Chapter 5 - Telescopes With Doctor Bones aka Don R. Mueller, Ph.D.

1. Astonishing Astronomy 101
With Doctor Bones (Don R. Mueller, Ph.D.)
Educator
Entertainer
J
U
G
G
L
E
R
Scientist
Science
Explorer

2. Chapter 5 - Telescopes

3. Refracting Telescopes
• Telescopes that use lenses to focus
light are called refracting telescopes.
• Large refractors are difficult to build.
• Glass is heavy, and glass lenses must
be supported only by their rims.
• Glass sags under its own weight,
defocusing the light.
• Refractors suffer from chromatic
aberration, a blurring effect due to
changes in the focal plane of the lens
for different wavelengths of light.

4. Reflecting Telescopes • Reflecting telescopes use a
curved mirror to focus light.
• Mirrors can be supported from
behind, and so can be much
larger than refractors.
• Larger means that more light
can be collected and focused,
allowing astronomers to image
dimmer or more distant
objects.
• Most modern telescopes are
reflectors.

5. Different styles of reflectors

6. X-Ray reflectors
• X-rays only reflect at glancing angles, otherwise they are
absorbed or pass through the mirror.
• X-ray mirrors are designed to reflect the incoming photons,
focusing them at the end of a long tube-shaped array of mirrors.

7. Very Large Mirrors: A marvel of engineering
www.keckobservatory.org
• Reflectors can be made very
large if multiple mirrors are
used as the primary mirror.
• The Keck Telescope uses 36
large mirrors to create a single
huge primary mirror.
• The positions of the mirrors are
precisely measured by lasers,
and can be individually adjusted
to keep them perfectly aligned.

8. • The ability of a telescope to collect light (more than
that of the human eye) is its light-gathering power.
• The objective is the part of the telescope, which
gathers light. Refracting telescopes use a lens as the
objective. The light is bent as it passes through the
lens. Reflecting telescopes use a mirror as the
objective with the light reflected after striking the
mirror.
• Telescopes must be precise to keep the light waves
coherent for maximum detection efficiency.
• The more light, the greater the chance to see fainter
objects.
Light gathering power

9. Size Matters
• Aperture size is important
when collecting light.
• A large collecting area
allows astronomers to
image dim and distant
objects.
• For a telescope with an
aperture of distance D in
diameter: D = 2r
2
D
4
Area



10. Radio Astronomers “do it” with Radio Telescopes
• Radio telescopes, like
the one in Arecibo,
Puerto Rico collect
radio waves from
astronomical objects
and events.
• The SETI Institute: to
explore, understand
and explain the origin,
nature and prevalence
of life in the universe.
• SETI: Search for Extra-
Terrestrial Intelligence.
http://www.seti.org/

11. Radio Astronomy
National Radio Astronomy Observatory (http://www.nrao.edu/) operates
major radio telescope facilities in West Virginia, New Mexico and Arizona.
• Learn the Basics of Radio Astronomy via the following links:
• http://www2.jpl.nasa.gov/radioastronomy/
• http://101science.com/rastronomy.htm

12. Telescopes: The World’s most popular
http://www.youtube.com/watch?v=fzv35gg4f4M
• Telescopes have been used
for years to collect light and
focus it into an eyepiece.
Astronomers would then look
through this eyepiece at
planets, nebulae, etc.
• The human eye, which is not
very sensitive to dim light was
replaced by the film camera.
• Film, however, is sensitive to
only around 10% of the
impinging light.

13. The Charge-Coupled Device (CCD)
• The CCD, similar to
those found in digital
cameras and phones,
utilizes the
photoelectric effect
(first introduced by
Einstein) to collect
around 75% of the
visible light it
receives.
• The CCD images can
be recorded and
downloaded
anywhere in the
world for analysis.
• The CCD's charge is created when photons
strike the semiconducting material, dislodging
electrons. As more photons strike the device,
more electrons are liberated, thus creating a
charge: proportional to the light's intensity.

14. Other types of telescopes
 Optical telescopes (Visible Light, Ultraviolet and Infrared)
 High-energy particle telescopes (X-ray and Gamma ray)

15. Outside the visible spectrum:
Ground-based and Earth orbit telescopes
• Many objects of astronomical
interest are outside the domain of
visible wavelengths.
• Much can be learned by studying
stars, planets and nebula using
multiple wavelengths.
• Ground-based radio telescopes can
be used to image pulsars and other
interesting bodies.
• Observations in other
wavelengths, requires
instrumentation outside of the
Earth’s atmosphere.
• X-ray, Gamma ray and infrared
wavelength telescopes are
currently in Earth orbit.

16. Ground- and Space-based Observatories

17. Dispersion
The degree light is diffracted depends on its wavelength.
• A prism spreads the light out, using this effect.
• This dispersion of light is a problem in refracting
telescopes. Dispersion is a phenomenon in which the
phase velocity of a wave depends on its frequency.
• This leads to chromatic aberration, a blurring effect.

18. Lenses
• A lens is a specially shaped
piece of glass that bends light
rays passing through it so that
they focus a particular
distance away (the focal
length) at a particular location
(the focal plane).
• A sensor such as a human eye,
a camera or CCD, if placed in
the focal plane can image the
light.

19. Thin Lens Equation
Question:
• An object is placed p = 20.0 cm
in front of a double-convex
lens of focal length f = 10.0 cm.
What is the image distance q in
cm?
1/p + 1/q = 1/f
Answer:
1/q = 1/f - 1/p
= 1/10.0 cm - 1/20.0 cm
= 1/20.0 cm.
Therefore, q = 20.0 cm
double-convex lens

20. Types of Lenses: Figure 1• The bi-convex lens is known as a positive
lens (positive focal length) because it
causes light rays to converge in forming a
real and inverted image.
• A bi-concave lens is called a negative lens
because the light waves that pass through
diverge from the focal point. Bi-concave
lenses operate similarly to concave
mirrors, whereby light waves are
refracted as if emitted from a point
behind the lens. However, because the
light does not actually converge at this
point, it is called a virtual focus with a
virtual image. Virtual images appear with
the same orientation as real objects, but
can only be viewed or projected with the
aid of another lens.

21. More Lens Info: Figure 2
a) Bi-convex
b) Plano-convex
c) Concavo-Convex
d) Bi-concave
e) Plano-Concave
f) Convexo-Concave

22. The Lens Lineup

23. Diffraction and Resolution
• The telescope’s resolution
is superior to that of the
human eye because it has a
larger aperture and the
light is diffracted less as it
passes through.
• An interference pattern
is produced whenever
diffraction occurs.
• To produce an
interference pattern,
the waves involved
must be coherent.
• Diffraction is a rippling effect due to
the finite size of an aperture.
• Light waves approach the aperture as
flat plane waves, similar to the straight
water waves seen above.
• As the waves pass through the
aperture, the waves become curved.

24. From Incoherent to Coherent: A
Simple Illustration

25. Diffraction Effects • Diffracted light waves can
interfere with each other.
• This results in a diffraction
pattern, a blurring of the image .
• Larger apertures have less
diffraction and higher resolution
than smaller apertures.
• Observing light of wavelength
nm, the smallest separation
angle arcsec a telescope can
resolve is related to the
telescope aperture Dcm by:
cm
nm
D
02.0
arcsec

 

26. Interferometers
• To counter diffraction
effects astronomers use
interferometers.
• Signals from these
arrays of widely-
separated telescopes
are added together to
create images with very
high resolution.
• In fact, the resolution is
equivalent to that of a
single telescope with an
aperture as large as the
separation in the array.

27. Before and After
• Before:
– What looks like a single star…
• After
– …is actually two stars!

28. Atmospheric Effects: Windows for Astronomy
• The Earth’s atmosphere absorbs
much of the radiation from space.
• High energy photons would sterilize
the planet.
• For example, X-ray photons have
more energy than UV, visible
and infrared photons.
• Visible, radio and some infrared
wavelengths are not absorbed
readily by the atmosphere, thus
– Optical and radio telescopes
work well from the ground
• Gamma Rays, X-rays and UV
photons are absorbed, therefore
– Observatories for these
wavelengths must be kept above
the Earth’s atmosphere.

29. • The Earth's “dry air”
atmosphere consists mostly
of nitrogen (78.1%) and
oxygen (20.9%) with small
amounts of argon (0.9%) and
carbon dioxide (0.035%) and
very small amounts of other
gases.
Dry air, meaning absent of
water vapor:
( N2 > O2 ) >> (Ar > CO2 )
p = 1 atm at sea level
Gravity "pulls" the atmosphere
towards the Earth's surface.
Earth’s Atmosphere: Dry air ( N2 > O2 ) >> (Ar > CO2 )

30. The Greenhouse Effect: IR gases
Order of Abundance:
Greenhouse gases in the
Earth's atmosphere:
Water vapor (H2O)
Carbon dioxide (CO2)
Methane (CH4)
Nitrous oxide (N2O)
Ozone (O3)
CFC’s

31. Infrared Absorption and Re-radiation

32. Light Pollution
• Ambient light from cities
is a real problem for
optical astronomy as it
“washes out” images.
• Research telescopes are
built far from cities to
reduce the effects of
light pollution.
• It is getting harder to
find good locations for
telescopes.

33. Atmospheric Effects
• Air refracts light just like
glass or water, but to a
lesser degree.
Cool air refracts light more
than warm air.
Pockets of cool air in the
atmosphere create moving
lenses in the sky, shifting
the light rays randomly.
This causes a twinkling effect,
called scintillation.
• A stable atmosphere
causes less scintillation.

34. Observatories in Space

35. Images from the Hubble Space Telescope