Tools and Techniques of Modern Observational Astronomy In

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.
• Distancetime. 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 Earths diameter
  to 1.
• Used Aristotles 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.

Galileo Galilei (1564-1642)
The Father of Observational Astronomy, he showed
that Copernicus was right the Earth is not the
centre of the Universe.
8
Galileo to Newton

• Galileos 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.

• Large lens/mirror collects light from a distant
  object and brings it into focus.
• Small lens/mirror takes the focussed light and
  magnifies it so it looks larger (large enough to
  see)

Illustration of a 60-ft refracting telescope, 1673
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

The large lens collects a lot of light from a
distant object and brings it into focus
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 dont 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 dont 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

The Very Large Array
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.

• These kind of detector chips are now widely used
  in digital cameras most applications requiring
  the acquisition of images
• Amateur astronomers can buy CCDs off-the-shelf

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.

• If we see a spectral line at a wavelength
  associated with a particular element (determined
  from lab experiments), we can identify it
  measure its abundance in that object.

304 Angstroms
Wavelength emitted by Helium so if we see a peak
at that wavelength, we know there is Helium
present in the source here, the Sun.
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
Two maps illustrating the night sky light
pollution from cities.
Want dark skies but also need infrastructure for
telescopes. Thus deserts e.g. Arizona,
California, Australia or not-too-remote mountains
e.g. Hawaii, Canaries, Chile.
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
UK Infrared Telescope 4m
Canada-France-Hawaii 3.4m
Gemini 8m
Subaru 8m
Keck I 2 10m
NASA IRTF 3.8m
U. Hawaii 88-in
Big Island of Hawaii, altitude 4177 m
27
View from Inside the Dome
Slit
Secondary Mirror
Wind Baffles
Primary Mirror Platform
28
The Keck Telescopes

• Twin 10-m telescopes
• Largest optical/IR telescopes in the world
• Instead of one mirror (like the Gemini 8-m), the
  10-m mirror is actually made up of 36 hexagonal
  mirrors which fit together
• Fine computer control keeps the segments aligned
  so it acts as one mirror.

29
La Serena, Chile
Gemini South, Cerro Pachon
Cerro Tolelo Inter-American Observatory (National
Optical Astronomy Observatories, USA) 4-m, 2.5m,
1.5m telescopes
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

34
(No Transcript)
35
Other Optical-IR Observatories
Kitt Peak National Observatory, Tucson
Siding Spring, New South Wales, Australia
36
Radio Observatories
Arecibo Observatory, Puerto Rico
The Very Large Array, Socorro, New Mexico
Very Long Baseline Array (VLBA) whole Earth
telescope Interferometer that uses 10 telescopes
to get very high resolution radio data.
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.

Robert Goddard launched the worlds first
liquid-fuel powered rocket in 1926 he designed
built it, and patented many of the technologies
still used today
38
Current Space Telescopes
XMM-Newton (X-ray Multi-Mirror), Arianne 5
launch, 1999
Rossi X-ray Timing Explorer Delta rocket launch,
1995
Hubble Space Telescope, 2.4m Optical, Space
Shuttle launch, 1990
Spitzer Space Telescope Infrared, Delta rocket
launch, 2003
Chandra X-ray Observatory, Space Shuttle Columbia
launch, 1999
39
NASAs Observatories
40
Europe
The Venus express
Lisa-pathfinder
Rossetta comets
JWST
GAIA
41
The Planetary Explorers
42
The Planetary Explorers