1. Telescopes: Portals of Discovery

2. How do light and matter interact?
Emission
Absorption
Transmission
Transparent objects transmit light
Opaque objects block (absorb) light
Reflection or Scattering

3. Reflection and Scattering
Mirror reflects light in a particular direction
Movie screen scatters light in all directions

4. Reflection of light with mirrors
The flat surface enables an incoming light beam to perfectly bounce
Angle of Incidence = Angle of Reflection

5. Reflection and Scattering with Matter
Interactions between light and matter determine the appearance of everything around us

6. Thought Question Why is a rose red?
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.

7. Thought Question Why is a rose red?
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.

8. Refraction (occurs during transmission)
The universal speed of light is measured in a vacuum.
The speed of light is slower in different substances.

9. Refraction
E.g. Light travels slower through glass or water than through air.
Refraction is the bending of light when it passes from one substance into another

10. Focusing Light
The speed of light through the liquid in your eye is slower than the speed of light in air.
Refraction can cause parallel light rays to converge to a focus

11. Image Formation
The focal plane is where light from different directions comes into focus
The image behind a single (convex) lens is actually upside-down!

12. What have we learned? How do light and matter ineract?
4 forms of interaction, including reflection and transmission
Refraction is when light slows and bends when travelling through a substance
How does your eye form an image?
It uses refraction to bend parallel light rays so that they form an image.
Cameras focus light like your eye and record the image with a detector (CCDs in digital cameras).

13. Telescopes: Giant Eyes
Our goals for learning
What are the two most important properties of a telescope?
What are the two basic designs of telescopes?
What do astronomers do with telescopes?

14. What are the two most important properties of a telescope?
Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time.
Angular resolution: Telescopes that are larger are capable of taking images with greater detail.

15. Light Collecting Area
A telescope’s diameter tells us its light- collecting area: Area = π(diameter/2)2
i.e The larger the aperture size, the better the telescope
The largest telescopes currently in use have a aperture diameter of about 10 meters

16. Bigger is better
Tool from the Telescopes tutorial.

17. Thought Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope?
It’s 5 times greater.
It’s 10 times greater.
It’s 25 times greater.

18. Angular Resolution
Distant objects look very close together; i.e. a small 'angular separation'
Angular resolution is the minimum gap between objects that the telescope can distinguish.
Eventually the car is so far away that you see the headlights as one light.
-you have reached the limit of your eye to resolve the angular separation.
Emphasize that at a great distance the lights would look like one light rather than two --- but we could still see them. (Students sometimes think that below a particular angular resolution we see nothing at all.)

19. Angular Resolution
Ultimate limit to resolution comes from interference of light waves within a telescope.
Larger telescopes are capable of greater resolution because there’s less interference for the amount of light

20. What are the basic designs of telescopes?
Refracting telescope: Focuses light with lenses
Reflecting telescope: Focuses light with mirrors
Catadioptric telescope: Focuses light with both lenses and mirrors

21. Refracting Telescope
Refracting telescopes need to be very long, with large, heavy lenses

22. Basic Refractor Telescope 1608

23. Galileo’s Telescopes 1609

24. Chromatic Aberration
Light focuses imperfectly in a lens because the light must transmit though glass
All lenses suffer from chromatic aberration
(slight blurring caused by the short wavelength light focusing before the long wavelengths)

25. Reflecting Telescope
Reflecting telescopes do not suffer chromatic aberration
Reflecting telescopes can have much greater diameters.
Most modern telescopes are reflectors

26. Mirrors in Reflecting Telescopes
The largest mirrors in the world are in the Keck telescope on Hawaii
Twin Keck telescopes on Mauna Kea
Segmented 10-meter mirror of a Keck telescope

27. Designs for Reflecting Telescopes

28. Newtonian Telescope 1672
One of the most common amateur telescopes for deep sky observing.
Early Newtonian used by Herschel
Modern version with a Dobsonian mount

29. Cassegrain Reflector Telescope, 1672
More efficient, but requires a more complex secondary mirror.
(hyperbolic rather than flat)

30. Some Cassegrain Correctors -Catadioptrics
Schmidt-Cassegrain Telescope 1930
One of most common amateur telescopes
Maksutov-Cassegrain Telescope 1941

31. Catadioptric Telescopes
Corrector lens
Schmidt-Cassegrain telescope on an equatorial mount

32. What do astronomers do with telescopes?
Imaging: Taking pictures of the sky
Spectroscopy: Breaking light into spectra
Timing: Measuring how light output varies with time

33. Imaging
Early astronomers had no choice, they observed by eye and recorded with descriptions and drawings.
Visible light only.
Camera - permanent record, non-subjective, reproducible world-wide in publications.
Can be built to detect any wavelength.

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

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

36. Imaging True color image False color image
The colors are the same as if you were viewing with your own eyes
False color image
The colors have been arbitrarily assigned to assist image interpretation

37. The famous image of the Eagle nebula is false color!
(Nebula in true color)

38. Spectroscopy
A spectrograph separates the different wavelengths of light before they hit the detector
Diffraction
grating breaks
light into
spectrum
Light from
only one star
enters
Detector
records
spectrum

39. Spectroscopy
Graphing relative brightness of light at each wavelength shows the details in a spectrum

40. Timing
A light curve represents a series of brightness measurements made over a period of time

41. What have we learned?
What are the two most important properties of a telescope?
Collecting area determines how much light a telescope can gather
Angular resolution is the minimum angular separation a telescope can distinguish
What are the basic designs of telescopes?
Refracting telescopes focus light with lenses
Reflecting telescopes focus light with mirrors
Catadioptric telescopes focus light with both.
The vast majority of professional telescopes are reflectors

42. What have we learned? What do astronomers do with telescopes? Imaging
Spectroscopy
Timing