A CCD BASED CURVATURE WAVEFRONT SENSOR FOR ADAPTIVE OPTICS IN ASTRONOMY

Adaptive Optics (AO) is a technique for compensating the distortions introduced by atmospheric turbulence in astronomical imaging.  To measure the distortions a wavefront sensor is needed.  There are a number of fundamentally different types of wavefront sensors, one of which is the curvature wavefront sensor.  At the European Southern Observatory (ESO) in Garching, Germany, several adaptive optics systems using curvature wavefront sensors are being developed for the Very Large Telescope (VLT) and the VLT interferometer (VLTI).  Curvature AO-systems have traditionally used avalanche photodiodes (APDs) as detectors due to strict requirements of very short integration times (200 microsec) and very low readout noise.

Advances in charge-coupled device (CCD) technology motivated an investigation of the use of a specially designed CCD as the wavefront sensor detector in a 60-element curvature AO system.  A CCD has never been used before as the wavefront sensor in a low light level curvature adaptive optics system.  This thesis shows that a CCD can achieve nearly the same performance as APDs at a fraction of the cost and with reduced complexity for high order wavefront correction.  Moreover the CCD has higher quantum efficiency and a greater dynamic range than APDs.  A readout noise of less than 1.5 electrons at 4000 frames per second was achieved.  A back-illuminated thinned version of this CCD can replace APDs as the best detector for high order curvature wavefront sensing.

  • A new CCD designed for curvature wavefront sensing
 (pdf-version download)
  • Computer simulation comparison of CCDs and APDs for curvature wavefront sensing
 (pdf-version download)
  • Talk given at ESO (Garching), CalTech (California) and the University of Heidelberg in 2001
 (pdf-version download)
  • A CCD based curvature wavefront sensor for Adaptive Optics in Astronomy
    (Ph.D. thesis, University of Heidelberg/Germany)
 (pdf-version download)

Computer simulation of curvature wavefront sensing:

  • (a) wavefront distortion
  • (b) intrafocal image
  • (c) extrafocal image
  • (d) curvature signal at high resolution
  • (e) curvature signal binned into 60 subapertures

Simulation parameters:

  • 0.66 arc sec seeing (at 500 nm)
  • sensing wavelength = 700 nm (monochromatic)
  • out of focus distance = 25 cm
  • telescope focal length = 400 m
  • telescope diameter = 8 m with 14 % obscuration from 1.12 m diameter secondary
  • Photon noise has not been simulated - all signals are "infinite" light level.
Images of the CCD Images of the detector mount

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Reinhold Dorn, Apr/23/2002      (Webdesign by EVI)