Angel lecture 4 Sept 21

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Angel lecture 4Sept 21

• QED view of astronomical optics
• Simple optical systems, interferometry

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Ideal diffraction limited images by QED GMT
same FWHM as 24.4 m filled circle24.4 m Airy
GMT
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Starting point

• Per homework
• star photon detection probability for circular
  aperture parabolic telescope
• classic Airy diffraction pattern
• FWHM of pattern in focal plane corresponds to
  angle l/D on sky HST resolution

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Example of more complex telescope aperture

• Giant Magellan Telescope (GMT)
• Next generation ground telescope
• Being designed by Arizona, Carnegie
  Observatories, CfA, MIT, Michigan, Texas
• Aperture synthesized from 7 X 8.4 m mirrors

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How do you get paths across 25 m aperture
accurate enough for diffraction limit?

• Need to control whole surface to fit paraboloid
  to accuracy ltlt 1.6 mm wavelength
• Steel structure supporting primary segments bends
  much more than this in wind gusts
• Hard to servo position of 18 ton segments to 100
  nm
• SOLUTION
• Use fact that paths reflect also off secondary
  mirror before coming to focus
• Make correction at much smaller, more agile
  secondary
• Secondary segments are 1.1 m diameter

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View of secondary from infrared instrument

• Secondary segments seen against sky
• In thermal infrared, beams or baffles glow much
  brighter than sky
• Need to eliminat background photons from sky that
  just add noise at detector.

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Exoplanet detection with GMT

• take advantage of very high angular resolution
• l/D 0.014 arcsec at l1.65 mm
• For reference, 1 AU at 10 pc 0.100 arcsec
• But how do we get very high contrast?
• Jupiter at 1 AU is 10-8 of sun

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Deform shape of secondary to change point spread
function (PSF)

• Waves across secondary will place added light at
  spot in halo
• Translation of wave across aperture changes phase
  of spot
• Match spot amplitude and angle to null local psf
  pixel
• superpose waves to match in detail each pixel of
  the diffraction pattern

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Speckles induced in a 1.6 micron real star image
from the MMT AO system
3 wave ripple induced by deformable secondary
across 6.5 m diameter Amplitude about 200
nm Little vectors biased in one direction, dont
curl up
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suppression of 180 degrees of diffraction pattern
with shape deformable secondary (Codona)
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Atmospheric wavefront aberration must also be
corrected dynamical deformation of secondary-
adaptive optics

• Thermal variations in atmosphere cause
  differences in path length across primary mirror
• Refractive index of STP air n 1.0003
• Pressure constant, so from gas law
  dT/Tdr/r
• dn/n n so dn10-6/K
• Representative atmospheric case
• boundary between air streams with DT 1 K and 1 m
  rms amplitude roughness on 1 m scale
• Yields 1mm rms path length variations on 1 m
  scale
• Actual boundary has variations on all scales
  given by Kolmogorov turbulence amplitude as
  x5/6

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path variations taken up by deforming shape of
secondary

• Done at MMT, LBT and GMT
• Typical turbulence is fixed screen moving by at
  wind speed like an invisible cloud carried by
  wind
• Speed 20 km/sec
• For 1 m turbulence time scale is 50 msec
• In practice servo needs 1 msec updates to follow

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How can we actually know the arrow direction for
a detected photon?

• We cant
• But we can if the aberration remains fixed, while
  a number of photons are detected
• Method of interferometry

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Numerical models of suppression with deformable
secondary