters. Telescope B
has a diameter of 8 meters.
The light gathering power of B is ___ times
the light gathering power of A
4
The angular resolution of an 8-inch diameter telescope
is better than a 2-inch diameter telescope by a factor of
4
Observatory Sites
Best observatory sites are high, dry mountain peaks.
* dark skies, f ar from city lights
* many clear nights
* dry conditions (f or infrared observations)
* steady atmosphere (less ‘twinkling’)
Radio telescopes use
reflecting
dish to focus waves onto an antenna.
For radio telescopes, as f or optical telescopes,
BIGGER is BETTER.
Visible light
= 500 nanometers
microwave
> 1 millimeter
radio
> 10 centimeters
Two or more radio telescopes can be
combined to
to make a radio interferometer.
Example:
Very Large Array ,
New Mexico
27 dishes, each 25-m across
Light-gathering power =
Resolv ing power =
VLA, Socorro, NM
dish 130 meters across
dish 36 kilometers across
Hubble Space Telescope
Height abov e ground: 600 kilometers
Diameter of mirror = 2.4 meters
Resolution =
0.05 arcseconds
(Resolution on ground is limited
to 0.5 arcseconds)
Not all EM radiation can penetrate Earth’s atmosphere.
only radio and visible can fully penetrate
Earth’s atmosphere is transparent in
“visual
window” and “radio window.
Infrared light is absorbed by
water vapor in the atmosphere.
IR light is less af fected by gas and dust
Ultraviolet
Emitted by hot stars, gas (50,000 K)
* Absorbed in the ozone lay er, 25 kilometers up
X-rays
X-ray s are absorbed by most molecules in the atmosphere.
X-ray mirrors ref lect at very
shallow angles (‘grazing incidence’)
X-ray s are emitted by
extremely hot gas:
T > 300,000K
X-rays
X-ray s are absorbed by most molecules in the atmosphere.
X-ray mirrors ref lect at very
shallow angles (‘grazing incidence’)
X-ray s are emitted by
extremely hot gas:
T > 300,000K
Gamma rays:
absorbed by nearly everything.
Gamma ray s can’t be
f ocused, so images
are low resolution.
Produced by exotic
objects in v ery energetic
processes.
The tendency of a wave to bend as it passes from one
transparent medium to another is called
refraction
What causes stars to twinkle?
turbulence in the atmosphere
doppler Effect
Christian Doppler (1842): If a source of waves moves toward you or
away f rom you, the wavelength is changed
radial velocity
how f ast an object is moving
toward y ou or away f rom you
The Doppler Effect in Light
The Doppler Effect in Light
Shif t depends upon the radial speed of the object:
Source moving toward you:
– Wav elength gets shorter = BLUESHIFT
Source moving away:
– Wav elength gets longer = REDSHIFT
The Doppler Ef f ect causes light from a source moving away to
be shifted to longer wavelengths
