2. Light: The Cosmic Messenger

3. Radiation: Information from Space
Radiation: Information from Space
In astronomy, unlike many other sciences, we cannot perform “experiments” with stars, galaxies, etc.!
The only way to investigate is by analyzing radiation they emit.

4. Astronomical Telescopes
Primary objective: gather large amounts of light
=> large telescopes!
The Gemini-North optical telescope on Mauna Kea in Hawai’i stands over 19 m (60 ft) high, and the primary mirror at the bottom is 8.1 m (26.5 ft) in diameter.

5. Astronomical Telescopes
Other forms of radiation (other than visible light) can also be observed, but very different telescope designs are needed.
Build DIFFERENT telescopes to detect different kinds of light from the “Electromagnetic Spectrum”

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

7. Our first key idea is that visible light is only a small part of the complete spectrum of light. You may wish to spend some time explaining the various things shown in this figure… You may also want to repeat this slide at various points to summarize other ideas.

8. OPTICAL Telescopes

9. OPTICAL Telescopes
James Webb Space Telescope (JWST)

11. James Webb Space Telescope

12. OPTICAL Telescopes
European EXTREMELY LARGE Telescope (E-ELT)

13. Refracting / Reflecting Telescopes
Refracting / Reflecting Telescopes

14. Refracting / Reflecting Telescopes
Refracting / Reflecting Telescopes
Refracting telescope: Lens focuses light onto the focal plane
Focal length

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

16. Refracting Telescopes bend light through lenses
FIGURE 3-19 Essentials of a Refracting Telescope
A refracting telescope consists of
a large, long-focal-length objective lens that
collects and focuses light rays and a small, short-focal length
eyepiece lens that re-straightens the light rays.
The lenses work together to brighten, resolve, and magnify
the image formed at the focal plane of the objective
lens.
Heavy glass lenses, bending different colors to different points (“Chromatic aberration”) & imperfections in glass, limit practical size

17. FIGURE 3-20 The Largest Refracting Telescope
This giant refracting telescope, built in the late 1800s, is housed at
Yerkes Observatory near Chicago. The objective lens is 102
cm (40 in.) in diameter, and the telescope tube is 19 1⁄3 m (63 1⁄2
ft) long. (Yerkes Observatory)

18. Newton’s Reflecting Telescope
FIGURE 3-8 Replica of Newton’s Reflecting Telescope
Built in 1672, this reflecting telescope has a spherical primary
mirror 3 cm (1.3 in.) in diameter. Its magnification was
40. (Royal Greenwich Observatory/Science Photo Library.)
Newton’s Reflecting Telescope

19. Different types of Reflecting Telescopes
FIGURE 3-9 Reflection
(b) A concave, parabolic
mirror causes parallel light rays to converge and meet at the
focal point. The distance between the mirror and focal point
is the focal length.
Different types of Reflecting Telescopes

20. Buying a Telescope
There are four basic types of telescope mounts that
steer the telescopes around the sky: The Fork Equatorial
Mount, the German Equatorial Mount, the Altitude-
Azimuth (Alt-Azimuth) Mount, and the Dobsonian
Mount (see figures). The

21. Functions of Telescopes!
Gather Light
Resolve Sharp Details
Magnify Resulting Images
Regardless of Wavelength range & size

22. Small Telescope image of
Andromeda Galaxy
FIGURE 3-13 Resolution
The larger the diameter of a telescope’s
primary mirror, the greater the detail the telescope
can resolve. These two images of the Andromeda Galaxy,
taken through telescopes with different diameters, show this
effect. Increasing the exposure time of the smaller diameter
telescope (a), will only brighten the image, not improve the
resolution. (AURA)

24. FIGURE 3-14 Magnification
The same telescope can magnify
by different amounts, depending on the focal length of the
eyepiece. (a) An image of the Moon taken with magnification
4 times greater than image (b). Note in this case that the
increased magnification leads to increased resolution (i.e.,
more detail can be seen in the larger image). (a & b: NASA)

25. #1 Function: Gathering Light
FIGURE 3-16 Photography Versus CCD Images
These three views of the same part of the sky, each taken with the same 4-
m telescope, compare CCDs to photographic plates.
(a) A negative print (black stars and white sky) of a photographic
image. (Patrick Seitzer, NOAO.)
(b) A negative CCD image. Notice that many faint stars and galaxies that are invisible in
the ordinary photograph can be seen clearly in this CCD
image.
(c) This (positive) color view was produced by combining
a series of CCD images taken through colored filters.
(Patrick Seitzer, NOAO.)
The Human Eye!

26. #1 Function: Gathering Light
Photographic Films
FIGURE 3-16 Photography Versus CCD Images
These three views of the same part of the sky, each taken with the same 4-
m telescope, compare CCDs to photographic plates.
(a) A negative print (black stars and white sky) of a photographic
image. (Patrick Seitzer, NOAO.)
(b) A negative CCD image. Notice that many faint stars and galaxies that are invisible in
the ordinary photograph can be seen clearly in this CCD
image.
(c) This (positive) color view was produced by combining
a series of CCD images taken through colored filters.
(Patrick Seitzer, NOAO.)

27. #1 Function: Gathering Light
Electronic Light Detection & Amplification
CCD = “Charge-coupled device”
FIGURE 3-16 Photography Versus CCD Images
These three views of the same part of the sky, each taken with the same 4-
m telescope, compare CCDs to photographic plates.
(a) A negative print (black stars and white sky) of a photographic
image. (Patrick Seitzer, NOAO.)
(b) A negative CCD image. Notice that many faint stars and galaxies that are invisible in
the ordinary photograph can be seen clearly in this CCD
image.
(c) This (positive) color view was produced by combining
a series of CCD images taken through colored filters.
(Patrick Seitzer, NOAO.)

28. Photographs vs. CCD chips vs.
Multi-color filtered CCD composite images
FIGURE 3-16 Photography Versus CCD Images
These three views of the same part of the sky, each taken with the same 4-
m telescope, compare CCDs to photographic plates.
(a) A negative print (black stars and white sky) of a photographic
image. (Patrick Seitzer, NOAO.)
(b) A negative CCD image. Notice that many faint stars and galaxies that are invisible in
the ordinary photograph can be seen clearly in this CCD
image.
(c) This (positive) color view was produced by combining
a series of CCD images taken through colored filters.
(Patrick Seitzer, NOAO.)

29. Orion in UV, Infrared, & Optical Wavelengths
FIGURE 3-29 Orion as Seen in Ultraviolet, Infrared, and Visible Wavelengths
(a) An ultraviolet view of the constellation
Orion obtained during a brief rocket flight on December 5,
1975. The 100-s exposure captured wavelengths ranging from
125 to 200 nm. (G. R. Carruthers, NRL.) (b) A false-color view
from the Infrared Astronomical Satellite uses color to display the
intensity of infrared wavelengths. (NASA/JPL-Caltech) (c) An ordinary
optical photograph. See also Margin Chart 3-2. (R. C.
Mitchell, Central Washington University.)
Orion in UV, Infrared, & Optical Wavelengths

30. #1 Function: Gathering Light
Depends upon the size of the objective mirror or lens.
Light gathering area increases by SQUARE of diameter
10 m telescope gather 4x more light than 5m
Subject to interference from other sources!

31. Los Angeles in 2001 FIGURE 3-25 Light Pollution
These two images of Tucson,
Arizona, were taken from the Kitt Peak National
Observatory which is 38 linear miles away. They show the
dramatic growth in ground light output between 1959 (top)
and 1989 (bottom). Since 1972, light pollution, a problem
for many observatories around the world, has been at least
partially controlled by a series of local ordinances.
(NOAO/AURA/NSF/Galaxy.)

32. FIGURE 3-25 Light Pollution
These two images of Tucson,
Arizona, were taken from the Kitt Peak National
Observatory which is 38 linear miles away. They show the
dramatic growth in ground light output between 1959 (top)
and 1989 (bottom). Since 1972, light pollution, a problem
for many observatories around the world, has been at least
partially controlled by a series of local ordinances.
(NOAO/AURA/NSF/Galaxy.)

33. #2 Function: Resolution
Depends upon the size of the objective mirror or lens.
Better resolution with more light
Depends upon wavelength of light, too!
Smaller wavelengths provide smaller details
UV images have more detail than Radio
Also subject to interference

34. Radio Telescopes gather long-wave, low-energy light
FIGURE 3-30 A Radio Telescope
Recall that the secondary mirror or prime focus on most telescopes blocks incoming light
or other radiation. This new radio telescope at the National
Radio Astronomy Observatory in Green Bank, West Virginia,
has its prime focus hardware located off-center from the telescope’s
100-m by 110-m oval reflector. By using this new
design, there is no such loss of signal. Such configurations are
also common on microwave dishes used to receive satellite
transmissions for home televisions. (NRAO/AUI/NSF.)
Radio Telescopes gather long-wave, low-energy light
Poor resolution unless made LARGE!

35. “Seeing” is the ability to resolve small details
Affected by:
Imperfections in optics (shapes of lenses/mirrors)
Atmospheric motion, density, temperature, moisture
Improved by:
Adaptive optics “subtracting out” atmospheric effects
Getting above atmosphere!
FIGURE 3-24 Effects of Twinkling
The same star field photographed
with (a) a ground-based telescope, which is subject
to poor seeing conditions that result in stars twinkling, and
(b) the Hubble Space Telescope, which is free from the effects
of twinkling. (NASA/ESA.)

36. FIGURE 3-26 The Hubble Space Telescope (HST)
This photograph of HST hovering above the Space Shuttle’s cargo bay
was taken in 1993, at completion of the first servicing mission.
HST has studied the heavens at infrared, visible light,
and ultraviolet wavelengths. (NASA.)
Improve seeing by getting above the atmosphere (and gather more types of light, too!)

37. 1 2 3 Ground-based image of Neptune
Ground-based image with adaptive optics
Hubble Space Telescope image
FIGURE 3-27 Images from Earth and Space
(a) Image of Neptune from an Earth-based telescope without adaptive optics.
(Courtesy of Center for Adaptive Optics, University of California)
(b) Image of Neptune from the same Earth-based telescope with
adaptive optics. (Courtesy of Center for Adaptive Optics, University of
California) (c) Image of Neptune from the Hubble Space
Telescope, which does not incorporate adaptive optics technology.
(NASA, L. Sromovksy, and P. Fry [University of Wisconsin-Madison])

38. #3 Function: Magnification
Least important
Without a bright, sharp image, no use!
Bigger, Dimmer, Fuzzier!
Depends upon EYEPIECE used
Small scopes: $ each
Easily swapped to magnify images
Depends upon telescope geometry, too

39. Active & Adaptive Optics!
Active optics (1980’s)
Put actuators on segmented mirrors to “bend” to right shape
Adaptive optics (1990’s to present)
“Deform” mirror in real time to compensate for atmospheric motion measured by Laser Guide Stars

40. VLT in Chile (4) combined 8.2 m telescopes
Tracking motions of stars at Milky Way Center

41. SALT in Africa
Largest current “single” surface scope

42. China’s latest achievement!
Largest radio telescope in the world!

43. Seeing in Stereo! FIGURE 3-28 The 10-m Keck Telescopes
Located on the dormant (and, hopefully, extinct)
Mauna Kea volcano in Hawaii, these huge twin
telescopes each consist of 36 hexagonal mirrors measuring
1.8 m (5.9 ft) across. Each Keck telescope has the light-gathering,
resolving, and magnifying ability of a single mirror 10
m in diameter. Inset: View down the Keck I telescope. The
hexagonal apparatus near the top of the photograph shows
the housing for the 1.4-m secondary mirror. (W. M. Keck
Observatory, Courtesy of Richard J. Wainscoat.)

44. FIGURE 3-31 The Very Large Array (VLA)
The 27 radio telescopes of the VLA system are
arranged along the arms of a Y in central New
Mexico. Besides being able to change the angles
at which they observe the sky, these telescopes can be moved
by train cars so that the array can detect either wide areas of
the sky (when they are close together, as in this photograph)
or small areas with higher resolution (when they are farther
apart). The inset shows the traditional secondary mirror
assembly in the center of each of these antennas. (Jim
Sugar/Corbis; inset: David Nunuk/Science Photo Library/Photo
Researchers.)
Interferometry – Combining signals simultaneously from 2 or more scopes

45. Visible & Radio wave views of Saturn
FIGURE 3-32 Visible and Radio Views of Saturn
(a) This picture was taken by a camera on board a spacecraft as it
approached Saturn. The view was produced by sunlight scattered
from the planet’s cloud tops and rings. (NASA) (b) This
false-color picture, taken by the VLA, shows radio emission from
Saturn at a wavelength of 2 cm. (Image courtesy of NRAO/AUI.)

46. FIGURE 3-37 Survey of the Universe in Various Parts of the Electromagnetic Spectrum
By mapping the celestial sphere
onto a flat surface (like making a map of Earth), astronomers
can see the overall distribution of strong or nearby energy
sources in space. The center of our galaxy’s disk cuts these
images horizontally in half. Because most of the emissions
shown in these diagrams fall in this region, we know that most
of the strong sources of various electromagnetic radiation as
seen from Earth (except X rays) are in our galaxy: (a) visible
light, (b) radio waves, (c) infrared radiation, (d) X rays, and
(e) gamma rays. (GFSC/NASA.)

47. Why build telescopes at all?
We already have enough!
Why do we need a more detailed picture of Mars?
Who cares?
This cost $100 Million dollars? You’ve got to be kidding me…