A great Power Point from Astronomy this semester, thought I would share it. This explains the properties of light and the use of Telescopes; their related properties and attributes.

1. 0
Chapter 5
Light and Telescopes

2. Astronomical Telescopes
0
• Primary objective: to
gather large amounts
of light
=> large telescopes!
• Other forms of
radiation (other than
visible light) can also
be observed, but
• The Gemini-North optical very different
telescope on Mauna Kea in Hawai’i telescope designs
stands over 19 m (60 ft) high, and are needed.
the primary mirror at the bottom is
8.1 m (26.5 ft) in diameter.

3. Radiation: Information
0
from Space
The Electromagnetic Spectrum
• In astronomy, we cannot perform experiments
with our objects (stars, galaxies, …).
• The only way to investigate them is by
analyzing the light (and other radiation) they
emit.

4. Light as a Wave (I) 0
• Light waves are characterized by a
wavelength λ and a frequency f.
• f and λ are related through
f = c/λ
c = 300,000 km/s
= 3 ×108 m/s

5. Wavelengths and Colors 0
Different colors of visible light correspond
to different wavelengths.

6. 0
Light as a Wave (II)
Wavelengths of light are measured in units of
nanometers (nm) or Ångström (Å):
1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm
Visible light has wavelengths
between 4000 Å and 7000 Å
(400–700 nm).

7. Light as Particles 0
Light can also appear as particles, called
photons (explains, e.g., photoelectric effect)
• A photon has a specific energy E,
proportional to the frequency f:
E = h×f
h = 6.626 x 10-34 J·s
is the Planck constant
The energy of a photon does not
depend on the intensity of the light!!!

8. The Electromagnetic Spectrum 0
Wavelength
Frequency
High
Need satellites flying air
to observe planes or
satellites

9. 0
Refracting / Reflecting Telescopes
• Refracting
telescope:
Lens focuses
light onto the
focal plane
Focal length
• Reflecting
telescope:
Concave
mirror focuses
light onto the
Focal length
focal plane
• Almost all modern telescopes are reflecting
telescopes.

10. 0
The Focal Length
Focal length = distance from the center of the lens to
the plane onto which parallel light is focused.

11. Secondary Optics 0
In reflecting
telescopes:
Secondary
mirror is
needed to re-
direct light path
towards back or
side of
incoming light
path
Eyepiece: to
view and
enlarge the
small image
produced in
the focal
plane of the
primary
optics

12. Disadvantages of 0
refracting telescopes
Chromatic aberration:
• Different wavelengths are
focused at different focal
lengths (prism effect)
• Can be corrected, but not
eliminated, by second
lens out of different
material
• Second lens is difficult and
expensive to produce; all
surfaces must be perfectly
shaped; glass must be
flawless; lens can only be
supported at the edges

13. The Powers of a Telescope:
0
Size Does Matter!
1. Light-gathering
power: Depends
on the surface
area A of the
primary lens /
mirror, D
proportional to
diameter
squared:
A = π (D/2)2

14. The Powers of a Telescope (II) 0
2. Resolving power: Wave
nature of light => the
telescope aperture
produces fringe rings that
set a limit to the resolution
of the telescope
Resolving power = minimum
angular distance αmin between two
objects that can be separated
αmin = 1.22 (λ/D) αmin
For optical wavelengths, this gives
αmin = 11.6 arcsec / D[cm]

15. Seeing 0
Weather
conditions and
turbulence in
the
atmosphere
set further
limits to the
quality of
astronomical
images.
Bad seeing Good seeing

16. 0
The Powers of a Telescope (III)
3. Magnifying Power: The ability of the
telescope to make the image appear bigger.
• The magnification depends on the ratio of focal
lengths of the primary mirror/lens (Fo) and the
eyepiece (Fe):
M = Fo/Fe
• A larger magnification does not improve the
resolving power of the telescope!

17. The Best Location for a Telescope 0
Far away from civilization–to avoid light pollution.

18. The Best Location for a Telescope (II) 0
On high mountain-tops–to avoid atmospheric
turbulence (→ seeing) and other weather effects.

19. Infrared Telescopes
0
• Most infrared radiation is absorbed in
the lower atmosphere.
• However, from high mountain tops or
high-flying air planes, some infrared
radiation can still be observed.

20. Infrared Telescopes
0
• Infrared
observations can
also be performed
form high-flying
aircraft.
• Infrared
detectors must
be cooled to
very low
temperatures.

21. 0
Traditional Telescopes (I)
Secondary mirror
Traditional primary
mirror: sturdy, heavy
to avoid distortions

22. 0
Traditional
Telescopes
The 4-m
Mayall
Telescope
at Kitt Peak
National
Observatory
(Arizona)

23. Advances in Modern 0
Telescope Design (I)
Modern computer technology has made possible
significant advances in telescope design:
1. Simpler, stronger mountings (“alt-azimuth
mountings”) to be controlled by computers

24. Advances in Modern 0
Telescope Design (II)
2. Lighter mirrors with lighter support structures,
to be controlled dynamically by computers
Floppy mirror
Segmented mirror

25. The Keck Telescopes 0
Pictured are the two Keck Telescopes on Mauna
Kea, Hawaii.
Each telescope has a mirror diameter of 10 meters.

26. Examples of Modern 0
Telescope Design

27. The Future of 0
Optical Telescopes
The Giant Magellan Telescope (2016)
The European
Extremely Large
Telescope (E-
ELT): 906
segments in a
42-m mirror!

28. Adaptive Optics 0
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence.
Distortions by the atmospheric
turbulence are measured using
a laser beam.

29. Interferometry 0
Recall: The resolving power of a telescope
depends on diameter D:
αmin = 1.22 λ/D
This holds true even
if not the entire
surface is filled out.
→ Combine the
signals from several
smaller telescopes
to simulate one big
mirror →
Interferometry

30. CCD Imaging 0
CCD = Charge-coupled device
• More sensitive than
photographic plates
• Data can be read
directly into
computer memory,
allowing easy
electronic
manipulations
False-color image to visualize
brightness contours

31. Negative Images
0
• The galaxy NGC 891
as it would look to our
eyes (i.e., in real
colors and brightness)
• Negative images (sky
= white; stars =
black) are used to
enhance contrasts

32. The Spectrograph 0
• Using a prism (or a grating), light can
be split up into different wavelengths
(colors!) to produce a spectrum.
• Spectral lines in a
spectrum tell us about
the chemical
composition and other
properties of the
observed object.

33. Radio Astronomy 0
Recall: Radio waves of λ ~1 cm to 1 m also penetrate the
Earth’s atmosphere and can be observed from the ground.

34. Radio Telescopes 0
• Large dish
focuses the
energy of radio
waves onto a
small receiver
(antenna).
• Amplified signals are
stored in computers and
converted into images,
spectra, etc.

35. Radio Maps 0
• In radio maps, the intensity of the
radiation is color-coded:
Red = high intensity;
Violet = low intensity
• Just like optical telescopes, radio
telescopes should be built in regions
with low average rainfall and cloud
cover.

36. Radio Interferometry 0
• Just as for optical
telescopes, the
resolving power
of a radio
telescope is αmin
= 1.22 λ/D.
• For radio
telescopes, this is
a big problem:
radio waves are
much longer than
visible light.
→ Use • The Very Large Array (VLA): 27
interferometry to dishes are combined to simulate a
improve resolution! large dish of 36 km in diameter.

37. Science of Radio Astronomy
0
Radio astronomy reveals several features,
not visible at other wavelengths:
• Neutral hydrogen clouds (which don’t
emit any visible light), containing ~90%
of all the atoms in the Universe
• Molecules (often located in dense
clouds, where visible light is
completely absorbed)
• Radio waves penetrate gas and
dust clouds, so we can observe
regions from which visible light is
heavily absorbed

38. Observatories in Space (I) 0
The Hubble Space • Launched in 1990;
Telescope maintained and
upgraded by several
space shuttle
service missions
throughout the
1990s and early
2000’s
• Avoids turbulence
in the Earth’s
atmosphere
• Extends imaging and spectroscopy to
(invisible) infrared and ultraviolet

39. The Future of Space-Based 0
Optical/Infrared Astronomy:
The James Webb Space Telescope

40. High-EnergyAstronomy: 0
X-rays and gamma-rays from space can also
be observed from astronomy satellites:
Combined visual + X-ray image (taken by
Chandra) of a supernova remnant