AST Senior Review Major Recommendations

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The SIFS IFU(courtesy CL de Oliveira, LNA)?
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Detectors for Opical/near-IR(current)?

• Photon Counters
• Image tube TV camera real-time discrimination
  (not solid state)?
• eg IPCS c1980s QE 15
• CCDs now dominate - Hi QE but
• Integrate signal on detector no time resolution
• Finite read-noise
• Finite read-time
• EMCCDs new generation of Photon Counters
• CCD-like QEs
• V. high frame-rates

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In some respect CCDs were a step backwards

• Photon Counting
• 1 digitized event 1 detected photon
• Frame rate 50 Hz
• Could see signal build up with time
• Terminate exposure based on continuous feed-back
• High frequency time resolution
• Zero dark noise
• Zero read noise
• But
• Low DQE small dynamic range easily saturated

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DQE - the key to gooddetectors

• Detector quantum efficiency - the fraction of
  incident photons detected - is the key measure
  for the effectiveness of a detector
• Traditional photographic plates, while large in
  size, have DQE of only about 5
• CCDs and similar semiconductor devices can have
  DQE as high as 90 (though wavelength dependent)?
• Like having a telescope with 9 times the
  collecting area

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Example CCD DQEs
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CCDs

• CCDs combine photon detection with integration
  and multiplexing
• Incident photons excite charge carriers which are
  stored and integrated in a capacitor
• CCDs are also uniquely effective in transferring
  charge from 2D to 1D
• charge clocked from pixel to pixel and read out
  at fixed point
• ideal for multiplexing

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CCD Array Camera

• Semiconductor fabrication limits the size of a
  CCD detector
• To get a large area need to mosaic detectors
  together

Subaru Mosaic CCD Camera
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Near-IR Detectors

• CCDs use Silicon as their substrate
• Valance to conduction bandgap in silicon is 1.1eV
  so restricted to detecting photons with
  wavelength lt 1 micron
• Need different materials for infrared
• InSb for 1 to 5 micron, HgCdTe for 1 to 2.5
  micron
• Detector elements bonded to Si CCD system to
  provide multiplexing readout

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IR Arrays vs. Optical

• IR arrays are smaller, more expensive (by factor
  of 10/pixel)?
• Readout has to be faster because of higher
  backgrounds
• Use of different materials can push to longer
  wavelengths
• More difficult to work with, less helpful
  characteristics, more expensive
• At longest wavelengths have to stress the
  detector to produce lower energy band gaps

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UVOIR Astronomy

• Definition
• UVOIR the "UV, Optical, Near-Infrared" region
  of EM spectrum
• Shortest wavelength 912 Å (or 91.2 nm) --- Lyman
  edge of H I interstellar medium is opaque for
  hundreds of Å below here
• Longest wavelength 3µm (or 3000 nm) --- serious
  H2O absorption in Earth's atmosphere above here
• Ground-based UVOIR
• 0.3µm (or 300nm) lt ? lt 2.5µm (or 2,500nm)?

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UVOIR Astronomy

• Uniqueness
• Best developed instrumentation
• Best understood astrophysically
• Highest density of astrophysical information
• Provides prime diagnostics on several of the most
  important physical tracers.
• gt UVOIR observations/identifications are
  almost always prerequisites to a thorough
  understanding of cosmic sources in other EM bands.

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Observational Priorities

• Is astronomy driven by discoveries rather than
  theoretical insights?
• Direction of field shaped by observations in
  vast majority of instances.
• Few important astronomical discoveries were
  predicted many were actually accidental
• If this is true (?) then the development of
  instrumentation (which includes telescopes)
  should play a major role.

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List of accidental discoveries(not my own!)?

• Uranus
• Expanding universe
• Pulsars
• Supermassive black holes/AGNs
• Large scale structure
• Dark matter in spiral galaxies
• X-ray emitting gas in clusters of galaxies
• Gamma ray bursts
• Extra-solar planets
• High redshift evolution of galaxies
• HST contributions were actually hindered by
  theoretical prejudice. A deep pencil-beam survey
  was delayed by 5 years.

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Counterexamples theory-driven discoveries

• Neptune
• General relativistic distortion of space-time
  near Sun
• 21 cm line of HI
• Helioseismology
• Cosmic microwave background
• Question
• Is Observational Astronomy a Science?
• (strictly speaking)

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ConclusionTechnology drives Discovery

• Key technology development for UVOIR astronomy
• 17th century telescopes
• 19th century spectroscopy, photography, quality
  lens making, large structure engineering
• 20th century large mirror fabrication,
  electronic detectors, digital computers, space
  astronomy
• Since 1980 array detectors

Detectors funded by the Military Industrial
Complex Instrumentation developed by you and me!
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Telescope size determines ultimate sensitivity

• Diameter doubling time 45 years
• Largest telescopes now 8-10m diameter
• Collecting area of 10m is 4106 that of the
  dark-adapted eye
• In planning 20m to 40m class
• For a given technology, cost ? D2.6
• Cost is roughly proportional to mass
• Even using new technologies, next generation of
  large ground-based telescopes will cross the 1
  billion threshold.

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The Future?

• NB Number of ground-based telescopes is NOT
  inversely proportional to their size
• Almost as many 8m telescopes as there are 4m
  telescopes (8)?
• How many 30m telescopes are there going to be in
  the next 50 years? (at US1B a pop)?

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cgs units get used to them!

• c ?? 3.1010 cm.s-1
• E h? (ergs)?
• F L/4?d2
• G 6.67.10-8 (cgs)?
• h 6.626.10-27
• eV 1.602.10-12 ergs
• Luminosity of Sun 4.1033 ergs/sec
• Mass of the Sun 2.1033 grams

Power in Watt 107 ergs.s-1 Surface Brightness
in ergs.s-1.cm-2.arcsec-2. Å-1 Flux density in
Jansky (Jy) 10-27.watt.m-2.Hz-1
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Flux measurements

• Sensitisity
• Can be roughly defined as the faintest source
  measurable --- it is not simply a matter of the
  size of the photon collector.
• It is instead a signal-to-noise (SNR) issue
• SNR measured value / uncertainty and is
  dependant on many things, including
• Structure of source (point vs. extended)?
• Nature of luminous background surroundings
• Foreground absorption
• Telescope instrument throughput
• Characteristics of detectors (quantum efficiency,
  noise)?

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SNRs in Astronomy

• Fundamental limit set by photon statistics
• Shot noise ie SNR lt vN, where N no. of
  detected source photons
• Typical SNR's in Astronomy
• Best precision SNR 1000 (0.1 error)
• Low by lab standards! Problems difficulty of
  calibration faintness of interesting sources.
• Typical "good" measures SNR 20-30
• Threshold detections SNR 5-10
• Velocity (red-shift) measures SNR gt3

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Noise Sources(an incomplete list)?

• Detector Noise (CCDs)?
• Read-noise (rms 3-10e-1/read)?
• Dark noise (3.10-4 e-1/s/pixel)?
• Determined by Temperature of detector
• Background Noise (Diffuse)?
• Artificial light pollution
• Earth's atmosphere
• Ecliptic scattered sunlight
• Scattered Galactic light
• Background Noise (Discrete)?
• Exclusion zone around bright stars caused by
  scattered light within instrument
• Source "confusion" caused by diffractive blending
  of multiple faint sources

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Magnitude System

• An ancient and arcane, but compact and by now
  unchangeable, way of expressing brightnesses of
  astronomical sources.
• Magnitudes are a logarithmic measure of spectral
  flux density (not flux!)?
• Monochromatic Apparent Magnitudes
• m? -2.5 log10 f? - 21.1
• where f? is in units of erg.s-1.cm-2.Å-1
• A system of monochromatic magnitudes per unit
  wavelength
• -1 mag is factor of 2.5 -5 mag is factor of 100

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Magnitude Normalization

• Normalization is chosen to coincide with the zero
  point of the widely-used visual or standard
  broad-band V magnitude system
• i.e. m?(5500Å) V
• Zero Point fluxes at 5500Å corresponding to
  m? (5500Å) 0, are (Bessell 1998)?
• f?0 3.63.10-9 erg.s-1.cm-2.Å-1 or
• f?0 3.63.10-20 erg.s-1.cm-2.Hz-1 or
• 3630 Janskys
• f?0/h? 1005 photons.s-1.cm-2.Å-1 is the
  corresponding photon rate per unit wavelength

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Surface Brightness

• Surface Brightnesses (extended objects)
• µ? m? 2.5 log10?
• where m? is the integrated magnitude of the
  source and ? is the angular area of the
  source in units of arcsec2.
• 1 arcsec2 2.35.10-11 steradians
• µ is the magnitude corresponding to the mean flux
  in one arcsec2 of the source.
• Surface brightness in flux density units
• (erg.s-1.cm-2.Å-1.arcsec-2)?

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Absolute Magnitudes

• M m - 5 log10(D/10), where D is the distance
  to the source in Parsecs (pc)?
• 1pc 3.258 light-years or 3.086.1013 kilometers
• 1 pico-pc a good days walk
• M is the apparent magnitude the source would have
  if it were placed at a distance of 10 pc.
• M is an intrinsic property of a source
• For the Sun, MV 4.83

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Source characterization

• Luminosity (L)?
• Power (energy/sec) radiated by source into 4?
  sterad
• Units ergs.s-1
• Flux (f)?
• Power from source crossing normal to unit area at
  specified location a distance D from source
• f L/4?D2 if source isotropic, no absorption
• Units ergs.s-1.cm-2
• Surface Brightness (I)?
• Power per unit area per solid angle
• Units ergs.s-1.cm-2.sterad-1 (f I.??)?
• I is independent of distance if source
  remains resolved

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Point Source Sensitivity

• Faintest UVOIR point sources detected
• Naked eye     5-6 mag
• Galileo telescope (1610)     8-9 mag
• Palomar 5m (1948)     21-22 mag (pg)         
                              25-26 mag (CCD)?
• Keck 10m (1992)     27-28 mag
• HST (2.4m in space, 1990) 29-30 mag
• NB current optical detectors approach 100 QE
• ie We can't improve sensitivity via detector
  development. Improvements require new
  instrumentation.

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Spatial Resolution

• Fundamental limit governed by diffraction in
  telescope/instruments
• Min. image dia. (?min) 2.2?/D rads(diffr.
  limit)?
• where D is the dia. of the telescope
• At 5500Å ?min 28/D(cm)?
• Inside Earth's atmosphere, turbulence strongly
  affects image diameter.
• Resulting image blur motion is called "seeing",
  and typically yields ?atm0.7-1.5
• i.e. spatial resolution in most instances is
  governed by the atmosphere, not the telescope.
• Good site Good environmental control
• Good AO approaches diffraction limit

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Spectral Resolution

• Theoretical maximum governed by diffraction limts
  set by optical components
• Practical limit set by photon rates
• High resolution devices are typically
  photon-starved (except for Sun).
• ID's, surveys, classification at low resolution
• 10-500Å or 10 ltRlt 500
• Physical analysis at moderate-to-high resolution
• 0.1-10Å or 500 ltRlt 50,000
• Highest to date 0.01Å or R 500,000

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Basic Lens formulae
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Basic Mirror formulae
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Optics and Focus

• Optics below represents a doublet lens
• Parallel rays from the left are made to converge
• Location where the rays cross is the focal
  point
• Distance from the fiducial point in the lens is
  the focal length (fl)?

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Images

• Object Plane
• Image Plane
• These are conjugates of each other
• Conjugate distances are
• s1 s2
• Lens formula
• 1/s1 1/s2 1/f
• Magnification (m) is given by
• m s1/s2

Object
s1
s2
Image
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Focal length and focal-ratio (f/)?

• Effective focal length (EFL fl) is the distance
  from the optics to the focal point
• f/ is the ratio focal length to the optic
  diameter (f/ d/fl)?
• f/1 is fast (v.difficult to control aberrations)?
• f/30 is slow (simple optics)?

d
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Plate Scale

• For a given optic with EFL fl, the image plane
  scale is given by
• P.S. 1c/fl (radians/m)?
• 206265 /fl (arcsec/mm)?
• For instance, a telescope with an EFL 10m (eg
  1.2m _at_ f/8), plate scale is
• 206265/104 20.6 arcsec/mm
• However, a telescope with an EFL 170m (eg 10m
  _at_ f/17), plate scale is
• 206265/1.7.105 1.2 arcsec/mm
• ie If you want a wide-field you have to have a
  small telescope

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Entendu

• Entendu A? (area solid angle)?
• Entendu is conserved for any optical system
• ie Conservation of Energy
• However, entendu can be lost in fibre systems
• High entendu is a figure of merit for an optical
  system
• Equivalent to more energy (or information)
  transport
• Telescope have generally to trade
• High A with High ?
• Spectrographs try to maximize A?.R

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Refraction
?1
n1

• Snells Law n1 sin(?1) n2 sin(?2)?
• ? n1 refractive index in region 1
• n2 refractive index in region 2
• where n c/v ?vacuum /??medium

?2
n2
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Refraction and Total Internal Reflection
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Constructive Interference
Destructive Interference
N2 interfering beams
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N gtgt 2 interfering beams n1 (eg Grating
Spectrograph)?
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Diffraction grating (N 100,000)?
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Michelson Interfermeter(N 2 interference n
gtgt1)?
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Fabry-Perot(N 500 n 100)?