Download Astro 201: Sept. 14, 2010

Astro 201: Sept. 16, 2010
• New: Copies of Lecture Notes and HW are
now on d2l, and should be faster to download.
• HW #3 on line, due Tuesday
• Midterm #1: Tuesday, Sept. 28 – more info
later
• Today:
– IR camera demo and lab – write-up due in one
week
– Telescopes
LIGHT
and
Telescopes
Spectrum of the Sun
X-ray spectrum
Summary
Astronomers take “images” of objects
Astronomers also take “spectra” of objects
 Temperature
 Type of atoms (hydrogen, helium, iron, etc)
 how fast the object is moving, at least radially
Sometimes astronomers take images though filters which isolate specific
wavelengths  (rough) spectral pictures
IR Light
The 10 micron camera contains a
detector which is sensitive to
infrared light.
All objects radiate "black body"
radiation, or "Planck radiation",
by virtue of their temperature.
• For objects near room temperature the radiation peaks in the
infrared. Your eyes, which are sensitive to optical light,
cannot see this radiation unless the object is VERY hot.
• The visible or optical light you see is reflected optical light
from the sun or lamps.
• Hotter things are brighter in the IR camera than cooler things.
• Some materials are opaque to IR light, but transparent to
visible light.
• Some materials are transparent to IR light, but opaque to
visible light.
• IR light can be reflected by a mirror, just like optical light.
Everyday Uses of IR light
• One everyday use of IR light is in remote
control devices
• IR cameras are used on ships and in buildings
to look for hot spots in electrical wiring
• night-time spotting of people (who are
warmer than their surroundings)
• for seeing "through" smoke in a fire
Astronomers use IR light
To measure temperatures; also to look "through" dust
Optical or Visible Wavelengths
IR wavelengths
The Trapezium in Orion: Stars are forming out of gas and dust
Why use telescopes?
(1) Light Gathering Power:
A large telescope can intercept and focus more light than does a small telescope.
A larger telescope will produce brighter images and will be able to detect fainter
objects.
(2) Resolving Power: A large telescope also increases the sharpness of
the image and the extent to which fine details can be distinguished.
(3) Detect types of light besides optical:
radio, X-ray, ultraviolet, infrared
put the telescope in space, above the atmosphere which
absorbs many wavelengths
Optical Light Telescopes:
Refracting (use a lens)
Reflecting (use a mirror)
REFRACTING TELESCOPE:
Examples Galileo’s telescope, our eyes
A CONVEX lens (thick in the middle) focuses light to a point.
Light gathered
From a large
Area is
Concentrated
Can see fainter
Objects than you
Can with your eye
Refracting Telescope
Objective Lens
Focal Length
Objective
Eyepiece Lens
Focal Length
of Eyepiece
Refracting Telescope:
Lens focuses light onto
the focal plane
Focal length
Reflecting Telescopes: Use mirrors as the
optics
A mirror shaped like a PARABOLA focuses light
to a point.
focus
Light from a large area is
concentrated in a small area.
Newton’s Telescope:
The first reflecting telescope
Secondary
Mirror
Primary
Mirror
The world’s biggest telescopes are
reflectors, not refractors.
What’s wrong with lenses?
(1)Lenses absorb light.
(2)Lenses sag.
(3)Lenses have chromatic aberration: colors
don’t focus at the same point.
Chromatic Aberration.
As light passes through a lens, just as a prism will disperse light, the lens will focus bluer
wavelengths differently than the redder wavelengths.
Blue Focus
Red Focus
World’s largest refracting telescope:
Yerkes Observatory, D = 1 meter, completed
In 1898.
Reflecting telescopes do not suffer from Chromatic
Aberration. All wavelengths will reflect off the mirror
in the same way.
Reflecting telescopes can be made very large because
the mirrored surfaces have plenty of support. Thus,
reflecting telescopes can greatly increase in light
gathering and resolving power.
Reflecting telescopes are often cheaper ($$$) to make
than similarly sized refracting telescopes.
Amount of light collected per second is is
proportional to the AREA of the lens or mirror.
Area 

4
D
D = diameter of
lens/mirror
2
A bigger lens or mirror is able to
resolve finer structures in the image
low resolution
high resolution
Two stars are “RESOLVED” if they
are seen as separate points.
Smallest angle resolved is proportional
to 1/D where D = the diameter of the
mirror
MAGNIFICATION is not as important:
Big, blurry image is less useful than
small, sharp image.
A MODERN REFLECTING TELESCOPE:
Large Binocular Telescope:
Mt. Graham, near Safford AZ.
Two mirrors, each 8.4m in diameter
Where to put a Telescope?
Far away from civilization – to avoid light pollution
“Seeing”
= twinkling
Weather conditions and
turbulence in the
atmosphere set limits to the
quality of astronomical
images from ground-based
observatories
Mountain top
observatories are put
on peaks where the
Atmospheric turbulence
is minimal
Bad seeing
Good seeing
Laminar vs. Turbulent Fluid Flow
Air becomes turbulent when it encounters
a barrier – e.g. a mountaintop
 bad seeing
Turbulent Flow
Laminar flow
The Hubble Space Telescope is 600 kilometers
above the Earth’s surface.
Hubble Space Telescope has great angular
resolution; it’s above the turbulent
atmosphere.
Light-gathering
ability? Not as
great; it’s only
D = 2.4 meters
in diameter.
Problem: It costs a lot of money to put a telescope in space!
Problem #2: It’s really hard to repair telescopes in space – only Hubble was designed to
be repairable
X-Ray Astronomy
X-rays are completely absorbed in the atmosphere.
X-ray astronomy has to be done from satellites.
NASA’s Chandra
X-ray Observatory
Gamma-Ray Astronomy
Gamma-rays: most energetic electromagnetic radiation; traces the most
violent processes in the Universe
The Compton
Gamma-Ray
Observatory
Infrared Astronomy
Although short wavelength IR gets through the atmosphere, longer wavelength IR does not.
In space, can cool the telescopes so it’s not a source of high background
Spitzer Space Telescope
Next Huge NASA mission, after
Hubble Space Telescope ends:
James Web Space Telescope (JWST)
Radio telescopes detect radio frequency
radiation which is invisible to your eyes.
Parabolic “dish” of a radio
telescope acts as a mirror,
reflecting radio waves to
the focus.
Radio telescopes can be huge
because they don’t have
to be as smooth as
optical telescopes: the
wavelength of radio light is
several cm’s and mirrors
only have to be smooth
to about 1/20 of a wavelength
to focus the light well
Surface of mirror
Arecibo Radio Observatory in Puerto Rico
Radio Interferometry
The Very Large Array
(VLA): 27 dishes are
combined to simulate
a large dish of 36 km
in diameter.
Even larger arrays consist of dishes spread out over the entire U.S.
(VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very
Long Baseline Interferometry)