TELESCOPES

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TELESCOPES

• The main tools astronomers use are, of course,
  telescopes.
• However, there are many types, depending on the
  band of the Electromagnetic Spectrum being
  studied.
• The instruments at the back end
• cameras, spectrometers, etc. are just as
  important as the light buckets.

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But First, The Eye

• Our natural way to collect and focus visible
  light
• The pupil admits light to the lens, which along
  with the cornea, focuses light on the retina
• The cones are retinal cells sensitive to color
  rods are more numerous but only give us shades of
  gray

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Refraction Focusing

• Light slows on entering glass, plastic, or cornea
  (n1.3376) and lens then it bends toward the
  normal
• This focuses incoming rays from a point to a
  point and creates an inverted image (which our
  brain turns right-side-up)

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Cameras Store Images

• Our eye refreshes images 30 times/s
• Lets us see motion, but prevents long exposures
  or seeing faint objects
• Film, or now, digital Charge Coupled Devices
  record for as long as the shutter is open

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What Are Telescopes Good For?

• Telescopes gather EM radiation and bring it to a
  focus.
• MOST IMPORTANT TO DETECT FAINT
  THINGS Observed BRIGHTNESS or INTENSITY
  is proportional to LUMINOSITY but declines
  inversely with the DISTANCE squared
• 
  so bigger
  telescopes let you see to greater distances.

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Better Sensitivity

• The bigger the aperture (diameter) of the
  telescope, the more light it gathers in the same
  time so it sees fainter objects. Bottom
  photo of Andromeda galaxy twice the aperture.

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What else do telescopes do?

• MAKE IMAGES of EXTENDED THINGS, or SEPARATE
  IMAGES of NEARBY THINGS (resolution)
• RECORD CHANGES OVER TIME
• MAGNIFY images. While important for
  bird-watching on earth, and of some importance
  for planetary observations, higher
  magnification yields more jiggling and often
  worse resolution.

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Focusing of Light

• LENSES USE CURVED SURFACES TO TAKE A PARALLEL
  BEAM OF RADIATION FROM A VERY DISTANT SOURCE TO A
  (PRIME) FOCUS.
• Such CONVERGING LENSES require CONVEX SURFACES
  the bending is greater for larger differences
  from the normal (perpendicular to surface).
  More sharply curved lenses produce shorter focal
  lengths.
• Images form as light from different parts of an
  object comes to a focus in different parts of the
  focal plane (inverted or upside-down).

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Refracting Lens Focuses Light
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Lenses or Mirrors Form Images
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Lenses Suffer Chromatic Aberration
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Mirrors of Paraboloidal Shape Focus Light of All
Colors to the Same Point and Invert Images Too
Mirror Focus Inversion
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Reflectors vs. Refractors
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Refractors vs. Reflectors

• Reflecting telescopes are superior because
• They only need perfect surfaces, not perfect
  solids. Therefore they are easier and
  cheaper to make -- MUCH cheaper for BIG
  telescopes!
• No chromatic aberration in reflection.
• Large lenses sag, and must be supported by their
  edges or else light is blocked. Big mirrors
  can be supported from behind w/o blocking light.
• Refractors need to be much longer than reflectors
  of the same aperture--again more

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Main Types of Reflecting Telescopes
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Larger Mirrors Gather More Light

• A telescopes area is proportional to the square
  of its diameter, or aperture.

Keck telescope D 10m has 2500 times the
aperture of human eye (around 4 mm) or an area
6.3 million times bigger. Thus, in one second it
can gather 6.3 million times the light, or see
an object only one/six-millionth as bright. Or,
the same object can be seen 2500 times further
away! Light Bucket Applet Plus, telescopes can
integrate (stare) for hours, while the human
brain produces a new image about 30 times a
second, wiping out the old one. Together, these
mean that big telescopes can make images of
objects billions of times fainter than we can
see.
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Big Telescopes Mauna Kea Observatory
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Larger Apertures Resolve Better

• Here ?, the angle between the closest objects
  that can be seen separately is in arcsec, while
  wavelength, ?, and aperture, D, are in the same
  units (meters).
• Example a 3.5m telescope working in yellow light
  500 nm 5x10-7m, has a resolution angle,
  ? 2.5x105 x (5x10-7m/3.5m) 0.036
  arcsec
• Resolution on earth limited by SEEING ---
  spreading of an image via turbulence in the
  atmosphere changes over lt 0.1 s, and smears
  images out to gt 0.5 arcsec. So little real
  improvement in resolution if D gt 0.25 m. This is
  also why STARS TWINKLE and PLANETS DON'T
  (usually, as seen by the naked eye).
• Angular Resolution Applet

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Resolution and Turbulence
Angular Resolution Car Lights Applet
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Avoiding Resolution Limits

• Go into Space Hubble Space Telescope ? 0.08
  Next Generation (James Webb) Space Telescope
  (? 0.02 arcsec )
• Speckle interferometry take very short exposures
  -- works only with very bright stars (? 0.002
  arcsec ). (Also see regular interferometry,
  below.)
• Active optics quickly tip and tilt mirrors to
  make image crisper (motors dome airflow
  pistons)
• Adaptive optics measure blurring of a bright
  star (or laser spot) and very quickly adjust
  mirror shape to reduce it then nearby images
  will also be crisper. ? 0.1 arcsec can be
  achieved in IR and visible

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Active Optics 3.5 m NTT
Star cluster R136 w/o w/ active optics
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Adaptive Optics Starfire Gemini
NGC 6934, a star cluster without (1, visible),
and with (0.1, IR) adaptive optics on 8-m Gemini
North
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Key Instruments (optical, UV, IR)

• Typical research telescopes have several
  instruments which are attached to the secondary
  focus (Cassegrain and/or Coude).
• INTENSITY (BRIGHTNESS)
• Phototube linear, but only one or two objects
  at once.
• Photographic plates non-linear, but compare
  many at once.
• Charged Coupled Device CCD -- linear and get
  many at once. CCDs now dominate intensity
  measurements.

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CCD Chip and Image
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Nobel Prize in Physics 2009

• Willard S. Boyle and George E. Smith who were at
  Bell Labs in 1969 share half the prize for the
  invention of the Charge Coupled Device sensor
  CCDs were used first in spy satellites, then by
  astronomers and today in digital cameras.
• The other half went to Charles K. Kao, who while
  working in England in 1966 demonstrated pure
  enough glass would allow fiber optic cables to
  work hence the internet.
• All are Americans, though Kao is also British and
  Boyle also Canadian

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Making Images

• Photographic plates, use multiple filters and
  combine for color images.
• CCD -- resolution now about as good and linearity
  far better data is DIGITAL and can be processed
  more easily to get more precise results.

Star cluster R136 in the Large Magellanic Cloud,
from
ground, original HST, processed HST,
corrected HST
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Spectrometers Polarimeters

• Most telescopes spend most of their time
  spreading the light out into all frequencies
  SPECTROSCOPY gives FAR MORE DETAILED INFORMATION
  than IMAGING.
• Temperatures Composition and
  abundances
• Pressures Velocities
  (Doppler shift)
• Rotation Magnetic fields
• POLARIMTERS Special materials can rotate
  different linear polarizations by different
  amounts and allow weak polarizations to be
  detected.

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Spectrograph Spectra

• Diffraction gratings, not prisms, actually used

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Timing Studies

• Along with IMAGING and SPECTROSCOPY measuring
  changes with time is a very important use of
  telescopes in brightness (light curve -- here
  Mira, P 331 days) positions (astrometry)
  spectra (binary stars and planets around stars)

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RADIO TELESCOPES

• INTERFEROMETRY OVERCOMES POOR RESOLUTION
• Single dish radio telescopes can't resolve
  better than ?20 arcsec --- Georgia as seen from
  the Moon.
• Combining and interfering signals can produce
  much better resolution the EFFECTIVE APERTURE
  becomes the MAXIMUM SEPARATION (BASELINE)
  between the telescopes. Very Large Array 27
  telescopes, up to 30 km baselines, so ?
  0.1 (at 1 cm) GSU from the Moon.
• Very Long Baseline Array 10 telescopes, 6000 km
  baseline so ? 0.0004 (at 1 cm) --- you from
  Moon.
• VLBI from Space HALCA -- up to 21,000 km
  baselines so ? 0.0001 arcsec (at 1 cm).

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Single Dishes Green Bank Telescope Arecibo
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Principle of Interferometry

• Constructive and destructive interference depend
  on the exact direction of incoming waves. As
  earth rotates, different path lengths are
  sampled, so images can be built up.
• Resolution equals that of telescope w/ aperture
  separation

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The VLA (Very Large Array)
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Optical Interferometry

• The same techniques can now be used in the
  optical band, where it is much more difficult to
  combine and interfere the signals. (A little
  easier in infrared.)
• CHARA ARRAY is the largest optical
  interferometer a GSU project, headed by Prof.
  Hal McAlister, its longest baseline is 330 m
  Light from 6 one-meter
  aperture telescopes can be combined to give
  resolutions of about 0.0004 arc sec
  (like VLBA) Measure sizes
  of stars (directly), separations between
  nearby stars, and even make images of
  nearby giant stars!

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CHARA Array on Mt. Wilson, CA
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First Results of the CHARA Array
Regulus, a nearby hot star had its size, shape
and temperature(s) accurately measured by CHARA.
Its fast rotation makes it much fatter
at its equator and it is also hotter at the
poles, so its brighter there. Compared to Sun.
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SHORT WAVELENGTH ASTRONOMY

• UV, X-ray and Gamma-ray astronomies are newer
  must be done in space, above the blocking
  atmosphere, so started in the 1960s
• Hubble Space Telescope (HST) works in the UV,
  along with IUE, FUSE, EUVE and others -- doing
  both imaging and spectroscopy.
• X-ray missions require grazing incidence mirrors
  as X-rays cant be focused current missions
  Chandra and XMM-Newton (past UHURU, Exosat, ASCA,
  Einstein)
• Gamma-rays cant be focused collimators and
  special detectors are used CGRO now HETE,
  SWIFT, FERMI
• IR astronomy also better done from space Spitzer
  is currently up there, but mountains and planes
  work too.

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Hubble Space Telescope
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X-ray Focusing
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Chandra X-ray Observatory
false color image of the supernova remnant Cas A
1 resolution!
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Compton Gamma-Ray Observatory
CGRO launched from shuttle Atlantis Image of the
blazar 3C 279
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Spitzer Space Telescope
4, 8, 24 ?m false color composite, of M81