Light and Telescopes

murmerlastUrban and Civil

Nov 16, 2013 (3 years and 11 months ago)

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Light and Telescopes

What do you think?


What is the main purpose of a telescope?


Why do stars twinkle?

What is Light and Why Would
Astronomers Want to Study the
Properties of Light?


Sometimes we say light is made of
waves


Sometime we say light is made of particles called
photons


Moves
very fast
, at 186 000 miles





per second


300,000 km per second


consider a prism ...

If you pass white light through a prism, it
separates into its component colors.

R.O.Y. G. B.I.V

spectrum

long wavelengths

short wavelengths

Analyzing The Properties of
Light


Visible Light is but one part of the entire
electromagnetic (EM) spectrum.


EM Spectrum includes all kinds of light


radio waves
(all light moves at the same speed
-


micro waves
light speed)


infrared light


visible light


ultra violet light
(some light photons have



x rays
shorter wavelengths


gamma rays
and more energy than others)

Visible light is only one
type of electromagnetic
radiation emitted by
stars

Each type of EM
radiation travels at
exactly the same speed
-

the speed of light!

Not all EM radiation can penetrate Earth’s atmosphere.

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 in Different Wavelengths of Light!

http://www.cnn.com/2001/LAW/02/20/scotus.heatdetector.01.ap/index.html

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

http://solar.physics.montana.edu/tslater/real
-
time/

Observations at wavelengths other than visible
light are revealing previously invisible sights

Visible light image

radio wavelength image

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!

Radio wavelength observations
are possible from Earth’s surface

The Very Large Array (VLA) in New Mexico

Different types of EM radiation
require different types of telescopes


A refracting telescope uses a lens to
concentrate incoming light


A reflecting telescope uses mirrors to
concentrate incoming starlight

Analyzing Spectra:
The Properties of Light


Visible Light is one part of the electromagnetic
(EM) spectrum.


EM radiation is described as a wave with
wavelengths in the range 1x10
-
15
m to 100 m.


Visible light is only a tiny small part of the entire
electromagnetic spectrum.


Little bits of light are called photons.

Dividing Light Into a Spectrum

Astronomers separate out light into its
individual components using a
diffraction grating or using a prism
-

then they analyze each part
independently!

Filter

Detector

blue

4600 A


81

Filter

Detector

blue

4600 A


81

green

5300 A


85

Filter

Detector

blue

4600 A


81

green

5300 A


85

yellow

5800 A


83

Filter

Detector

blue

4600 A


81

green

5300 A


85

yellow

5800 A


83

orange

6100 A


78

Filter

Detector

blue

4600 A


81

green

5300 A


85

yellow

5800 A


83

orange

6100 A


78

red

6600 A


70

The spectrum is continuous.

UV

IR

Spectra


Most light sources contain energy in lots
of different wavelengths.


We can measure the brightness in various
wavelength bands
--
the result is called the
spectrum.


The spectrum (total character of light
emitted) can tell us a lot about a source.

A refracting
telescope
uses a lens to
concentrate
incoming
light

Similar to a
magnifying glass

A larger
objective lens
provides a
brighter (
not
bigger
) image

lenses
reverse
images

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)

Functions of a Telescope


To gather light.


want a large objective


range of few inches to 10 meters!!


To resolve fine detail.


limited by size and atmospheric “seeing”


To magnify


least important


about 50x per inch of aperture

(rule of thumb)

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

Yerkes Observatory

-

40
-
inch Refracting Telescope:

The Largest Refracting Telescope in the World

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 stars


Light Pollution from Cities


Effects of Twinkling from Earth’s atmosphere

Tucson, Arizona in 1959 and 1980

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

Rapid changes in the density of
Earth’s atmosphere cause passing
starlight to quickly change
direction, making stars appear to
twinkle.


Advanced technology is
spawning a new generation of
equipment to view the universe


CCDs (charge
-
coupled devices)


Large telescopes on remote mountain tops


Mauna Kea in Hawaii


Cerro Pachon in Chile


Adaptive Optics to counteract the blurring
of Earth’s atmosphere


Orbiting space observatories

A Charge
-
Coupled Device
(CCD)

Ordinary Photographs vs. CCDs

Film (negative) CCD (negative) CCD (positive)

Same integration (I.e. exposure) time.

Different quantum efficiency: Film 1% CCD 70%

Matching 10
-
m, multiple
mirror Keck Telescopes in
Hawaii with adaptive optics

High above
Earth’s
atmosphere,
the Hubble
Space
Telescope
provides
stunning
details about
the universe

What did you think?


What is the main purpose of a telescope?



A telescope is designed to collect as much light
as possible. It also improves resolution and
magnifies images.


Why do stars twinkle?


Rapid changes in the density of Earth’s
atmosphere cause passing starlight to change
direction, making stars appear to twinkle.