telescopes and Doppler Effect Flashcards
(37 cards)
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)
