Light (Electromagnetic Spectrum)
and
Telescopes
[week 2 and 3]
A Telescope is a tool
used to gather light from
objects in the universe
OBJECTIVES
Our
Objective
• Treat the telescopes as an instrument
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–
–
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Learn telescope parameters
What makes a telescope useful?
Telescope operation
Different telescope types
There are two different types of
telescopes
• A refracting telescope uses a glass lens to
concentrate incoming light
• A reflecting telescope uses mirrors to
concentrate incoming starlight
Telescope Optics
The Refractor
Objective (lens)
eyepiece
• Common as small
telescopes
• $$$ in large apertures
• Superb image quality
2.4” Amateur Refractor
40” Refractor
The Newtonian Reflector
6” Amateur Newtonian
• Common as amateur
telescopes
• Lower cost
• Simple optical design
• Good image quality
• Central obstruction
The Cassegrain Reflector
• Large f-number in small package
• F/10 in a 24” long tube
• Good imagery for large f/#
• Design used in large telescopes
The Schmidt Cassegrain
Corrector plate
Spherical
primary
mirror
• Large f-number in small package
• F/10 in a 24” long tube
• Good imagery for large f/#
• Better spherical aberration control
8” Schmidt Cassegrain
Chromatic Aberration
The Problem
Chromatic Aberration
The Solution
Spherical Aberration
Spherical Aberration
A ____________ telescope uses a
lens to concentrate incoming light
Reflecting telescopes use mirrors to
concentrate incoming starlight
Three main functions (Powers)
of a Telescope
Most important!!
• Light Gathering Power: bigger aperture is
better making objects appear brighter
followed by
• Resolving Power: to see fine detail
RP(in arcsec) =
and least important,
• Magnifying Power:
magnification = M
LGP  Area 
 (nm)
d (mm)
 .2516
M 
fo
fe

4
d2
wavelength (nm)
diameter (mm)
SENSITIVITY
HOW MUCH LIGHT CAN THE TELESCOPE GATHER
DEPENDS ON THE
-APERTURESIZE OF THE MIRROR OR
LENS
Light Gathering Power
“The Power of a Telescope”
eye
Eye behind telescope
Telescope Resolving Power
Star
Double Star
DIFFRACTION
RESOLUTION AND THE AIRY DISC
RESOLUTION
 = 4.56 / D
 is the separation in arc seconds
D is the diameter of lens/mirror
in inches
A larger
objective lens
provides a
brighter (not
bigger) image
Magnification
Magnification = Telescope focal length ÷ eyepiece focal length
2000 mm ÷ 76 mm = 78 X
2000 mm ÷ 10 mm = 200 X
2000 mm ÷ 1 mm = 2000X
Maximum useful magnification:
- 60X per 1” of aperture
Practical magnification depends on
- Optics and seeing
The Job of a Telescope
See faint objects - Light gathering power
See detail on objects - Resolving power
Magnify otherwise small objects - Magnification
Reflecting telescopes use mirrors to
concentrate incoming starlight
Advantages and Disadvantages of
Various Telescopes
See One-Minute Astronomer worksheet and
notes below previous slide.
If you pass white light through a prism,
it separates into its component colors.
long wavelengths
R
O
Y
G
B
I
V
short wavelengths
spectrum
But visible light is only one
type of electromagnetic
radiation (light) emitted by
stars
Astronomers are truly
interested in the entire
spectrum of Light!
Consider This Class as Seen in Different Wavelengths of Light!
Consider Orion as Seen in Different Wavelengths of Light!
Observations at other wavelengths are
revealing previously invisible sights
UV
Ordinary
visible
infrared
Map of
Orion
region
Hubble Space Telescope Views of Orion Nebula showing stars hidden in clouds
http://oposite.stsci.edu/pubinfo/pr/97/13/A.html
TODAY’S Sun as seen in visible light from Earth and
from space in X-rays by satellites
Radio wavelength observations
are possible from Earth’s surface
The Very Large Array (VLA) in New Mexico
One such array is called the Very Large Baseline Array (VLBA): it consists of ten radio
telescopes which reach all the way from Hawaii to Puerto Rico: nearly a third of the way
around the world! By putting a radio telescope in orbit around the Earth, radio astronomers
could make images as if they had a radio telescope the size of the entire planet!
Why do some stars, sky objects appear to twinkle?
Differences in the temperature and density of small portions
of Earth’s atmosphere cause passing starlight to quickly
change direction, making stars appear to twinkle.
Earth’s atmosphere hinders astronomical research
Image of stars taken
with a telescope on
the Earth’s surface
Same picture taken with
Hubble Space Telescope
high above Earth’s blurring
atmosphere
High above
Earth’s
atmosphere,
the Hubble
Space
Telescope
provides
stunning
details about
the universe
• Hubble
orbits the
Earth at an
altitude of
about 353
miles and
in 97
minutes.
But visible light is only one
type of electromagnetic
radiation (light) emitted by
stars
Astronomers are truly
interested in the entire
spectrum of Light!
Astronomers use different instruments to look at
light of different wavelengths - sometimes, we even
have to go above Earth’s atmosphere.
Not all EM radiation can penetrate Earth’s atmosphere.
Which is the correct reasoning for why a gamma ray
telescope located in Antarctica that is to be used to
look for evidence of black holes in the centers of
galaxies would not get funded?
A. There is no way to detect the presence
of a black hole.
B. Gamma rays are too energetic to detect
with a telescope.
C. You can’t build a functioning telescope
in Antarctica.
D. Gamma rays don’t penetrate Earth’s
atmosphere.
Imagine you’re the head of a funding
agency that has a very tight budget for
building a telescope. Which of the three
proposed telescopes below would be best to
support?
A. A gamma ray telescope in Antarctica
B. A radio telescope in orbit above the
Earth
C. A visible telescope located high on a
mountain in Peru
D. An ultraviolet telescope located in the
Mojave desert
Poor Seeing:
Aberration
introduced
by the Atmosphere
Ideal
Aberrated
ACTIVE OPTICS
ADAPTIVE OPTICS - CORRECTS ATMOSPHERIC EFFECTS
ALTAZIMTH MOUNT
EQUATORIAL MOUNT
Telescope Field of View
Moon=0.5 deg.
Field of View = Eyepiece apparent FOV ÷ Magnification
Large Magnification = Small field of view
Small Magnification = Large Field of view
Plane Mirror
Normal:
Angle of Incidence
i
normal
Angle of Reflection 
r
The Law of Reflection:
 r  i
i r
Concave & Convex Mirrors
Focal Length:
Diverging (of light):
Converging (of light):
Convex
Radius of Curvature:
The radius of curvature of the
mirror is twice the
focal length.
Concave
Snell’s Law
[The Law of Refraction]
Refraction: The bending of light that occurs when it is incident on a
plane surface & is going from one medium to another medium. Why
does this happen?
ANSWER: Because the speed of light changes in various mediums!
Normal:
Angle of Incidence:
Angle of Refraction:
Snell’s Law
(carpet & hardwood analogy)
• If the light slows down, what happens to its wavelength, frequency and color?
(do they stay constant, increase or decrease)
Recall v   f.
• Different colors of light have different speeds and thus bend different
amounts. (n = v/c)
• If light is going from a less dense medium to a more dense one, it bends
_____________ the normal.
• If light is going from a more dense medium to a less dense one, it bends
_____________ from the normal.
• Be sure to observe Snell’s Law in lab today as it explains why the
lenses bend light or refract light.
Diverging & Converging Lenses
Focal Length:
Radius of Curvature:
The radius of curvature of the
mirror is twice the
focal length.
Refractor and Reflector
Lens
Mirror
Question from Last Week:
• Spherical Lenses are the most common type
of lens and compound lens systems are used
to lessen the spherical aberration.
Astrology: The belief that the
positions of the stars and planets
as seen from Earth impact human
events.
CONSTELLATIONS
(STAR CHARTS)
Use the winter
triangle to find
constellations
during winter
evenings
Anyone recognize any shapes here?
Star Names
Betelgeuse
Aldebaran
Rigel
SIRIUS
Using Orion to find other objects
Pleiades
Aldebaran
Great Orion Nebula
Sirius
Pleiades
Seven
Sisters
Subaru
Use the
Big
Dipper in
the
northern
sky as a
way to
find other
groups of
stars &
north
How to find stuff in the sky – Star Charts
http://skymaps.com/
http://www.fourmilab.ch/yoursky/cities.html
http://www.astro.wisc.edu/~dolan/constellations/constellationjavalist.html