Optics and Telescopes

1
Ohio University - Lancaster Campus
slide 1 of 71Spring 2009 PSC 100

• Optics and Telescopes

Credit www.sherwoods-photo.com
Credit www.telescopeguides.net
2
Ohio University - Lancaster Campus
slide 2 of 71Spring 2009 PSC 100

• This evening we will investigate
• how lenses and mirrors can be used to focus light
  and form an image.
• the 3 basic telescope designs and the advantages
  and disadvantages of each.
• some numbers that characterize a telescope
  f-ratio, light gathering power, resolution,
  magnification

3
Ohio University - Lancaster Campus
slide 3 of 71Spring 2009 PSC 100

• This evening we will investigate
• recording the images produced by a telescope.
• telescopes that use the other wavelengths of
  light.

4
Ohio University - Lancaster Campus
slide 4 of 71Spring 2009 PSC 100

• Optics The science of reflecting and/or
  refracting (bending) light so as to produce an
  image of an object. The image is usually
  recorded so that it can be studied more
  extensively.

5
Ohio University - Lancaster Campus
slide 5 of 71Spring 2009 PSC 100

• Regarding Mirrors
• The Law of Reflection When a ray of light
  strikes a shiny or specular surface, the ray
  reflects away at the same angle at which it
  struck the surface. The angle of incidence
  equals the angle of reflection, as measured from
  a normal to the surface.

6
Ohio University - Lancaster Campus
slide 6 of 71Spring 2009 PSC 100
?i ?r
a shiny or reflective surface
7
Ohio University - Lancaster Campus
slide 7 of 71Spring 2009 PSC 100

• If the reflecting
• surface is curved
• correctly, the light
• can be focused to
• a point, called the
• focal point. An
• image forms near
• the focal point.

Credit www.antonine-education.co.uk
8
Ohio University - Lancaster Campus
slide 8 of 71Spring 2009 PSC 100

• Regarding Lenses
• The Law of Refraction When light moves from a
  less dense medium (empty space or air) to a
  denser medium (glass), the light slows down and
  bends INTO the denser medium.

9
Ohio University - Lancaster Campus
slide 9 of 71Spring 2009 PSC 100
speed of light in air 3 x 108 m/s
speed of light in glass 2 x 108 m/s
10
Ohio University - Lancaster Campus
slide 10 of 71Spring 2009 PSC 100

• Glass can be formed into a convex lens which will
  also focus light. An image forms near the focal
  point. The focal length is the distance from the
  centerline of the lens to the focal point.

focal length
11
Ohio University - Lancaster Campus
slide 11 of 71Spring 2009 PSC 100

• The f-ratio is a way to compare or rate convex
  (converging) lenses.
• The f-ratio is the focal length of the lens
  divided by the lens diameter.

12
Ohio University - Lancaster Campus
slide 12 of 71Spring 2009 PSC 100
Thicker lenses tend to focus closer to the lens
and give brighter images. These are fast
lenses. Do these lenses have low or high
f-ratios? But these lenses have other problems.
13
Ohio University - Lancaster Campus
slide 13 of 71Spring 2009 PSC 100
Thinner lenses focus farther from the lens, give
less-bright images, and are described as slow
lenses.
14
Ohio University - Lancaster Campus
slide 14 of 71Spring 2009 PSC 100

• When taking photographs of space objects, using a
  fast lens with a low
• f-ratio means less time is needed for the
  photograph. This results in less blurring due to
  vibration of the telescope and the motion of the
  stars.

Credit Gemini Observatory/AURA
15
Ohio University - Lancaster Campus
slide 15 of 71Spring 2009 PSC 100

• Chromatic Aberration a problem with lenses
• The edges of lenses act like prisms. They split
  white light into all the colors of the rainbow.
• Problem the different colors focus at different
  focal points. This means that if you focus the
  blue color of an object, the red is fuzzy, and
  vice versa.

16
Ohio University - Lancaster Campus
slide 16 of 71Spring 2009 PSC 100
Chromatic Aberration
17
Ohio University - Lancaster Campus
slide 17 of 71Spring 2009 PSC 100

• Theres always a trade-off in optics. The
  problem of chromatic aberration is worst with
  fast or low f-ratio lenses. These are the
  lenses wed like to use most!
• The problem is fixed by making compound lenses
  out of 2 or more different kinds of glass.
• Mirror-based telescopes dont have this problem
  a definite advantage!

18
Ohio University - Lancaster Campus
slide 18 of 71Spring 2009 PSC 100

• 3 Types of Telescopes
• Refractors (gathers light with a lens)
• Reflectors (gathers light with a mirror)
• Mixed (uses a combination of lenses and mirrors)
• Schmidt-Cassegrain Telescopes
• Maksutov-Cassegrain Telescopes

19
Ohio University - Lancaster Campus
slide 19 of 71Spring 2009 PSC 100

• Refracting Telescopes
• The original type, invented in the 1500s and
  first used by Galileo to explore space.
• Sharpest, brightest images.
• Lenses are heavy and expensive!
• Prone to chromatic aberration.
• Give an inverted (upside-down) image.
• Can only be made up to about 40 inches in
  diameter.

20
Ohio University - Lancaster Campus
slide 20 of 71Spring 2009 PSC 100
Credit library.thinkquest.org
21
Ohio University - Lancaster Campus
slide 21 of 71Spring 2009 PSC 100

• Reflecting TelescopesAdvantages
• Mirrors are much cheaper to make than lenses, and
  are very light-weight, easy to carry.
• Mirrors can be VERY large. Multiple mirrors can
  be combined to simulate a single gigantic mirror.
• No chromatic aberration.

22
Ohio University - Lancaster Campus
slide 22 of 71Spring 2009 PSC 100

• Reflecting TelescopesDisadvantages
• Not quite as sharp or bright an image as the same
  size refractor.
• Large scopes get currents of different
  temperature air inside their tubes. This can
  make images blurry.
• Mirrors will oxidize (corrode) over time.

23
Ohio University - Lancaster Campus
slide 23 of 71Spring 2009 PSC 100
24
Ohio University - Lancaster Campus
slide 24 of 71Spring 2009 PSC 100

• Combination scopesthe Cassegrains
• Very short tube length, because the light gets
  folded back on itself twice. This makes the
  scope easy to handle transport.
• Moderately expensive.
• Best choice for amateur astrophotography, because
  the tube doesnt vibrate or shake very much.

25
Ohio University - Lancaster Campus
slide 25 of 71Spring 2009 PSC 100
The corrector plate is a type of lens. A
secondary mirror is glued to its inner surface.
26
Ohio University - Lancaster Campus
slide 26 of 71Spring 2009 PSC 100

• The telescope mount is as important as the
  optics! There are two types
• Altitude-Azimuth. Like aiming a tank. Point it
  in the compass direction (azimuth) you want, then
  point it up to the angle (altitude) you want.
• Easy to use, but image rotates over time.

27
Ohio University - Lancaster Campus
slide 27 of 71Spring 2009 PSC 100

• Equatorial. Part of the mount is aimed at the
  north celestial pole. The mount then swivels
  east-west to follow an object through the sky.
• Disadvantage a real bear to use!
• Advantage the picture in the telescope doesnt
  appear to rotate over time.

28
Ohio University - Lancaster Campus
slide 28 of 71Spring 2009 PSC 100

• What is the function of a telescope? Its not
  just to make the image bigger!
• Gathering light
• Resolving details
• Magnifying the image

29
Ohio University - Lancaster Campus
slide 29 of 71Spring 2009 PSC 100

• A Telescope is a Light Funnel
• Gathering light from dim objects is the MOST
  important function of a telescope.
• Which would you rather see, a large but very dim
  image or a smaller, but very bright image?

30
Ohio University - Lancaster Campus
slide 30 of 71Spring 2009 PSC 100

• Light-gathering power (LGP)
• How much light can the human eye gather? A
  typical human eye has a pupil that is about 0.5
  cm in diameter when fully dilated at night.
• Area of the pupil ? r2 ? (0.25 cm)2
  about 0.2 cm2.
• The main purpose of the telescope is to take
  light from a much larger area and funnel it
  into your pupil.

31
Ohio University - Lancaster Campus
slide 31 of 71Spring 2009 PSC 100

• How much light can a telescope gather?
• A 10 inch diameter scope (25 cm diameter) gathers
  ?(12.5cm)2 490 cm2.
• This is 490 cm2 / 0.2cm2 almost 2500 times more
  light than the naked eye.

32
Ohio University - Lancaster Campus
slide 32 of 71Spring 2009 PSC 100

• To compare a telescopes LGP to that of a
  typical eye, use the formula
• LGP 4D2
• where D is the telescopes lens/mirror diameter
  in centimeters. (2.54 cm/inch)
• What is the LGP of a 6 inch telescope?

33
Ohio University - Lancaster Campus
slide 33 of 71Spring 2009 PSC 100

• Seeing Small Details Resolution
• Resolution is defined as the minimum angle
  between 2 objects, that will allow you to see
  them as 2 separate objects and not one big blob.
• Units are arcseconds (1/3600th of a degree)
• The smaller the theoretical resolution number is,
  the smaller the details you can see.

34
Ohio University - Lancaster Campus
slide 34 of 71Spring 2009 PSC 100

• Theoretical Resolution (?)
• (2.1x105)(wavelength in m)
• (diameter of objective mirror or lens)
• The diameter is in meters, not inches!
• What is the resolution of a 10 inch scope for
  blue light (450 nm or 4.5 x 10-7 meters)?
• Calculate the resolution again for red light (7.0
  x 10-7 meters)

35
Ohio University - Lancaster Campus
slide 35 of 71Spring 2009 PSC 100

• Resolution not the same for all light!
• What color of visible light would have the
  poorest resolution? The best?
• What color of all the types of light would have
  the poorest resolution? How is this limitation
  overcome?

36
Ohio University - Lancaster Campus
slide 36 of 71Spring 2009 PSC 100

• Theres a practical limit to resolution for a
  ground-based telescopethe Atmosphere!
• Air currents in the atmosphere will make the
  image blurry. Think twinkling stars!
• The best time for viewing is in the hours before
  dawn, since the air currents are least.
• Are there any other accommodations that could be
  made?

37
Ohio University - Lancaster Campus
slide 37 of 71Spring 2009 PSC 100

• Magnification the least important function of a
  telescope
• M focal length of the objective lens or mirror
• focal length of eyepiece lens
• What is the magnification factor (power) of a
  telescope with a 1000 mm focal length, using an
  eyepiece with a 2.5 cm focal length?

38
Ohio University - Lancaster Campus
slide 38 of 71Spring 2009 PSC 100

• My 10 inch (25 cm) Schmidt-Cassegrain telescope
  has a 250 cm focal length. If I use an eyepiece
  with a 1.25 cm focal length, what is the
  magnification?
• If I want to increase the magnification, should I
  use a 2.5 cm focal length eyepiece, or a 0.75 cm
  focal length eyepiece?

39
Ohio University - Lancaster Campus
slide 39 of 71Spring 2009 PSC 100

• A bit of review
• If you doubled the size of a telescopes
  objective mirror without making any other
  changes, how would the telescopes properties
  change?

40
Ohio University - Lancaster Campus
slide 40 of 71Spring 2009 PSC 100

• Why do astronomers no longer use film in their
  cameras?
• Film has been replaced by CCD chips
  (Charge-Coupled Device).

41
Ohio University - Lancaster Campus
slide 41 of 71Spring 2009 PSC 100
Credit rst.gsfc.nasa.gov/Intro/ccd.jpg
42
Ohio University - Lancaster Campus
slide 42 of 71Spring 2009 PSC 100
The surface of a CCD chip is divided up into rows
of rectangular light-sensitive pixels (picture
elements). Films have irregularly shaped and
distributed grains of light-sensitive chemicals.
The pixels are usually much more sensitive than
the chemical grains.Advantage???
43
Ohio University - Lancaster Campus
slide 43 of 71Spring 2009 PSC 100
Film Emulsions
Credit www.imx.nl/photosite/technical/Filmbasics/
grainshapes.jpg
44
Ohio University - Lancaster Campus
slide 44 of 71Spring 2009 PSC 100
Individual pixel
Light-sensitive layer (gives off electrons when
struck by light) Semi-conductor layer (acts as
an electron filter) Collector layer (holds the
electrons until counted)
This stack of 3 layers is one pixel.
45
Ohio University - Lancaster Campus
slide 45 of 71Spring 2009 PSC 100
Why use CCDs instead of film?

• CCD Detector
• 70 efficient
• Shorter exposures
• Resolution can be higher (8 Mpixels or higher)

• Film
• 5 to 10 efficient
• 7 to 14 times longer exposures
• Resolution is limited by grain size

46
Ohio University - Lancaster Campus
slide 46 of 71Spring 2009 PSC 100

• Pictures are available in seconds.
• Pictures can be digitally added together.
• Initial cost is similar to film but operating
  costs are much lower.

• Pictures must be developed (hours to days)
• Digital techniques are possible, but more
  difficult.
• Operating costs higher.

47
Ohio University - Lancaster Campus
slide 47 of 71Spring 2009 PSC 100
A typical, high-res image produced by a CCD.
Credit solarsystem.nasa.gov/multimedia/gallery/PI
A02888.jpg
48
Ohio University - Lancaster Campus
slide 48 of 71Spring 2009 PSC 100

• All astrophotographs are black white.
• Photographs can be taken in color, but you lose
  resolution.
• 4 pixels must be binned or clustered for color
  photographs (1 BW, 1 red, 1 green, 1 blue) This
  makes the overall pixel size 4 times bigger
  lower resolution.

49
Ohio University - Lancaster Campus
slide 49 of 71Spring 2009 PSC 100
1 big pixel if the photo is taken in color
4 smaller pixels if the photo is taken in
B/W. Better resolution.
50
Ohio University - Lancaster Campus
slide 50 of 71Spring 2009 PSC 100

• So how can we see all those beautiful color
  photographs?

NGC 2393 The Eskimo Nebula Credit Andrew
Fruchter (STScI) et al., WFPC2, HST, NASA
51
Ohio University - Lancaster Campus
slide 51 of 71Spring 2009 PSC 100

• We take 4 pictures in succession then combine
  them into a single image
• one photo through a red filter.
• one photo through a green filter.
• one photo through a blue filter.
• one photo in B/W (often called a Luminance
  filter) for overall brightness levels.

52
Ohio University - Lancaster Campus
slide 52 of 71Spring 2009 PSC 100
M57 Ring Nebula taken through red, green, and
blue filters. Notice the different details which
come out. Credit Chris Brown, University of
Manitoba
The composite color photo.
53
Ohio University - Lancaster Campus
slide 53 of 71Spring 2009 PSC 100

• Telescopes which see at other wavelengths than
  visible light.
• Not all objects are visible at optical
  wavelengths (400 700 nm).
• Many hot objects are only visible at shorter
  wavelengths (UV, X-rays, ?-rays)
• Many cool objects are only visible at longer
  wavelengths (IR, microwaves, radio waves)

54
Ohio University - Lancaster Campus
slide 54 of 71Spring 2009 PSC 100

• Radio Telescopes
• Detect cool gases H H H2
• Can detect molecules out in space
• oxygen O2,
• carbon dioxide CO2
• hydrogen cyanide HCN
• formaldehyde H2CO
• Ethanol CH3COOH

55
Ohio University - Lancaster Campus
slide 55 of 71Spring 2009 PSC 100

• Advantages Problems
• Operate night or day
• Atmosphere doesnt absorb radio waves
• Poorest resolution of any type of light (doesnt
  see details well)
• Solution is to make antennas (dishes) VERY large

56
Ohio University - Lancaster Campus
slide 56 of 71Spring 2009 PSC 100
The Arecibo Radio Telescope, Puerto Rico.Credit
National Radio Astronomy Observatory
57
Ohio University - Lancaster Campus
slide 57 of 71Spring 2009 PSC 100
The Green Bank Telescope(GBT) in Green Bank,
W.Va. The largest steerable dish in the world.
As tall as the Statue of Liberty, the dish would
hold the building youre in. Credit National
Radio Astronomy Observatory
58
Ohio University - Lancaster Campus
slide 58 of 71Spring 2009 PSC 100
The Very Large Array (VLA), Socorro, N.M.Credit
National Radio Astronomy Observatory
59
Ohio University - Lancaster Campus
slide 59 of 71Spring 2009 PSC 100

• Infrared Telescopes
• Very similar to visible wavelength telescopes,
  except for the detector, called a bolometer.
• IR scopes detect heat from warm gas or warm
  objects. Warm means not hot enough to glow in
  visible light.
• These scopes must be kept very cold or the heat
  that the scope itself radiates will swamp out
  what is being observed.

60
Ohio University - Lancaster Campus
slide 60 of 71Spring 2009 PSC 100

• What kinds of objects do IR telescopes observe?
• IR telescopes see molecules dust. In some
  cases, they can look through cooler dust to see
  whats inside the dust clouds!
• Since stars form where theres lots of dust,
  these scopes are used for for looking inside
  dusty nebulas where new stars form.

61
Ohio University - Lancaster Campus
slide 61 of 71Spring 2009 PSC 100
Star-forming regions around Orion, in visible and
IRCredit Akira Fujii / NASA
62
Ohio University - Lancaster Campus
slide 62 of 71Spring 2009 PSC 100
The Spitzer Space Telescope, part of the Great
Telescope SeriesCredit NASA/JPL-Caltech
63
Ohio University - Lancaster Campus
slide 63 of 71Spring 2009 PSC 100
The Sombrero Galaxy (in Leo) in IR and in visible
light.Credit JPL / NASA (top) Credit NASA/ESA
and The Hubble Heritage Team STScI/AURA)
64
Ohio University - Lancaster Campus
slide 64 of 71Spring 2009 PSC 100

• Ultraviolet Telescopes
• Look for hot, young stars.
• These stars help us better define star-forming
  regions, which contributes to a better
  understanding of the evolution of our galaxy.
• They also look for hot, distant galaxies, as they
  looked in the early universe.
• What famous scope is also a UV telescope?

65
Ohio University - Lancaster Campus
slide 65 of 71Spring 2009 PSC 100
GALEX Telescope (Galaxy Evolution
Explorer)Credit JPL / NASA
66
Ohio University - Lancaster Campus
slide 66 of 71Spring 2009 PSC 100
Galaxy NGC 300 in Sculptor Constellation, 7
million light years away Credit
NASA/JPL-Caltech/Las Campanas
67
Ohio University - Lancaster Campus
slide 67 of 71Spring 2009 PSC 100

• X-ray and Gamma Ray Telescopes
• See very hot objects
• Black Holes
• Pulsars Neutron Stars
• Supernovas
• VERY good resolution great ability to observe
  fine details

68
Ohio University - Lancaster Campus
slide 68 of 71Spring 2009 PSC 100
Core of the Elliptical Galaxy NGC 4261
(accretion disk of a black hole.) Credit
NASA/ESA and The Hubble Heritage Team
STScI/AURA)
69
Ohio University - Lancaster Campus
slide 69 of 71Spring 2009 PSC 100
Cassiopeia A - the remnant of a supernova
which exploded about 300 years ago.Credit
X-ray NASA/CXC/SAO Optical NASA/STScI
Infrared NASA/JPL-Caltech
70
Ohio University - Lancaster Campus
slide 70 of 71Spring 2009 PSC 100
The Chandra X-ray Telescope, part of the Great
Telescopes series. Credit chandra.nasa.gov
(artists conception)
71
Ohio University - Lancaster Campus
slide 71 of 71Spring 2009 PSC 100
A gamma ray burst beginning.Credit NASA
(artists conception)
The GLAST ( Gamma-ray Large Area
Space Telescope, renamed FERMI )Credit General
Dynamics for NASA (artists conception)