ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: jorosz@mail.sdsu.edu
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Text:
“Discovering the Essential Universe,
Fifth Edition”
by
Neil F. Comins
Course WWW Page
http://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2013.html
Note the underline: … ast101_fall2013.html …
Also check out Nick Strobel’s Astronomy Notes:
http://www.astronomynotes.com/
Homework
• Homework due September 19: Question 4
from Chapter 3 (What are the three main
functions of a telescope?)
• Write down the answer on a sheet of paper and
hand it in before the end of class on September
19.
Homework
• Go to a planetarium show in PA 209:
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The days and times of the shows will be (all shows last less than 1 hour):
Thursday, September 5 : 5 PM
Friday, September 6 : 2 PM
Monday, September 9 : 1 PM and 5 PM
Tuesday, September 10 : 1 PM and 5 PM
Wednesday, September 11 : 5 PM
Thursday, September 12 : 5 PM
Friday, September 13 : 3 PM
• Get 10 points extra credit for homework part of grade.
• Sign up for a session outside PA 209.
• Hand in a sheet of paper with your name and the data and time of
the session.
Coming Up
• This week: Chapter 3 (Telescopes and light)
• Tuesday, September 24: wrap-up, review
• Thursday, September 26: Exam #1
Fall 2013
No appointment needed!
Just drop by!
Where: Room 215, physics-astronomy building (PA-215).
When: All semester long, at the following days and times:
• Monday:
12 – 2 PM; 5 – 6 PM
• Tuesday:
12 – 2 PM; 5 – 6 PM
• Wednesday: 12 – 2 PM; 5 – 6 PM
• Thursday: 1 – 2 PM; 3 – 6 PM
In the News…
• http://www.latimes.com/science/sciencenow/la-sci-sn-harvest-moon-video20130918,0,525553.story
• http://24timezones.com
Coming Up:
• The 4 forces of Nature
• Energy and the conservation of energy
• The nature of light
– Waves and bundles of energy
– Different types of light
• Telescopes and detectors
Light is a form of energy.
Why is this important?
With very few exceptions, the
only way we have to study
objects in Astronomy is via the
light they emit.
What is the nature of light?
Light can be thought of as a
wave in an electric field
or
as discrete particles of energy…
What is the nature of light?
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Light can be thought of as a wave. The wavelength
(usually denoted with a l) is the distance from crest to
crest.
What is the nature of light?
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Light can be thought of as a wave. The frequency
(usually denoted with n) is the number of crests that pass
a given point each second.
What is the nature of light?
Light can be thought of as a wave. The frequency
(usually denoted with n) is the number of crests that pass
a given point each second.
What is the nature of light?
The velocity of the wave is the wavelength times
the frequency:
The velocity of light in vacuum is constant for
all wavelengths, regardless of the relative
velocities of the observer and the light source.
What is the nature of light?
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
The above animation shows waves with different
wavelengths moving with the same speed. Their
frequencies are different.
What is the nature of light?
Light can be thought of as a
wave in an electric field
or
as discrete particles of energy…
What is the nature of light?
Light can also behave like discrete particles called
photons. The energy of a photon depends
on the frequency (or equivalently the
wavelength):
The value of h is constant for all situations.
What is the nature of light?
Photons of higher energy have higher frequencies
and shorter wavelengths, since
Different “types” of light.
What light can tell us.
Visible light
• White light is made up of different colors
Visible light
• Different colors correspond to different
frequencies (or wavelengths).
• The colors of the rainbow are ROY G BIV:
red orange yellow green blue indigo violet.
Visible light
• In the visible,
 red has the longest wavelength, the smallest
frequency, and the lowest energy.
 violet has the shortest wavelength, the highest
frequency, and the highest energy.
The Electromagnetic Spectrum
• Visible light is only a tiny
fraction of the
Electromagnetic Spectrum.
• For example, there is invisible
radiation with wavelengths
longer than red light that heats
the thermometer.
The Electromagnetic Spectrum
• As we go to wavelengths slightly longer
than visible (i.e. smaller frequencies and
lower energies), we find infrared radiation,
which is basically perceived as heat.
• As we go to longer wavelengths still, we
find microwave radiation, which is often
used to pop popcorn.
The Electromagnetic Spectrum
• At the longest wavelengths, corresponding
to the smallest frequencies and the lowest
energies, we have radio waves, including
AM/FM, shortwave, TV, etc.
The Electromagnetic Spectrum
• Visible light is only a tiny fraction of the
Electromagnetic Spectrum.
• If we go to shorter wavelengths (higher
frequencies and energies), we find
ultraviolet light. With higher energies, UV
photons can damage skin cells.
The Electromagnetic Spectrum
• As we go even shorter in wavelength
(higher in frequency and energy), we get Xrays. With their high energies, X-rays can
be used to image our insides.
• As the shortest wavelengths and the highest
energies, we have gamma rays. Gamma
rays are sometimes used to sterilize food.
The Electromagnetic Spectrum
• Visible light is only a tiny
fraction of the
Electromagnetic Spectrum.
The Electromagnetic Spectrum
• Gamma rays, X-rays, UV light, visible light,
infrared radiation, microwaves, and radio waves
are all different manifestations of
electromagnetic energy.
• The range in wavelengths typically encountered
span a factor of 1014.
• All forms of electromagnetic radiation travel
with the same velocity.
• The Earth’s atmosphere is transparent to visible
light, some infrared, and the radio. It is opaque to
UV, X-rays, and gamma rays.
Coming Up:
• The 4 forces of Nature
• Energy and the conservation of energy
• The nature of light
– Waves and bundles of energy
– Different types of light
• Telescopes and detectors
With very few exceptions, the
only way we have to study
objects in Astronomy is via the
light they emit.
So we need to collect photons,
and detect them.
Telescopes
Telescopes
• A flat surface reflects incident light at the
same angle.
• A curved surface can focus light.
Telescopes
• Glass alters the path of light.
• A curved piece of glass can focus light.
Telescopes
• A telescope uses mirrors or lenses to
collect and focus light.
• The area of the lens or mirror can be
considerably larger than the area of the
eye’s pupil, hence much fainter objects can
be seen.
Telescopes
• A refracting telescope
uses a large lens to bring
the light to a focus, as in
Figure (a).
• A reflecting telescope
uses curved mirrors to
bring the light to a focus,
as in Figure (b).
Telescopes
• The largest lenses that can
be built have a diameter of
about 1m, and have very
long focal lengths.
• A lens must be held by its
edges, and large lenses sag
under their own weight.
Also lots of light is lost in
the glass.
• For these and more reasons,
all modern telescopes use
mirrors.
Telescopes
• Using an objective mirror, plus some additional
mirrors and lenses, light is collected and focused
to a point.
• This is a Newtonian telescope.
Telescopes
• Using an objective mirror, plus some additional
mirrors and lenses, light is collected and focused
to a point.
• This is a Cassegrain telescope.
Telescopes
• A telescope’s main job is collecting photons.
• The light gathering power is proportional to
the area of the mirror or lens. The area of a
circle is
•If you double the diameter of the mirror, the
light gathering power goes up 4 times.
Telescopes
• Modern mirrors can be made thin.
Their shapes are maintained using
pistons under computer control.
• The Gemini telescope in Hawaii has
primary mirror 8.1m in diameter.
Telescopes
• Modern mirrors can be
made thin. Their shapes
are maintained using
pistons under computer
control.
• The Gemini telescope in
Hawaii has primary mirror
8.1m in diameter.
• These thin mirrors are cast
in special rotating ovens.
Telescopes
• Mirrors can also be
made out of smaller
segments.
• The Keck telescopes
in Hawaii have
primary mirrors 10m
in diameter.
What a Telescope Does
• A Telescope is used to collect photons, so you
can see fainter objects.
Seeing Detail
• What does the next line say?
–
If you can read this, thank a teacher.
• Why is so hard to read?
• Why do binoculars help?
• It is hard to read because the angular size is
small. The binoculars magnify the angular
size.
What a Telescope Does
• A telescopes magnifies angular sizes.
What a Telescope Does
• A telescopes magnifies angular sizes and allows
you to see more detail.
Telescopes at other Wavelengths
• Recall that there other forms of “light”,
including radio waves, X-rays, UV light, etc.
• The goal of “collect and detect” is still the
same.
• However, the technologies used to collect and
detect are different at different wavelengths.
Radio Telescopes
• Radio telescopes use
“mirrors” made from
steel plates.
• Radio receivers collect
the focused radio
waves.
• The radio telescopes
are huge because of
the long wavelengths
of the radio waves.
Radio Telescopes
• Radio telescopes use
“mirrors” made from
steel plates.
• Radio receivers collect
the focused radio
waves.
• The radio telescopes
are huge because of
the long wavelengths
of the radio waves.
Radio Telescopes
• The GBT is the largest
steerable radio
telescope in the world,
with a diameter of 100
meters. It is perhaps
the largest movable
land-based object in
the world.
Radio Telescopes
• With modern
computers and
electronics, one can
combine the signals
from several radio
telescopes to
“synthesize” a much
larger telescope.
• The Earth’s atmosphere is transparent to visible light,
some infrared, and the radio.
• It is opaque to UV, X-rays, and gamma rays. To detect
these wavelengths, one must go to space.
X-ray Telescopes
• For example, X-ray light cannot be reflected like
visible light can. X-ray telescopes use “grazing
incidence” mirrors to collect X-rays.
Telescopes in Space
• The Hubble Space Telescopes observes in
the ultraviolet, visible, and infrared
Telescopes in Space
• The Hubble Space Telescopes observes in the
ultraviolet, visible, and infrared.
• It is also above the blurring atmosphere.
Telescopes in Space
• The Spitzer Space Telescopes observes in
the infrared
Telescopes in Space
• The image on the left is at optical wavelengths,
and the wavelength on the left is at infrared
wavelengths. Different features are seen.
Telescopes at other Wavelengths
• For most wavelengths,
you need to go into
space to observe.
Next:
Light Detection
Detection of Light
• Once the telescope has collected the light,
we need to detect the photons.
• A typical detector will not record all of the
incoming photons, i.e. the efficiency is less
than 100%.
• The observing efficiency is proportional to
the product of the telescope area and the
detection efficiency.
Detection of Light
• A bigger telescope gives a higher incoming
photon rate.
• A more efficient photon detector means
more photons are recorded for a given
incoming rate.
Galileo Galilei (1564-1642)
• Galileo was one of the
first people to use a
telescope to
systematically study
astronomical objects,
starting in about 1609.
• Galileo showed by
observing the patterns
of light and shadows
that the moon had
craters and mountains.
• Galileo made very detailed drawings and notes.
This page describes his observations of Jupiter and
its 4 moons that he discovered.
• Galileo observed spots on the Sun. He showed
that the Sun rotates about once every 28 days.
Disadvantages of the Eye.
• Sometimes your eyes “play tricks” on you,
i.e. you hallucinate.
• It is hard to be quantitative about some
things, for example how bright one star is
compared to another.
• Unless you are a good artist, it is difficult to
share your observations. There is no direct
permanent record.
Photography
• Certain light-sensitive chemicals (usually silver oxide) are placed
on glass plates or on plastic film.
• The chemicals are altered when exposed to light, the degree to
which depends on the intensity of the light.
• A chemical “development” process “freezes” the chemicals in
their altered states, making a record of the image.
• It is important to note that only about 5% of the exposed light is
actually recorded on film. Lots of photons are simply “wasted.”
Photography
•
Instead of your eye, put a camera on the
telescope. There are some practical
difficulties, as well as some advantages…
A difficulty…
• Stars move in the sky,
they rise and set. If
the telescope is not
moving, the images
appear streaked.
• Fortunately, it is
relatively easy to track
the telescope.
A difficulty…
• The curvature of the tracks depend on where you
look.
…The Solution:
• If you align one axis
towards the celestial
pole, then motion
around that axis will
“cancel” out the
apparent motion
caused by the
rotation of the Earth.
…The Solution:
• The gears needed to
track the telescope are
visible near the top of
the mount.
• Most telescopes are so
well balanced that they
can be moved by
hand, in spite of the
fact that they can
weigh several tons!
Advantages of Photography
• The film can be exposed over long periods
of time, allowing one to collect more
photons.
• Example: your eye takes an “exposure”
roughly 30 times per second. Suppose you
exposed film for 30 seconds. During that
time, your eye would have taken 900
exposures, erasing everything at the start of
each one.
Advantages of Photography
• Example: your eye takes an “exposure” roughly
30 times per second. Suppose you exposed film
for 30 seconds. During that time, your eye
would have taken 900 exposures, erasing
everything at the start of each one.
• Thus a 30 second exposure could potentially
detect 900 times more photons than you would
have seen with your eye, neglecting the different
efficiencies in detection (film is less sensitive
than the eye).
Advantages of Photography
• A camera on a small telescope can easily be
more effective than looking with the eye
through a big telescope.
History of Photography
• Film (usually glass plates) was first used for
astronomy.
• In the late 1970s, digital cameras came into
use. Digital cameras are more efficient at
detecting light than photographic film.
Photographic film detects about 5% of the
incoming light, whereas digital cameras can
detect well over 90% of the incoming light.
Digital Photography
• In the late 1970s, digital cameras came into use.
Digital cameras are more efficient at detecting
light than photographic film. Photographic film
detects about 5% of the incoming light, whereas
digital cameras can detect well over 90% of the
incoming light.
Digital Photography
• Digital cameras are more efficient at detecting
light than photographic film.
Color Photography
• Color information is obtained by placing various
filters in front of the camera.
Color Photography
• The separate images are digitally processed to
obtain the final color image.
Color Photography
Color Photography
The faintest stars and
galaxies in this picture
are about one billion
times fainter than the
faintest stars you can see
without a telescope.