The GLAO module for HAWK-I



HAWK-I is a wide field imager currently under development at ESO which will work from 0.85 to 2.2 microns. This 7.5 by 7.5 armin imager will be first installed on the VLT without any AO system but with the Deformable secondcary mirror and the 4 LGSs tool installations it is foreseen to add an Ground Layer Adaptive Optics module to the imager.
The goal of the AO module is to reduce by 15 % in Y and 30% in Ks band the diameter collecting 50% of the encircled energy (for 1 arcsec visible seeing conditions) over the entire FOV

Concept baseline:
  • Wavefront tomography using 4 sodium LGSs
  • Four Shack-Hartmann WFS for LGSs
  • Ground layer correction: one Deformable Secondary Mirror
  • On-axis NIR tip-tilt NGS using the HAWK-I detector
  • As alternative, off-axis visible tip-tilt NGS
  • One NGS WFS for DSM commissioning & maintenance

    		•  Functional Block Diagram
    LGS wave front sensors:
  • Systems mounted on rotating structure (to keep WFS pupil position aligned with DSM M2 actuator patterns + LGS position rotation)
  • 4 pick up mirrors placed at 5.5 arcmin from HAWK-I center FOV
  • 90 degrees configuration
  • 32×32 sub-apertures WFSy
  • 8×8 pixel per sub-aperture (256×256 CCD, 24?m pixel size)
  • 4 arcsec FOV per sub-aperture (0.5 arcsec/pixel scale)
  • Each LGS WFS mounted on linear stage (altitude focus varying between 80 and 180 km)
  • Calibration fiber system available


    • Infrared Tip-tilt Sensing
  • HAWK-I camera size: 4k×4k, 7.5×7.5 arcmin, 0.1063 arcsec/pixel
  • Scientific frame readout: 2.6 sec without TT sensing (at 10?s/pix)
    		•  Field of view geometry
    For tip-tilt sensing (baseline):
  • Sub array of 16×16 pixel or 1.7×1.7 arcsec
  • Readout noise double correlated: 17 e- RON
  • 32 channels for scientific readout, 33rd for TT window
  • With TT sensing readout: 3.93 sec with 65Hz TT frame rate and 5.12 msec integration time
  • Photon limit due to science filters not studied yet
  • Location TT window: programmable via OS command, 1 px accuracy.


    • Visible Wave front sensor
  • Back-up solution without vignetting the HAWK-I FOV.
  • Accept up to 25 arcsec dithering
  • Need the HAWK-I rotation for NGS selection
  • NGS selected in annular FOV between 13.2 and 15.2 arcmin
  • A 50mm pick-up mirror reflects a 54’’ FoV to the TT NGS detector
  • 256×256 detector, 0.2 arcsec/pixel
  • Window size: 3” FOV, 16×16 pixel
    		• Opto-mechanical Design of the GLAO module
    DSM commissioning mode
  • 40x40 subapertures Shack-Hartmann WFS
  • 6x6 pixels per subaperture, pixel scale: 0.3 arcsec/pix
  • 1.8 arcsec FOV per sub-aperture
  • Focal Elongator in front of HAWK-I
  • Pixel scale up to 10 milli arcsec/pixel on a 10’’ FoV
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