Electromagnetic Radiation and Telescopes
(How we get information about the cosmos
and how we gather the information)
Clicker Question
a) bend around corners and edges.
Diffraction is the
tendency of light
to
b) separate into its component colors.
c) bend through a lens.
d) disperse within a prism.
e) reflect off a mirror.
Clicker Question
a) bend around corners and edges.
Diffraction is the
tendency of light
to
b) separate into its component colors.
c) bend through a lens.
d) disperse within a prism.
e) reflect off a mirror.
Diffraction affects all
telescopes and limits the
sharpness of all images.
Telescopes
Light Hitting a Telescope Mirror
*
*
huge mirror
near a star
small mirror far
from a star
In the second case (reality), light rays from any single point
of light are essentially parallel.
Light rays from a distant source, parallel to the
"mirror axis“, all meet at one point, the focus.
CCD
*
*
Optical Telescopes
Reflecting and refracting telescopes
Optical Telescopes - Refracting vs. Reflecting
Refracting telescope
Focuses light with a lens (like a camera).
object (point of light)
image at focus
Problems:
- Lens can only be supported around edge.
- "Chromatic aberration".
- Some light absorbed in glass (especially UV, infrared).
- Air bubbles and imperfections affect image quality.
Reflecting telescope
Focuses light with a curved mirror.
<-- object
image
- Can make bigger mirrors since they are supported from behind.
- No chromatic aberration.
- Reflects all radiation with little loss by absorption.
Refracting Telescope
Reflecting Telescope
Yerkes 40-inch (about 1 m).
Largest refractor.
Cerro-Tololo 4 -m reflector.
Chromatic Aberration
Lens - different colors focus at different places.
white light
Mirror - reflection angle doesn't depend on color.
Types of reflecting telescopes
Clicker Question
Modern telescopes
use mirrors rather
than lenses for all
of these reasons
EXCEPT
a) light passing through lenses can be absorbed
or scattered.
b) large lenses can be very heavy.
c) large lenses are more difficult to make.
d) mirrors can be computer controlled to
improve resolution.
e) reflecting telescopes aren’t affected by the
atmosphere as much.
Clicker Question
Modern telescopes
use mirrors rather
than lenses for all
of these reasons
EXCEPT
a) light passing through lenses can be absorbed
or scattered.
b) large lenses can be very heavy.
c) large lenses are more difficult to make.
d) mirrors can be computer controlled to
improve resolution.
e) reflecting telescopes aren’t affected by the
atmosphere as much.
Reflecting instruments like
the KECK telescopes can
be made larger, and more
capable, than refractors.
Mirror size
Mirror with larger area captures more light from
a star. Can look at fainter objects with it.
Keck 10-m optical telescope
30-100 m optical telescopes now being considered!
Image of Andromeda galaxy
with optical telescope.
Image with telescope of twice the
diameter, same exposure time.
The Two Main Types of Observation
Imaging (recording pictures)
Spectroscopy (making a spectrum) usually using a
diffraction grating
In both cases, image or spectrum usually recorded on a
CCD ("charge-coupled device")
Resolving Power of a Mirror
(how much detail can you see?)
fuzziness
you would
see with
your eye.
detail you
can see
with a
telescope.
(a) 10′; (b) 1′; (c) 5″; (d) 1″
"Angular resolution" is the smallest angle by which two objects
can be separated and still be distinguished.
For the eye, this is 1' (1/60th of a degree). Or 100 km at
distance of the Moon.
angular resolution 
wavelength
mirror diameter
For a 2.5-m telescope observing light at 5000 Angstroms (greenish),
resolution = 0.05".
But, blurring by atmosphere limits resolution of optical telescopes to
about 1". This is called seeing.
Seeing
*
Air density varies => bends light.
No longer parallel
Parallel rays enter
atmosphere
dome
No blurring case.
Rays brought to
same focus.
Blurring. Rays
not parallel. Can't
be brought into
focus.
CCD
*
Sharp image
on CCD.
Blurred
image.
Example: the Moon observed with a 2.5 m telescope
1" => 2 km
0.05" => 100 m
Hubble Space
Telescope image,
0.05” resolution
Ground-based telescope image, 1” resolution
North America at night
So where would you put a telescope?
Kitt Peak National
Observatory, near Tucson
Mauna Kea Observatory,
Hawaii
Clicker Question
a) larger telescopes & longer wavelengths.
Resolution is
improved by
using
b) infrared light.
c) larger telescopes & shorter wavelengths.
d) lower frequency light.
e) visible light.
Clicker Question
a) larger telescopes & longer wavelengths.
Resolution is
improved by
using
b) infrared light.
c) larger telescopes & shorter wavelengths.
d) lower frequency light.
e) visible light.
Diffraction limits resolution; larger telescopes and
shorter-wave light produces sharper images.
Clicker Question
An advantage of
CCDs over
photographic
film is
a) they don’t require chemical development.
b) digital data is easily stored & transmitted.
c) CCDs are more light sensitive than film.
d) CCD images can be developed faster.
e) All of the above are true.
Clicker Question
An advantage of
CCDs over
photographic
film is
a) they don’t require chemical development.
b) digital data is easily stored & transmitted.
c) CCDs are more light sensitive than film.
d) CCD images can be developed faster.
e) All of the above are true.
Clicker Question
a) the quality of the telescope’s optics.
Seeing in
astronomy is a
measurement of
b) the transparency of a telescope’s lens.
c) the sharpness of vision of your eyes.
d) the image quality due to air stability.
e) the sky’s clarity & absence of clouds.
Clicker Question
a) the quality of the telescope’s optics.
Seeing in
astronomy is a
measurement of
b) the transparency of a telescope’s lens.
c) the sharpness of vision of your eyes.
d) the image quality due to air stability.
e) the sky’s clarity & absence of clouds.
Smeared overall image of star
“Good Seeing” occurs when the
atmosphere is clear and the air is still.
Turbulent air produces “poor seeing,”
and fuzzier images.
Point images of a star
Radio Telescopes
Large metal dish acts as a mirror for radio
waves. Radio receiver at prime focus.
Surface accuracy not so important, so easy
to make large one.
But angular resolution is poor. Remember:
Effelsberg 100-m (Germany)
Andromeda galaxy –
optical
angular resolution 
wavelength
mirror diameter
D larger than optical case, but wavelength much
larger (cm's to m's), e.g. for wavelength = 1 cm,
diameter = 100 m, resolution = 20".
Andromeda radio
map with
Effelsberg telescope
Radio Astronomy
Longer wavelength means poorer angular
resolution.
Advantages of radio astronomy:
• Can observe 24 hours a day.
• Clouds, rain,
and snow don’t
interfere.
• Observations
at an entirely
different
frequency; get
totally different
information.
Parkes 64-m (Australia)
Green Bank 100-m telescope (WV)
Jodrell Bank 76-m (England)
Arecibo 300-m telescope (Puerto Rico)
Interferometry
A technique to get improved angular resolution using an array of
telescopes. Most common in radio, but also limited optical interferometry.
D
Consider two dishes with separation D vs. one dish of diameter D.
By combining the radio waves from the two dishes, the achieved
angular resolution is the same as the large dish.
Example: wavelength = 1 cm, separation = 2 km, resolution = 1"
Very Large Array (NM). Maximum
separation 30 km
Very Long Baseline Array. Maximum
separation 1000's of km
VLA and optical image of Centaurus A
Clicker Question
Radio
dishes are
large in
order to
a) improve angular resolution.
b) give greater magnification.
c) increase the range of waves they can collect.
d) detect shorter waves than optical telescopes for
superior resolution.
Clicker Question
Radio
dishes are
large in
order to
a) improve angular resolution.
b) give greater magnification.
c) increase the range of waves they can collect.
d) detect shorter waves than optical telescopes for
superior resolution.
Resolution is worse with
long-wave light, so radio
telescopes must be large
to compensate.
Clicker Question
a) observations can be made day & night.
Radio telescopes
are useful because
b) we can see objects that don’t emit visible
light.
c) radio waves are not blocked by interstellar
dust.
d) they can be linked to form interferometers.
e) All of the above are true.
Clicker Question
a) observations can be made day & night.
Radio telescopes
are useful because
b) we can see objects that don’t emit visible
light.
c) radio waves are not blocked by interstellar
dust.
d) they can be linked to form interferometers.
e) All of the above are true.
The Very Large Array
links separate radio
telescopes to create
much better resolution.
Clicker Question:
When multiple radio telescopes are used for
interferometry, resolving power is most
improved by increasing:
A: the distance between telescopes;
B: the number of telescopes in a given area;
C: the diameter of each telescope;
D: the power supplied to each telescope
Astronomy at Other Wavelengths
Telescopes also observe infrared, UV, X-rays and gamma rays.
Mostly done from space because of Earth's atmosphere.
Infrared allows
you to see
radiation from
warm dust in
interstellar gas.
Spitzer Space
Telescope - infrared
Infrared also allows you to see through dust. Dust is good at
blocking visible light but infrared gets through better.
Trifid nebula in visible light
Trifid nebula with Spitzer
GLAST – Gamma Ray Large Area Space Telescope
Gamma rays are the most energetic
photons, tracing high-energy events in
Universe such as “Gamma-ray Bursters”.
Latest mission is GLAST – successfully
launched this year.
X-ray Optics
X rays and gamma rays will not reflect off mirrors as other
wavelengths do; need new techniques.
X rays will reflect at a very shallow angle, and can therefore be
focused.
Gamma Rays cannot be focused at all; images are coarse
Hubble Space Telescope and its successor-to-be: the James Webb
Space Telescope
Advantage of space for optical astronomy:
get above blurring atmosphere – much
sharper images.
Center of M51: HST (left; 0.05” resolution) vs.
ground-based (right; 1” resolution)
The James Web Space Telescope
Mock-up of JWST
Will have diameter 6.5 meters (vs. HST 2.5 meters) – much higher
resolution and sensitivity. Will also observe infrared, whereas
Hubble is best at visible light. Expected launch 2013.
Clicker Question:
The biggest telescopes on Earth are:
A: Gamma-ray telescopes.
B: X-ray telescopes.
C: Optical telescopes
D: Radio telescopes
E: Infra-red telescopes
Much can be learned from
observing the same
astronomical object at many
wavelengths. Here, the Milky
Way.
Summary of Chapter 3
• Refracting telescopes make images with a lens.
• Reflecting telescopes make images with a
mirror.
• Modern research telescopes are all reflectors.
• CCDs are used for data collection.
• Data can be formed into image, analyzed
spectroscopically, or used to measure intensity.
• Large telescopes gather much more light,
allowing study of very faint sources.
• Large telescopes also have better resolution.
Summary of Chapter 3, cont.
• Resolution of ground-based optical telescopes
is limited by atmospheric effects.
• Resolution of radio or space-based telescopes
is limited by diffraction.
• Active and adaptive optics can minimize
atmospheric effects.
• Radio telescopes need large collection area;
diffraction is limited.
• Interferometry can greatly improve resolution.
Summary of Chapter 3, cont.
• Infrared and ultraviolet telescopes are
similar to optical.
• Ultraviolet telescopes must be above
atmosphere.
• X rays can be focused, but very differently
than visible light.
• Gamma rays can be detected but not
imaged.