The document describes astronomical telescopes, emphasizing their primary function of gathering light and observing various forms of radiation through designs like refracting and reflecting telescopes. It details the characteristics of light, including wavelength and frequency, and discusses modern advancements in telescope technology, such as adaptive optics and radio astronomy. It also highlights the significance of telescope location for optimal viewing conditions and the role of space observatories in extending our observation capabilities beyond atmospheric interference.

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