The Very Large Array (VLA) in New Mexico
Observations at wavelengths other than visible light are
revealing previously invisible sights
Visible light image
radio wavelength image
Astronomers use different instruments to look at
light of different wavelengths - sometimes, we even
have to go above Earth’s atmosphere.
SOFIA - the Stratospheric Observatory for Infrared
Astronomy
Observations at other wavelengths are
revealing previously invisible sights
UV
Ordinary
visible
infrared
Map of
Orion
region
Consider Orion as Seen in Different Wavelengths of Light!
http://www.cnn.com/2001/LAW/02/20/scotus.heatdetector.01.ap/index.html
High above
Earth’s
atmosphere,
the Hubble
Space
Telescope
provides
stunning
details about
the universe
Hubble Space Telescope Views of Orion Nebula showing stars hidden in clouds
http://oposite.stsci.edu/pubinfo/pr/97/13/A.html
Yesterday’s Sun as seen in visible light from
Earth and from space in X-rays by satellites
High Energy Gamma Rays - Compton Gamma Ray Observatory (GRO) Satellite
The Sky’s emission of Gamma Rays
But, we receive GRBs from every direction !!
The fact that GRBs come from every direction imply
that GRBs don’t come from our galaxy, but from
other galaxies spread in every direction!
Three main functions of a
telescope
• brighten
(called light gathering power)
• see fine detail
(called resolution)
and least important,
• magnify
magnification = (objective lens focal length / eyepiece lens focal
length)
magnification = (objective lens focal length / eyepiece lens focal length)
A larger
objective lens
provides a
brighter (not
bigger) image
Two Fundamental Properties of a Telescope
1. Angular Resolution
• smallest angle which can be seen
 = 1.22  / D
where  = wavelength; D = diameter of the aperture
Angular Resolution
• The ability to separate two objects.
• The angle between two objects
decreases as your distance to them
increases.
• The smallest angle at which you can
distinguish two objects is your
angular resolution.
Two Fundamental Properties of a Telescope
1. Angular Resolution
• smallest angle which can be seen
 = 1.22  / D
2. Light-Collecting Area
•
The telescope is a “photon bucket”
A =  (D/2)
2
D
A
Angular Resolution:
  1.22

where  is in radians.
D
 radians  180 deg so 57.30 deg/radian
  1.22  57.30 

D
 69.91



  69.91  deg    3600
D


 251676

D
arcsec

D
hence:
deg 
 seconds  
 deg  


Parts of the Human Eye
• pupil – allows light to
enter the eye
• lens – focuses light to
create an image
• retina – detects the
light and generates
signals which are sent
to the brain
A camera works in the same way where the shutter acts
like the pupil and the film or CCD acts like the retina!
Lenses bend Light
Focus – to bend all light waves coming from the same
direction to a single point
Light rays which come from different directions converge
at different points to form an image.
Telescope Types
• Refractor
– focuses light using lenses
• Reflector
– focuses light using mirrors
– used exclusively in professional
astronomy today
A refracting
telescope
uses a lens
to
concentrate
incoming
light
Similar to a
magnifying glass
Refracting telescopes have
drawbacks
• Spherical aberration
Too spherical
Refracting telescopes have
drawbacks
• Spherical aberration
• Chromatic aberration
Special achromatic compound lenses and lens
coatings can often fix this aberration
Refracting telescopes have
drawbacks
• Spherical aberration
• Chromatic aberration
• Sagging due to
gravity distorting the
lens
• Unwanted
refractions
• opaque to certain
wavelengths of light
Refractor
Yerkes 40-inch telescope; largest refractor in the world
Reflector
Gemini 8-m Telescope, Mauna Kea, Hawaii
Reflectors
MMT – Mt. Hopkins, AZ
SUBARU – Mauna Kea, HI
Reflecting telescopes use
mirrors to concentrate incoming
starlight
Newtonian Focus
Prime Focus
Cassegrain focus
coude’ focus
Astronomer’s face two major
obstacles in observing the
skies
• Light Pollution from Cities
Tucson, Arizona in 1959 and
1980
Astronomer’s face two major
obstacles in observing the
skies
• Light Pollution from Cities
• Effects of Twinkling from Earth’s
atmosphere
Rapid changes in the density
of Earth’s atmosphere cause
passing starlight to quickly
change direction, making stars
appear to twinkle.
Irregularities in the atmosphere’s density
cause the light to not arrive at the
telescope as plane parallel waves.
Atmosphere
Lens
Advanced technology is spawning
a new generation of equipment
to view the universe
• CCDs (charge-coupled devices)
• Large telescopes on remote mountain
tops
– Maunakea
– Cerro Pachon in Chile
• Orbiting space observatories
• Adaptive Optics to counteract the
blurring of Earth’s atmosphere
Adaptive Optics (AO)
• It is possible to “de-twinkle” a star.
• The wavefronts of a star’s light rays are deformed by the
atmosphere.
• By monitoring the distortions of the light from a nearby
bright star (or a laser):
– a computer can deform the secondary mirror in the opposite way.
– the wavefronts, when reflected, are restored to their original state.
AO mirror off
AO mirror on
• Angular resolution
improves.
• These two stars are
separated by 0.38
• Without AO, we see
only one star.
A.O. Movie…
Instruments in the Focal Plane
How do astronomers use the light collected by a telescope?
1. Imaging
–
–
use a camera to take pictures (images)
Photometry  measure total amount of light
from an object
2. Spectroscopy
–
use a spectrograph to separate the light into its
different wavelengths (colors)
Imaging
• Filters are placed
in front of a camera
to allow only
certain colors to be
imaged
• Single color images
are superimposed
to form true color
images.
A little color theory:
So why do TV’s use RGB?
Shouldn’t they use RYB?
Spectroscopy
• The spectrograph
reflects light off a
grating: a finely ruled,
smooth surface.
• Light interferes with
itself and disperses into
colors.
• This spectrum is
recorded by a CCD
detector.