Tools and Techniques of Modern Observational Astronomy

1. Tools and Techniques of Modern Observational Astronomy

2. In the last episode� What is Astronomy? What is light? The EM spectrum. The dual nature of light. Detectors. Distance units used in astronomy. Distance=time. More distant objects are further back in time. What is a galaxy. Our Galaxy: the Milky Way. A tour from the Earth to superclusters in the universe.

3. Outline Measuring the circumference of the Earth Galileo and the telescope Galileo to Newton Refracting and reflecting telescopes Reflecting telescopes Radio telescopes: interferometry From eyes to electronics Telescopes and sites: Gemini, the VLT Rocket science Space telescopes

4. Measuring the Earth 200 BC: Eratosthenes calculated Earth�s diameter to 1%. Used Aristotle�s idea that, if the Earth was round, stars would appear at different positions to observers at different latitudes He knew that on the 1st day of summer, the Sun passed directly overhead at Syene, Egypt. At midday on the same day, he measured the angular displacement of the Sun from overhead at the city of Alexandria, 5000 stadia (1 �stadium� ~ 0.15 km) away from Syene

5. Measuring the Earth

6. Measuring the Earth Found the angular displacement = 7.2 degs, 1/50 of a circle. Geometry: this is the same as ratio of the distance between Syene & Alexandria to the total circumference of the Earth. Circumference then equals the distance between the 2 cities multiplied by 50 = 250,000 stadia = 40,000 km. Earth measured by spacecraft = 40,070 km

7. Galileo & the Telescope Galileo was 40 when he heard of a Dutch optician who in 1608 had invented a glass that made distant objects appear larger. Using these lenses, Galileo crafted his own telescope. He discovered that the Moon has craters, that Jupiter has its own moons, that the Sun has spots, and that Venus has phases. He realized that his observations only made sense if all the planets revolved around the Sun, not around the Earth.

8. Galileo to Newton Galileo�s telescope used lenses - a �refracting� telescope The problem with this type of telescope is that to get higher magnifications you need a longer distance between the two lenses � becomes impractical. In 1672, Newton designed a telescope which used a mirror instead of lenses � the �reflecting� telescope. This design does not suffer from the same limitation and is what astronomers still use today.

9. Refracting Telescopes Use two glass lenses to focus light Lenses are in a �convex� (curved outward) shape, which bend light inwards to make the image Need bigger lenses and larger distances between the two lenses (�focal length�) to get higher magnification Glass lenses produce colour distortions because light of different wavelengths bends at different angles

10. Reflecting Telescopes Use mirrors to focus light Mirror shape is �concave� (curved inward): bends reflected light together. Uses two mirrors to reflect focussed light down to the �eyepiece�, which has a small lens which magnifies image All wavelengths of light reflect off mirror in same way so don�t have colour effect problems as with the refractors

11. Cassegrain Reflector Mirrors can be made very large � easier than lenses Telescope �tube� does not have to be as long due to positional flexibility of the secondary mirror Cassegrain design has a hole in the middle of the primary, through which the secondary bounces the light back to the detector � much more convenient design for large telescopes Images don�t have holes or shadows because the light rays from the unblocked parts of the primary are all added together when the light is focussed

12. An 8-meter Mirror

13. Radio Telescopes Until 1930s, all telescopes were optical. Began to explore another part of the EM spectrum: the radio Large metal or wire mesh dish to reflect radio waves to antenna above dish Much larger than optical telescopes because radio wavelengths are much longer (lower energy) To collect enough radio photons to detect a signal, radio dishes must be large

14. Interferometry Can increase resolution of images by connecting telescopes together to make an interferometer

15. Interferometry Very hard to do at wavelengths other than radio, because other wavelengths too short � though are beginning to try it in the optical/IR. Radio waves from an object reach each telescope at slightly different times, so the waves are out of sync with one another Knowing the distances between the telescopes and how out of sync the waves are, the signals can be combined electronically to create a very high resolution image.

16. From Eyes to Electronics Originally the only way to record astronomical information and images was to sketch them Photography was introduced to astronomy in the middle of the 19th C., and was ubiquitous for more than a century as the primary method of recording astronomical information Can expose � �integrate� - for much longer with a photo plate than the eye, enabling us to study much fainter objects by accumulating more light However response of photo plates was non-uniform (�analog�) and so not easy to calibrate or standardize

17. From Eyes to Electronics Charge-Coupled Devices (CCDs): the standard detectors used in telescopes since the mid-80s. Light-sensitive semiconductor chip Each �pixel� is an individual photon detector. Photon arriving on a pixel generates an electrical charge, which is stored for later readout. Size of charge increases as more photons strike the pixel: brighter object = greater charge Together an array of pixels makes an image. More pixels = higher resolution (more detailed) images.

18. How Powerful a Telescope Is Light-gathering power: most important �Light bucket�: bigger mirror, more photons Resolving power Ability to see small details & sharp images Again depends on large mirror: the more waves that can be packed on to the mirror, the more info is detected by the telescope, the more detailed the eventual image Magnifying power � least important Increases size of image in field of view However, spreads light out so image becomes fainter; and enlarges any distortions due to atmosphere

19. Photometry: Monitoring Light A �light curve� is a graph of intensity (brightness) over time, made by counting the number of photons coming from a source over a period of time. The light curve tells you how bright your source is and the amount of time it remained at that brightness. Can then track variations in the light coming from the source.

20. What Causes Variability? Many types of stars are intrinsically variable Others are in binary orbits & so eclipse each other Some types of stars have non-periodic variability � flaring � others with jets Transient phenomena � bursts of activity from stars etc which are usually quiet and then �go off�, maybe just once �Active Galaxies� and �quasars� which have jets and other types of variability � supermassive black holes Gamma-ray bursts � most energetic events known

21. Spectroscopy The technique of measuring the intensity of light at different energies by splitting the light into a spectrum. A spectrum gives us info about the composition of the object we are observing � e.g. what elements are in it. Particular elements emit light at particular energies (wavelengths), so we can identify their presence in a spectrum.

22. How to Choose a Telescope Site Weather. Want reliably clear nights � at least 75% of nights/year should be clear Dark site � minimal/no effects of light pollution e.g. nearby cities Need air to be stable. Called having good �seeing�. Atmosphere produces distortion in the light coming from space � twinkling stars � and also blocks some of the light (�extinction�) Even �clear� air can have lots of turbulence, with layers of different temperatures in the atmosphere. High altitude gets us above much of the atmosphere and the distorting effects of water vapour etc. Needs to be a good place for a holiday.

23. Light Pollution

24. Why is the Sky Blue? Redder (long wavelength) light is scattered less by molecules in atmosphere than bluer light This is why Sun looks orange/red near horizon � other sunlight colours are scattered away so we see only red/orange Also why sky is blue � blue light scattered more, so you see more blue light scattered back to your eyes when looking away from Sun

25. Adaptive Optics Air is constantly in turbulent motion so light from celestial objects is bent randomly in many ways thousands of times per second � twinkling � which produces blurred images Much like how ripples in water distort view of objects below the surface New technique which allows us to compensate for atmospheric distortion Requires high-performance computing

26. Mauna Kea, Hawaii

27. View from Inside the Dome

28. The Keck Telescopes

29. La Serena, Chile

30. Gemini South in Action

31. Gemini South Cooling Down

32. The Very Large Telescope (VLT)

33. The VLT The VLT is an array of four 8-m telescopes which operate in optical and infrared wavelengths. Located on Cerro Paranal

35. Other Optical-IR Observatories

36. Radio Observatories

37. Rocket Science 3 fathers of modern rocketry: Robert Goddard (engineer, US), Hermann Oberth (physicist, Rumanian/German), Kanstantin Tsiolkovsky (mathematician, Russian) Prior to the 20th C., all rockets were solid fuel � gunpowder (e.g. fireworks) Werner von Braun � German rocket scientist Civilian group was pulled into the Nazi war effort, developed the V2 rocket. Realized in early 1945 that the Axis would lose; made arrangements to surrender to the Americans (Hitler had ordered their execution to prevent their capture by the Allies) Then became the leader of the US rocket program which developed everything from weaponry to the rockets used for the manned space missions (Mercury, Gemini, Apollo) � and of course the rockets used to launch space astronomy missions.

38. Current Space Telescopes

39. NASA�s Observatories

40. Europe

41. The Planetary Explorers

42. The Planetary Explorers