Telescopes

1. Telescopes

2. Why Telescopes are Important • We have just spent a lot of time talking about light • But none of that does us any good if we don’t have a way of collecting that light in the first place • This is where telescopes come in

3. What is a Telescope? • So just what is a telescope? • An object that uses lenses or reflecting surfaces to collect light from a distant object

4. The History of the Telescope • It is a common misconception that Galileo invented the first telescopes • In fact, Hans Lippershey, a Dutch spectacle make, is credited with designing the first telescope • Galileo is the first person known to have turned a telescope to the sky

5. Galileo’s Telescope • He was astonished by what he saw • The rings of Saturn • Stars in the Milky Way • The moons of Jupiter • Spots on the sun • He also went blind

6. Can You Imagine? • Just imagine what he must have felt, being the first person to see these things • It was all completely unexpected • Galileo’s observations, and those of others, would literally change the course of human history

7. Telescopes Today • Telescopes have come a long way since then • The biggest single telescopes have main mirrors that are over 12 meters in diameter! • Some telescopes are actually arrays that are made of dozens of smaller telescopes linked together

8. 0 of 5 Today, there are telescopes that are designed to see radio, microwave, infrared, visible, ultraviolet, x-ray and gamma-ray light? • True • False

9. How Telescopes Work • There are two main types of telescopes • Refracting telescopes use lenses to focus light to a point • Reflecting telescopes use mirrors to focus the light • Catadioptric telescopes are a combination of the two

10. Refracting and Reflecting Telescopes

11. Anatomy of a Telescope • Although there are many types of telescopes, all have some basic key parts

12. Anatomy of a Telescope • Aperture • Primary • Secondary • Eyepiece • Optical Tube • Finder • Detector

13. Anatomy of a Telescope • The aperture is simply the part of the telescope that lets light in • The primary bends the light, bringing the rays to a point • The secondary aids in this process

14. Focusing Light • The idea of focusing light is important • Telescopes collect light from a large area • By focusing the light, we concentrate its power • The focal plane is the plane where the light rays meet • The focal length is the distance from the primary lens (or mirror) to the focal plane

15. Focusing Light

16. Focusing Light

17. The Focal Plane • If we put our eye at the focal plane, we would only see a bright point • The eye piece straightens out the rays of light so our eye can see the image • If we move the eyepiece out of the focal plane, the image will be distorted

18. Anatomy of a Telescope • The optical tube protects the rest of the telescope and blocks stray rays of light • The finder is a small telescope used for honing in on objects • The detector is the thing that actually records the light • Could be your eye

19. Common Reflectors

20. Some Other Telescope Properties • There are some other properties of telescopes that astronomers are interested in • Some of these things are • Collecting Area • Resolution • Magnification • Cost

21. 0 of 5 What is the most important property of a telescope for astronomers? • Magnification • Collecting Area • Focal Length

22. Collecting Area • Collecting area is the area of the surface that the telescope uses to collect light • This is usually the primary lens or mirror • Many people think that magnification is the most important property of a telescope, but it is actually collecting area

23. Collecting Area • Why is collecting area important? • Suppose you are trying to see a very faint source • If it is faint, then it is not sending many photons to Earth • We need to collect as many photons as we can • Having a large collecting area does this – like casting a big net to catch more fish

24. Collecting Area • Most astronomical objects are pretty faint, so we need to collect as much light as we can

25. Resolution • The resolution of a telescope defines how much detail we can see • All optical systems have some resolution limit • Eventually, the separation between objects becomes too small for us to see

26. Angular Size • We measure the separation of objects in terms of angles • Remember that there are 360° in a full circle • There are 60 arc minutes in a degree, and 60 arc seconds in an arc minute

27. Back To Resolution • Consider looking at a distant mountain • Although the mountain is covered in trees, we cannot make out the trees themselves • Our eyes have too low of a resolution

28. Telescope Resolution • Telescopes have a much improved resolution, but there is a limit • The resolving power of a telescope (with a round primary) cannot be greater than

29. Telescope Resolution • Notice that this depends on both wavelength and the diameter of a telescope • This is a natural limit…it is set by nature and we simply cannot do better

30. Telescope Resolution • Other things affect the resolution of a telescope • Atmospheric distortions are notoriously bad • Think of the “waves” that come off of hot asphalt • But there are things we can do to compensate for these effects

31. Telescopes in Space • The most straightforward is to put our telescope above the atmosphere • There is another good reason for doing his, and it has to do with how light travels through the atmosphere

32. Atmospheric Transparency

33. Why Do We See What We See? • This provides a great explanation of why our eyes are sensitive to visible light • The Sun puts out most of its light in the visible part of the spectrum • Only visible light and radio make it through the atmosphere • Radio waves have very long wavelength, so we would need very big eyes to resolve anything

34. Magnification • Astronomer’s do like magnification, too • But note that it does not matter how much you magnify something…if you cannot resolve it, magnification does you no good • Think of a pixelated image

35. Pixelation

36. Magnification • With that in mind, here is the jist on magnification • Magnification is defined as the increase in angular size • The formula to calculate magnification is

37. Magnification • fT is the focal length of the telescope • fE is the focal length of the eyepiece

38. 0 of 5 What will give you a larger magnification? • A short eyepiece focal length • A long eyepiece focal length

39. Magnification • A longer focal length for the telescope, or shorter focal length for the eyepiece, means a larger magnification

40. Cost • Cost is of course a practical concern, but it needs to be taken into account • Telescopes can cost as much as hundreds of millions of dollars, and guess who pays for most of it… • Space telescopes are especially expensive, so we need to make compromises

41. Nickels and Dimes… • Since it costs lots of money to put a telescope in space, space telescopes are usually smaller • No atmosphere, but less collecting area, lower limiting resolution • There are also lots of practical concerns

42. Precision Instruments • If astronomers want the best possible data, their telescopes have to be very, very good • Consider that in order to get a clear image, your mirrors or lenses need to be perfect on scales of about the wavelength of light you are looking at • This can be nanometers

43. Some Famous Telescopes

44. Some Famous Telescopes

45. Some Famous Telescopes

46. Some Famous Telescopes

47. Some Famous Telescopes

48. Some Famous Telescopes

49. Some Famous Telescopes

50. Some Famous Telescopes