1. Telescopes

2. Types of Telescopes There are telescopes in all emag. wavelengths.
Optical telescopes use visible light.
Examples: Refracting Telescope, Reflecting Telescope,Cassegrain Telescope

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

5. The larger the telescope, the more light it gathers.
Optical Telescopes
Astronomers use telescopes to gather more light from astronomical objects.
The larger the telescope, the more light it gathers.

6. Why do we use telescopes?
1. to brighten an image
2. to magnify
3. to get better resolution
Common misconception: magnification is the most important thing….not necessarily

7. 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.

8. Refraction

9. Focal Point Focal Length
the place where light rays converge to a point
Focal Length
the distance from a curved mirror or lens to its focus

10. Typical Small Refractor

13. Reflection

14. The 100” Telescope (old school)

15. Multiple Mirror Telescope

16. Why use a telescope?
Brighten
Magnify
Resolve

17. Why are large telescopes reflectors?
Large glass lenses sag and warp over time.
Cost—it’s cheaper (only one side’s polished and it’s easier to smooth it out)

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

20. The Powers of a Telescope:
1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: D
A=  (r)2

21. Telescopes Brighten
Light-Gathering Power - cross sectional area of the telescope tube (the aperture)
Light gathering power α (diameter)2
Ex. If telescope A is 3 inches in diameter and it is compared to telescope B that is 6 inches in diameter…..
B is 2x as large as A….so the light gathering power is (2)2 = 4 x light gathering power
(If it was 3x larger…then (3)2 = 9 x power…etc.)

22. Light Gathering Power
10.7 cm camera
15.2 cm camera

23. Telescopes Magnify
Magnification - the number of times larger an object appears through a telescope than as seen by the naked eye

24. Magnification Telescopes are usually labeled as Aperture -- f / #
Example: A telescope is labeled as
f / 7
Q: What is the aperture?
A: 500 mm
Q: What is the focal length ?
A: f = (500 mm) (7) so f = 3500 mm
Q: What would the magnification be if we used a 50mm eyepiece?
A: mag. = mm / 50 mm = 70 x

25. Example Problem
How would you change the magnification for this telescope?
Change magnification by changing out eyepieces.

26. Telescopes Resolve
Angular Resolution - measure of the clarity of images
Telescope with larger diameters are able to resolve smaller objects.

27. Resolution

28. Things that Detect Light for Astronomers
Human Eye and Photographic Film
Photometers - an electronic device that measures the brightness of stars
CCD’s (charge-couple device) - an electronic imaging device that records the intensity of light falling on it

29. CCD Camera and Color Filters

30. CCD Imaging CCD = Charge-coupled device
CCD = Charge-coupled device
More sensitive than photographic plates
Data can be read directly into computer memory, allowing easy electronic manipulations
Negative image to enhance contrasts
False-color image to visualize brightness contours

31. Observing Problems Bad Weather Light Pollution Dispersion
Scintillation
“twinkling”

32. Disadvantages of 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.
Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges

33. Observing Problems
Atmospheric dispersion is the spreading out of light into a spectrum by Earth’s atmosphere.
Venus

34. Seeing
Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images.
Bad seeing
Good seeing

35. The Best Location for a Telescope
Far away from civilization – to avoid light pollution

36. Why do stars twinkle?
Scintillation is the twinkling of stars caused by turbulence in the Earth’s atmosphere.
Turbulence - hot air rising and cool air falling
Note: Planets seem to not twinkle because so bright.…but can be “fuzzier”.

37. Radio Telescopes
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.

39. Radio Maps Radio maps are often color coded:
Radio maps are often color coded:
Like different colors in a seating chart of a baseball stadium may indicate different seat prices, …
colors in a radio map can indicate different intensities of the radio emission from different locations on the sky.

40. Radio Interferometry
Just as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D.
For radio telescopes, this is a big problem: Radio waves are much longer than visible light
 Use interferometry to improve resolution!

41. Interferometry Interferometry
Recall: Resolving power of a telescope depends on diameter D:
amin = 1.22 l/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

42. Radio Interferometry (2)
The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter.
Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!

43. The Largest Radio Telescopes
The 300-m telescope in Arecibo, Puerto Rico
The 100-m Green Bank Telescope in Green Bank, WVa.

45. The Hubble Space Telescope...
…is the largest telescope in space.
...is 30 times more sensitive than ground based telescope (resolves 0.05 arcseconds).
...orbits the Earth every 95 minutes.
…gives high resolution images because it does not suffer from the effects of atmospheric turbulence.

46. Hubble’s best images
                                                            

47. Advances in Modern Telescope Design (1)
Modern computer technology has made possible significant advances in telescope design:
Segmented mirror
1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers
Floppy mirror

48. Examples of Modern Telescope Design (2)
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes

49. Science of Radio Astronomy
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.

50. Infrared Astronomy
Most infrared radiation is absorbed in the lower atmosphere.
NASA infrared telescope on Mauna Kea, Hawaii
However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed.
Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen.

51. NASA’s Space Infrared Telescope Facility (SIRTF)
Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be observed from space.

52. Ultraviolet Astronomy
Ultraviolet radiation with l < 290 nm is completely absorbed in the ozone layer of the atmosphere.
Ultraviolet astronomy has to be done from satellites.
Several successful ultraviolet astronomy satellites: IUE, EUVE, FUSE
Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the Universe.

53. The Hubble Space Telescope
Launched in 1990; 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

54. Gamma-Ray Astronomy
Gamma-rays: most energetic electromagnetic radiation;
traces the most violent processes in the Universe
The Compton Gamma-Ray Observatory

55. X-Ray Astronomy X-rays are completely absorbed in the atmosphere.
X-rays are completely absorbed in the atmosphere.
X-ray astronomy has to be done from satellites.
X-rays trace hot (million degrees), highly ionized gas in the Universe.
NASA’s Chandra X-ray Observatory