Chapter 6:
The Tools of the
Astronomer
Telescopes gather light and
concentrate it.
They come in two types:
Reflectors and Refractors
Reflectors use mirrors to
reflect the light to a focus
Refractors use lenses to
bend the light to a focus
The most important property
of any telescope is to gather
lots of light and concentrate.
The Light Gathering Power (LGP) of a telescope depends
on the area and the area is proportional to the square of
the diameter
2
LGP  diameter
Refraction is the bending of
light when it goes from one
material to another
n1 sin 1  n2 sin 2
cvacuum
n
cmaterial
The Law of Refraction
relates the incident angle,
1, and the refracted angle,
2, to the index of refraction
of each of the two materials
Recall that the index of refraction is the ratio of the speed
of light in vacuum to the speed of light in the material
If we curve the surfaces of a
piece of glass, we can get
parallel light rays to focus to
a point
A refracting telescope uses
two lenses
Since the eye already has a lens, the eyepiece is needed
to bring the light rays back to parallel for the eye to see
Large refractors
can be very long
and bulky
The Largest Lens is 40”
The large refractor at the Yerkes Observatory in
Wisconsin was built in the 1897.
Lenses and refractors suffer
from Chromatic Aberration
This applies to camera lenses, your eye, telescopes
and anything else that uses a lens to focus light
Chromatic Aberration causes
rainbows around objects
Correcting for Chromatic
aberration can be expensive
The compound lens takes two lenses of
different materials and combine them to
correct for color distortion
Chromatic aberration is used
in a prism to separate light
into its colors
Since it is meant to
be separated we
don’t call it an
aberration.
Instead, it is called
dispersion
A diffraction grating works on
interference of light waves
Diffraction is much
more efficient at
separating light
into its colors than
dispersion
Unfortunately, diffraction
also leads to problems
Look closely enough and points
aren’t just points but rings, too
Reflection is the bouncing of
light off a surface
i  r
A concave
mirror
focuses light
to a focal
point
Telescope mirrors are
made so that the focus is
over a small area called
the focal plane rather
than a point
There are
several
types of
reflecting
telescopes
The resolution of a telescope
depends on its size and the
wavelength of the light
  2.06 10
5

D
The atmosphere limits the
resolving power of a
ground-based telescope
Adaptive Optics can
clear up most of the
distortions caused
by the atmosphere
The
distortions
are caused by
differences in
the air above
the telescope
Once a sight with good “seeing”
is found everyone wants to use it
Kitt Peak
Arizona
Mauna Kea Hawaii
The Largest Optical Telescope
Keck I and II (for now)
The 10 meter Keck Mirror
The Earth’s Atmosphere
Blocks Many Wavelengths
Only visible light, radio waves and some Infrared can be
“seen” from the ground. To see the rest, you must go get
outside Earth’s atmosphere by going into space
Early telescopic observations
were done by eye and sketch
By the early 1900’s photographic
plates were the dominant
scientific way to observe
Today all scientific observations
are done with a CCD Camera
A CCD converts photons into electrons
and then counts the electrons
Each pixel acts like a light bucket, catching photons,
converting them to electrons and storing them until
they are read out by the electronics.
Watch ClassAction Telescopes and Astronomical
Instruments module CCD Simulator animation
CCD’s only take black &
white images
To make a color
image we either
take images with
red, green and
blue filters or use
microfilters
Some CCD Cameras are
HUGE
The LSST
telescope is
being built
in Chile
Another common measuring
device is the spectrograph
Spectrographs can use prisms
or diffraction gratings
The mirror just before the
prism rotates to put the
different colors on the
photomultiplier tube
The diffraction grating
spectrometer just takes a
picture of the spectrum
produced by the grating
The spectrograph produces a
spectrum of the object
The CCD only takes a black and white image so it
must be calibrated. The lines at the top and bottom
are produced by a calibration light source and are
used to calibrate the wavelengths.
Spectra can be displayed as a
graph or rainbow of color
Observing In Radio Waves
Radio waves have long wavelength
and so have poor resolution
Under the dish at Arecibo
As long as the spacing in the mess is much less than the
wavelength, the “light” sees the mesh as a solid surface
The Very Large Array (VLA)
Instead of building one huge dish, the VLA ties
many smaller radio dishes together in an
interferometer which gives much higher resolution
The individual
dishes of the
VLA are still
large
Optical Interferometry is
extremely difficult but it is
being tried
The Very Large
Telescope in Chile
combines several
telescopes
together to form
an interferometer.
The atmosphere blocks
most IR so you have to get
above it to see in IR
The SOFIA is an IR telescope mounted in a 747
Some wavelengths require
observing in space
The Chandra
Observatory
sees in x-rays
The GALEX
mission observes
the sky in far
ultraviolet
The Spitzer
Space Telescope
looks at the
universe in far
infrared
To learn about a planet we
need to go there
The Cassini spacecraft is
currently orbiting Saturn
The Voyager I and II missions
were fly-by missions.
Voyager I flew by Jupiter and Saturn. Voyager made
the grand tour and flew by Jupiter, Saturn, Uranus
and Neptune
We learn even more when we
land on a planet
Huygens
landed on
Titan in
2005
The Mars
Curiosity
Rover is
roaming
around on
Mars
The Russian
Venera missions
landed on Venus
in the 1970’s and
1980’s
We learn the most if we go there
and bring something back
NASA plans to launch a
Mars sample return
mission in 2018
The US Apollo missions
brought back a total of
382 kilograms of moon
rocks. The Soviet Luna
robotic missions brought
back 362 grams
Observing Neutrino’s opens up
a new window on the universe
Neutrino’s are very hard to
detect since they don’t interact
with normal matter very much
Looking for gravity waves is
another new technique
LIGO uses an interferometer to detect the
passing gravity waves and has two sites
We also use computer models
to simulate astronomical events
The collision of
two galaxies
takes billions of
years but can
be simulated
with a
supercomputer