1. Galileo’s telescope

2. Refractor or Reflector

3. Refraction
Refraction is the bending of light when it passes from one substance into another
Your eye uses refraction to focus light

4. Refraction makes a pencil appear to be bent when placed in water.
When light passes through a glass slab it first refracts towards the normal then away from the normal.

5. Example: Refraction at Sunset
Sun appears distorted at sunset because of how light bends in Earth’s atmosphere

6. Refraction telescope images
The main lens in a refracting telescope is called the primary lens (or the objective lens). This lens is a part of the telescope and is fixed.

7. Problems with refractors.
Light of differing frequency travel at different speeds in glass. This results in a spectrum from a prism and leads to chromatic aberration in lenses.
Large achromatic lenses are expensive.
Lenses must be supported at the edges. Large lenses sag in the middle under their own weight.
Long focal length= long tubes. Costly mounts and domes.

8. Chromatic Aberration

9. Achromatic Lenses

10. Yerkes Observatory Williams Bay Wis.
Worlds largest at 40 inches in diameter
63ft long

11. Reflectors Most large telescopes are reflectors.
Isaac Newton presented a reflecting telescope to the Royal Society in 1671.
Mirrors avoid chromatic aberration.
Objective mirror instead of an objective lens.
Early reflectors were made of polished metal alloys. Tarnished rapidly dimming images.

12. Reflectors 2
1850 method for depositing silver on glass. Most telescopes became reflectors
Still tarnished, normal glass changes shape with temperature changes
1940’s technique for casting Pyrex glass and aluminum coatings (more durable)
Now Pyrex or Fused Quartz
Palomar 200 inch mirror took 11 years to build and required 5 tons of glass.

13. Reflecting telescope images
Light in a reflecting telescope does not have to pass through glass at all.
The main mirror in a reflecting telescope is called the primary mirror (objective mirror).

14. Images are inverted

15. Types of reflectors

16. Prime or Cassegrain
Prime focus is often used in photography. It places the observer in a cage.
The Cassegrain focus is at the bottom and requires a secondary mirror.

17. Cassegrain Telescopes

18. Earl of Rosse’s mid –nineteenth century telescope
Observers stood at the prime focus.

19. Newtonian
A Newtonian focus is inconvenient for large telescopes.

20. SCT

21. Telescope Mounts
The choice of mount depends on the telescopes main function and size.
Good optics are useless if you can’t control the telescope.
Good control is useless if the optics are poor.

22. Alt-Azimuth Mounts

23. Equatorial Mounts

24. Equatorial Mounts
200in

25. Russian 6m
First large telescope to use alt-azimuth mount. A key move to the new generation of telescopes.
For a given aperture this permits a cheaper lighter structure in a more compact dome.

26. Telescope Rating Characteristics
Powers of a telescope
Light gathering power
Resolving power
Magnifying power.

27. Low
High
Light Gather Power
Low
High
Magnification
Low
High
Resolution

28. Light Gathering Power A telescope is a light bucket.
Like a bucket catching rain the diameter is the determining factor in hiw much light gets caught.
Area= π r2
The large the radius, diameter, the greater the area.
Bigger is better. The more light you collect the farther out you see.

29. Resolving Power … ability of a telescope to see fine detail.
Defined as the angular distance between two objects that are barely visible as separate.
Due to the wave nature of light magnified images have diffraction fringes.

30. Diffraction Limits
The edge of a telescope acts as slits causing diffraction patterns. These are only seen at the highest magnification for a telescope.

31. Overlap
Closely spaced objects begin to overlap, becoming indistinguishable.

32. Increasing resolving power.
The smaller the number the better the resolution.

33. Resolution formula Angular resolution = 0.25 *
For visible wavelengths in the middle of the visible spectrum
“ α” ≈
Note aperture is in the denominator. Large D smaller resolution.
Bigger telescope is better.

34. Magnifying Power
Usually the big sell. Least important. It can be changed.
Magnification is calculated by the focal length of the objective divided by the focal length of the eyepiece.
To Increase magnification just change to a shorter focal length eyepiece.
M =

35. Maximum Magnification
As magnification of an instrument is increased the image will be dimmer.
As a rule of thumb the maximum magnification of a telescope can be found by
Mmax = 20(X/cm) *D(cm)

36. F5_4 Focal Length of a Lens
The focal length of a lens or mirror is the distance from the center of the lens to the image formed from an object placed at a great distance.

37. Observing Problems: Light Pollution
Many of the most interesting objects are dim. Bright skies wash out the images. The darker the sky the better.

40. Seeing

41. Seeing
To reduce the effects of the atmosphere observatories are placed at high elevations and in regions where the air is dry.

42. Paranal Observatory ESO
The best seeing is from remote, high and dry locations.

43. Telescopes
New Generation telescopes use advances in technology to correct images for bad seeing conditions.

44. New Generation Telescopes
Large mirrors had to be made thick to avoid sagging.
A mirror can be supported from the back. Traditional telescopes were big, heavy, and expensive. Control devices had to be massive too.
High speed computers have helped to reduce costs and improve performance.
Computer control makes alt-azimuth mounts usable.
Computer control makes it possible to control the shape of thin mirrors rapidly. This reduces the costs of making the mirror, smaller mounts and allows for…

45. Thin floppy mirrors
Mirrors are backed by movable pistons able to change the shape of the mirror quickly under computer control.
One of the Keck hexagonal mirror segments.
Thin mirrors are lighter require less support and they change temperature faster (less trouble with convection currents at surface)

46. Floppy Mirrors

47. Nordic 2.6 M Canary Islands 1989
First large instrument whose dome and primary mirror shape are continually adjusted for the sharpest possible image.

48. Gemini 8.1 m mirrors
Rapid control of mirror shapes makes it possible to correct some distortions caused by poor seeing.
Real time mirror control achieved by 120 actuators under the mirror and 60 around the edge.
Right: The Gemini mirrors have adaptive optics

49. Adaptive optics
AO allows this 1.5m telescope to reach 0.1” resolution

50. Adaptive Optics Corrected Image

51. Adaptive Optics
Without adaptive optics
With adaptive optics
Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence

52. Another example of adaptive optics.

53. Pluto and Charon by adaptive optics

54. Star by AO
The central star is blocked out to show a disk of matter ..

55. Inside Gemini

56. Gemini open
The air inside and outside the dome must be the same temperature so the dome is opened shortly before sunset.

57. Inside looking out

58. Mauna Kea Observatory Hawaii
At 4 km altitude sky is clear. Keck 10 m mirror

59. Keck
Keck (I and II) have 9.8 m segmented mirrors[ 36 of them, 90cm on each side]. A major advance in telescope design.

60. Keck I Segmented Mirror

61. Segmented Mirrors

62. Moons by Keck

63. Segmented Mirror

64. Texas Hobby-Eberly Telescope, HET:
9.1 effective aperture mirror is made from 91 spherically segmented mirrors forming a hexagon 11 m x 10 m.

65. Plans
Giant Telescopes are proposed. The California Extremely Large Telescope would be a 30 m telescope featuring 1080 segments 40cm on a side.

66. More plans
XLT
Extremely large telescope

67. The mirror

68. Enormously large telescope

69. OWL
Overwhelmingly Large Telescope

70. Optical Interferometry
Combining signals requires control of signal path lengths to a fraction of the wave length of the e-m radiation being combined.
Only radio waves could be used until recently.
The resulting image has the resolution of the distance between mirrors.

71. Very Large Telescope Array
Paranal Observatory at Atacama Chile
Four 8.2 m reflecting telescopes .
Used together have the effective area of a 16 m .

72. VLT

73. Large Binocular Telescope

74. Binoc mirror
Spinning oven makes mirror closer to end shape, requires less finish grinding.

75. Giant Magellan Telescope

76. CCD (charge coupled device) Images
CCDs can detect both dim and bright objects in the same exposure, are more sensitive the a photographic plate, and can be read directly into a computer.

77. CCDs,2
CCDs can not only image but give relative intensity data that was once required a photometer. This leads to interesting contour plots and the use of false color images.

78. Imaging
Astronomical detectors generally record only one color of light at a time
Several images must be combined to make full-color pictures

79. Imaging
Astronomical detectors can record forms of light our eyes can’t see
Color is sometimes used to represent different energies of nonvisible light

80. Early 90s

81. Spectra Spectrograph: Most use a grating instead of a prism.
Details about the intensity at specific wavelengths gives a wealth of information.

84. Earth’s Atmosphere
The Earth’s atmosphere is mostly transparent for visible light and radio waves.
For that reason, there are two major types of telescopes:
Common Optical Telescopes
Radio Telescopes

85. The other window
Radio ?
Why don’t we see in the radio region?

86. Components of Radio telescope
because you would have to have huge eyes. Remember the resolution formula ?

87. Radio telescopes “α” = 0.25 Still bigger is better
To get the same resolution radio telescopes must be huge.
“α” = 0.25

88. Green Bank 100 m telescope
Largest steerable radio telescope in the world.

89. 300 m Radio Telescope
Arecibo Puerto Rico

90. Very Large Array
Radio interferometry increases the resolution, by combining signals. VLA consists of 27 dishes has the resolution of a 22 mile diameter radio telescope. Soccoro NM

91. Radio images

92. Very Long Baseline Array VLBA
Combines electronically signals from Hawaii and the Virgin Islands. This gives the resolution of an Earth sized telescope

93. IR Infrared Telescopes
Top:Longer wave length light doesn’t see the smog particles.
Dust doesn’t block IR as easily in space either.

94. NASA 3 m Infrared Telescope
Water absorbs IR
IR Telescopes must be at high elevations, in the mountains, on balloons or in space. Still only the near infrared is visible under even the thin atmosphere.

95. NASA’s Great Observatories

96. Figure 6.22

97. Hubble launched April 1990. Visible light and UV ( Shuttle Discovery)
Compton Gamma-Ray Observatory.(Shuttle Atlantis). April 1991 to June 2000 (lost gyroscope)
Fermi Gamma-Ray Observatory (formerly GLAST) 2008 replaces Compton
Swift Has gamma ray detectors, as well as x-ray and visible light telescopes.
Chandra X-ray Observatory July 1999 (Shuttle Columbia)
Space Infrared Telescope Facility (SIRTF), now called the Spitzer Space Telescope

98. SIRTF Space Infrared Telescope Facility [Spitzer]

99. SIRTF, Spitzer Space Telescope
Was launched by Delta rocket first expected in mid April Saved a lot of money, but required a redesign. Was to have been launched by a shuttle.

100. IRAS
Infrared Astronomy Satellite surveyed cooler gas and dust.

101. Dust gets in the way

102. Resolution

103. Orbit

104. Chandra Xray Telescope

105. Gamma ray telescopes
Design

106. X ray mirrors
From Chandra Ed at Harvard

107. Extreme Ultraviolet Explorer

108. F5-23a Hubble

109. F5-23bMars by Hubble

110. James Webb Space Telescope – 2013… I mean 2018 launch…