1. Telescopes & Light

2. The Powers of a Telescope
Light Gathering Power: Astronomers prefer *large* telescopes. A large telescope can intercept and focus more starlight than does a small telescope. A larger telescope will produce brighter images and will be able to detect fainter objects.
Resolving Power: A large telescope also increases the sharpness of the image and the extent to which fine details can be distinguished.
Magnification: The magnifying power is the ability of the telescope to make the image appear large in the field of view.

3. Light-Gathering Power:
Size Does Matter
Light-Gathering Power:
Depends on the surface area (A) of the primary lens and is proportional to the telescopes diameter. D

4. The Powers of a Telescope
Resolving Power: Wave nature of light: The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.

5. The Powers of a Telescope
Magnifying Power
The ability of the telescope to make the object’s optical image appear bigger while being observed
A larger magnification does not improve the resolving power of the telescope!

6. Types of Telescopes
Refracting Telescopes: Use lenses as the optics to focus and bend light.
Galileo used a refracting telescope.
The human-eye is partly a refracting telescope.

7. Refracting Telescope
Objective Lens
Eyepiece Lens
Focal Length Objective
Focal Length of Eyepiece
Refracting Telescope: Lens focuses light onto the focal plane
Focal length

8. Disadvantages
Refracting telescopes suffer from Chromatic Aberration. As light passes through a lens, just as a prism will disperse light, the lens will focus bluer wavelengths differently than the redder wavelengths.
Blue Focus
Red Focus

9. Disadvantages Cannot be made very large for a multitude of reasons.
Get to be very expensive to maintain.
Lenses can grow “cloudy” over time.
Lenses can distort over time.

10. 140-ft Hevelius telescope 1673

11. Most modern telescopes are reflecting telescopes.
Types of Telescopes
Reflecting Telescopes: Use mirrors as the optics to focus and bounce light.
The rear view mirror on your car is a simple reflecting telescope.
Reflecting Telescope: Concave Mirror focuses light onto the focal plane
Focal length
Most modern telescopes are reflecting telescopes.

12. Reflecting Telescope

13. Advantages
Reflecting telescopes do not suffer from Chromatic Aberration. All wavelengths will reflect off the mirror in the same way.
Reflecting telescopes can be made very large because the mirrored surfaces have plenty of support. Thus, reflecting telescopes can greatly increase in light gathering and resolving power.
Reflecting telescopes are often cheaper ($$$) to make than similarly sized refracting telescopes.

14. Newton’s Telescope: The first reflecting telescope

15. Hey! Where are you going? The Doppler Effect
The wavelength of light is effected by the relative motion between the source and the observer. Christian Doppler ( ), a professor of mathematics in Prague, pointed out in 1842 that the observed wavelength of light is affected by motion.
The frequency increases when the source and observer are approaching each other, and decreases when they are moving away from each other.

16. The Doppler Effect
Observer
Stationary
Towards
Blueshift
Away
Redshift

17. Observer
Observer

18. How does this help? Star moving towards from us Stationary Star
Star moving away from us
Strength of the Light with this Wavelength
Wavelength

19. Telescopes

20. Where to put a Telescope?
Far away from civilization – to avoid light pollution

21. Where to put a Telescope?
On high mountain-tops – to avoid atmospheric turbulence and other weather effects

22. On high mountain-tops – to avoid atmospheric turbulence and other weather effects

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

30. Hubble Space Telescope

31. NASA’s Chandra X-ray Observatory
X-Ray Astronomy
X-rays are completely absorbed in the atmosphere.
X-ray astronomy has to be done from satellites.
NASA’s Chandra X-ray Observatory

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

33. Radio Astronomy

40. Radio Interferometry

41. Radio Interferometry
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)!