Light / Telescopes - Ann Arbor Earth Science

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

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 parrallel
waves.
Atmosphere
Lens

Haleakalä Observatory,
Maui
(Faulkes Telescope North)

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.
•
Angular resolution
improves.
•
These two stars are
separated by 0.38
•
Without AO, we see only
one star.
AO mirror off
AO mirror on

A.O. Movie…
ao-animation.mov

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)

•
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.
Imaging

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.

Blocking the Aperture

•
Based on the idea of
filling the aperture of
a camera:
Interferometry

kbk

•
Based on the idea of
filling the aperture of
a camera:
Interferometry

Green Bank Interferometer
© 2004 Pearson
Education Inc., publishing

The Very Large Array (VLA)

Traffic Jam, VLA Style

VLA Synthesized Aperture
•
Synthesized
aperture in a
few minutes,
as “seen” by
the radio
source
•
Composed
of 351 pairs
of antennas

Synthesized Aperture—1 hour
•
Synthesized
aperture after
1 hr of Earth
rotation
•
Each pair
“fills in” more
of the
aperture

Synthesized Aperture—10
•
Synthesized
aperture
after 10 hrs
of Earth
rotation
•
Aperture is
“filled in”
•
Very high
fidelity
imaging
hours

Radio Galaxy Cygnus A: Pre-VLA

Cygnus A Imaged by the VLA