NOTES: Arvind Borde The Bending of Light and Telescopes. Light travels in straight lines... except when it bends (refraction).

Transcription

1 Arvind Borde The Bending of Light and Telescopes Light travels in straight lines except when it bends (refraction). 1 The bending of light causes lensing. 2 And lensing is what our eyes, cameras, telescopes rely on. 3

2 Focal length Image Size If an object occupies an angle θ (in degrees) in the field of view, then the image size, s, it makes on a seeing device with focal length f is s = θ 180 πf. s has the same units as f. 4 5 (1) If f eye 17mm, what is s? 6

3 That s the diameter of the image the moon makes on your retina. (2) A telescope with a focal length that s a thousand times as long (17m), will make an image that s how large? Image Resolution Simply increasing the size of an image does not mean you ll see more detail. 7 You also need to increase the resolution. The angular resolution of a device is the smallest angle between two objects that the device sees as two. The formula (for resolution in degrees) is θ res = π λ D where D is the diameter of the device and λ the wavelength of the light. A smaller value of θ res means better/sharper resolution. 8 9

4 (3) Do you get better resolution with red light or blue? (4) For a given wavelength, what can you change in a device to get better resolution? (5) Visible light has an average wavelength of λ = m. What is the resolution of a telescope with a 1 m lens (diameter)? 10 Small angles: ( ) 1 = 1 (one minute). 60 ( ) 1 = 60 ( ) 1 = 1 (one second)

5 How to make better refracting telescopes Longer telescopes bigger images. Larger diameter of lens better resolution. Also, collect more light, so see fainter objects. And avoid diffraction (light/dark bands). Problems Long tubes tend to flex. Larger lenses are hard to build and support; they sag in the middle. Blurring due to the atmosphere for earthbound telescopes; the maximum useful lens diameter for sharpness is 0.5m. Chromatic aberration: different wavelengths focus at slightly different spots. 13 Reflecting Telescopes 14 15

6 Advantages of Reflecting Telescopes Can support mirror at the back: No sag, so can be as big as needed. No chromatic aberration. Can use truss tubes, instead of closed. The mirror backing is often glass: can be shaped with high precision. 16 Polishing the mirror for the Hubble space telescope For observations at wavelength λ the bumps and other imperfections need to be smaller than λ/4. The reflective surface depends on use: aluminum for visible wavelength, gold for infra-red, etc. 17 (6) What tells us that gold is a better reflector at the red end of the spectrum than the blue? 18

7 (7) There seems an obvious flaw in reflecting telescopes. What is it? 19 The blocking of light in the middle does not obscure part of the image. It just lowers the amount of light collected. 20 The next few slides show you some telescopes used by astronomers... 21

8 We travel first to the South Pole:

9 What has BICEP2 done? Has been collecting for over seven years lowtemperature microwave radiation from the very early Universe (around the big bang ). Still ongoing. (Early announcement last year was premature.) 25 Summit of Mauna Kea 4,205m high; dry, stable air 13 telescopes, 11 countries 26 27

10 What has Subaru done?

11 What has Keck done?. March 5, 2015: 4-image cosmic lens.. March 5, 2015: Evidence for a past ocean on Mars from a spectral analysis of the composition of the atmosphere.. January 16, 2015: Three almost earth size planets around a nearby star.. Ongoing: Studies of the galactic center by the group at UCLA 31 Barcelona Rooftops 32 33

12 Satellite dishes Wavelengths: λ 1.5 3cm What has Arecibo done? (Earthquake in 2014.) Ongoing: Asteroid hunter. 2011: Cold brown dwarfs. 2010: Images of comet. 1974: Attempted CETI. 1974: Taylor & Hulse, binary pulsar. 1968: 33 millisecond Crab pulsar. 1964: Rotation of Mercury 59 days

13 What has ALMA done? February 2015: Star formation in the Sculptor Galaxy 37 December 2014: Black hole jets blowing away the Hydrogen from entire galaxies. November 2014: Formation of stars > M. These were telescopes based on earth. They have limitations: Atmospheric blurring. Rotation of the earth (except for one which?). Some radiation simply does not reach the earth s surface

14 At certain wavelengths, we need telescopes based in space: Hubble, 1990 (optical)

15 What has Hubble done? 2001: Planet forming disks common around stars; found planets : Different methods to get estimate of the age of the Universe: 13 billion years : accelerated expansion of Universe 1997: Supermassive black holes are at the centers of galaxies. 1996: Hubble deep field pictures of thousands of galaxies allowed study of their evolution. Chandra, 1999 (x-ray) 44 45

16 46 What has Chandra done? January 2015: Bright X-ray flare from MW black hole. 2012: Halo of hot gas around MW 2006: Strong evidence for dark matter. 2002: Possible evidence for quark stars 2000: Possible mid-sized black holes