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Method for Accurate Determination of Off-Axis PSFs Using Crowded Star-Field Observations

This study presents a robust method for accurately determining off-axis point spread functions (PSFs) in crowded star fields. It utilizes both on-axis PSF data and calibration images from faint guide stars to derive off-axis PSFs at various distances and azimuthal angles. Through a combination of deconvolution and convolution techniques, the research estimates the mean Strehl ratio and full-width half maximum (FWHM) for scientific observations. The method shows promise for future enhancements, including the use of laser guide stars and more comprehensive simulations to improve PSF accuracy in challenging observational conditions.

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Method for Accurate Determination of Off-Axis PSFs Using Crowded Star-Field Observations

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  1. A Method for Determining AOOff-Axis PSFs Eric Steinbring (UCSC, UCO/Lick, CfAO, DEEP) Sandra Faber, Sasha Hinkley

  2. Scientific Observations CFHT Pueo/KIR image of 3C68.2 field (~ 1 square-arcminute FOV) Guide-star is at the faint limits for full correction Scientific target is 20 arcseconds away Suitable PSF stars are rare

  3. Calibration Observations CFHT Pueo MONICA/KIR observations of M5 (~ 1 square-arcminute FOV) Unsaturated image of guide-star in center of field Many PSF stars in field One set of J, H, K images per night

  4. Generating Off-Axis PSFs • Obtain on-axis PSF for each scientific observation • Obtain crowded star-field calibration image • Deconvolve calibration image with its on-axis PSF • Convolve the residual calibration image with the on-axis PSF for the scientific observation

  5. Off-Axis PSFs PSFs at 5, 10, 20, 30 arcseconds off-axis PSFs at 20 arcseconds off-axis at different azimuthal angles

  6. Average Off-Axis PSF The average PSF at 20 arcseconds off-axis Correct mean Strehl and FWHM for scientific observation Does not account for azimuthal anisoplanatism

  7. Improved Test of the Method • Obtain several star-field calibration images with a faint guide-star • Obtain calibration PSFs for bright guide-stars at zenith and star-field airmass

  8. Lick AO/IRCAL Observations of M15 Faint guide-star calibration images in J, H, and K over 2 nights (30 ~ 1 arcminute long) Bright guide-star PSFs at zenith and cluster airmass Estimates of r0 from WFS information (Don Gavel, LLNL)

  9. Isoplanatic Angle Single layer turbulence with D/r0>>1 Obtain on-axis Strehl (J: 4%, H: 6%, K: 10%) and isoplanatic angle (J: 150 arcsec, H: 80 arcsec, K: 60 arcsec) as a function of airmass Obtain estimate of mean altitude of layer (1000 – 3000 m)

  10. Modeling / Simulations Single layer turbulence modeled with a synthetic phase-screen Inputs are r0 and mean altitude of layer Model number of corrected modes, subapertures of WFS and number of degrees of freedom in DM (Sasha Hinkley, UCSC/CfAO)

  11. Summary • A single observation of a crowded star-field can determine off-axis PSF for entire night given on-axis PSF information • A single low-altitude layer is sufficient to explain isoplanatic angle for Lick AO

  12. Future Work • Better determination of Strehl in crowded star-fields • Work repeated with laser guide-stars • More sophisticated simulations (Miska Lelouarn, UCSC/CfAO) • Method would benefit greatly from an independent measurement of mean altitude of turbulence (SCIDAR / DIMM for Lick or Mauna Kea?)

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