NGAO Photometry

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NGAO Photometry

• Matthew Britton, Rich Dekany, Ralf Flicker,
  Claire Max, Chris Neyman, and Knut Olsen

NGAO Meeting 6, April 2007
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Why is SCAO Photometry Hard?

• The Cn2 and wind profiles vary significantly on
  timescales of minutes to hours.
• This temporal evolution induces variations in the
  PSF in time, field location, and wavelength.
• Variability in the PSF presents significant
  challenges in performing photometry on
  observations of crowded fields and resolved
  objects.

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A Simple Example

• Seeing Limit

With AO
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Pathways to PSF Variability

• Residual guide star wavefront errors
• Anisoplanatism
• Scintillation
• Telescope and instrument optics

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Residual Guide Star Wavefront Errors

• Fitting error
• Servo error
• Measurement error

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Strehl vs. Wavelength
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Anisoplanatism
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Scintillation
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Telescope and Instrument Optics

• Non-common path errors
• Changing gravity load
• Field rotation

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Strehl Ratios vs. Time
c.f. Christou, Gladysz
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Astronomical Applications

• Planetary astronomy
• Volcanoes on Io, rings of Uranus, Titans
  clouds, asteroids.
• Stellar photometry
• Sag A flares, followup of Ogle-type surveys,
  stellar population surveys.
• Extragalactic astronomy
• Spatially resolved spectroscopy of galaxies.

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Photometry

• We can do aperture photometry when sources are
  sufficiently well separated so that there is no
  PSF blending.
• If we knew the PSF delivered by the AO system, we
  could deconvolve our images and IFU spectra and
  do photometry directly.
• Observe a calibration star or field
• Stinebring
• Find some way to estimate the PSF

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PSF Estimation Methods

• Guide star PSF estimation from telemetry data
• PSF estimation from image data
• PSF estimation from Cn2 profiles

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Guide Star PSF Estimation from Telemetry Data
OTF approach to estimate in-band errors from
telemetry data, out of band errors by integrating
Kolmogorov power spectrum
Curvature Sensors

• Jean Pierre Veran, CFHT

Shack Hartman Sensors

• Laurent Jolissaint, Gemini
• Mike Fitzgerald, Lick
• Jose Marino, National Solar Observatory
• Yann Clenet, VLT

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PSF Estimation from Image Data

• IDAC, Mistral, AIDA
• Simultaneously solve for PSF and deconvolved
  image
• Constrained to operate within an isoplanatic
  patch
• Suffer from many degrees of freedom
• Starfinder, parametric blind deconvolution, Swan
• Parametric models can account for anisoplanatism
• MTF approach
• A one dimensional model

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PSF Estimation from Cn2 Profiles

• Use measured guide star point spread function and
  measured Cn2 profile to estimate field dependent
  PSF.
• Factors the problem of guide star PSF estimation
  from that of field dependent PSF evolution due to
  anisoplanatism.
• No free parameters in estimating the field
  dependent PSF

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A Crowded Field Observation
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Modeling a Crowded Field
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Deconvolution with a Field Dependent PSF

• Divide image into subfields with dimension of
  order the isoplanatic patch size
• Use local estimate of the PSF to perform
  deconvolution using some method (FFT,
  Richardson-Lucy, matrix inversion)

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Deconvolution Pipeline

• Find a way to successfully estimate the guide
  star PSF from Shack Hartmann telemetry data
• Use Cn2 profiles and estimated guide star PSF to
  derive the field dependent PSF
• Apply deconvolution algorithm using field
  dependent PSF estimates
• This method can be automated

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Photometry with Advanced AO Architectures

• MCAO
• MOAO (?)

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Photometric Considerations for NGAO

• SC LTAO architecture will deliver the same
  variable PSF as SCAO.
• MCAO architecture will deliver a PSF that is more
  stable over the field, but still varies in time.
• Photometric performance for wide field MOAO mode
  is really an open question.

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Working Groups Recommendations

• Secure a turbulence profile monitor for Keck
  Observatory
• Consider an auxiliary PSF monitoring camera for
  NGAO
• Consider assessing current level of photometric
  performance using NIRC2 and/or OSIRIS
• Consider providing a deconvolution pipeline for
  NGAO