Curvature sensing - how does it work?
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Curvature sensing - how does it work? |
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Curvature wavefront sensing looks at the intensity between pupil image and image plane. A curved wavefront comes to focus before and after the nominal focal plane and thus is brighter or dimmer in an out-of-focus image. It must be sensitive on both sides of the focus to calibrate the scintillation. |
Curvature wavefront sensingA curvature wavefront sensor measures the intensity I1 in an intrafocal plane and the intensity I2 in an extrafocal plane and compares these intensities to determine the curvature of the wavefront. The normalised difference(I1-I2)/(I1+I2) is used to reconstruct the wavefront. An oscillating membrane mirror is used to modulate the location of the plane being imaged on a single detector. During one half-cycle of the membrane motion, the detector records the intrafocal distribution of light, and during the other half-cycle it records the extrafocal distribution. |
A realistic curvature signalIt presents a Kolmogorov atmospheric wavefront distortion, the infrared focal plane image, intrafocal and extrafocal images and the curvature signal. |
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 wavefront distortion |
 infrared focal plane image |
 intrafocal image |
 extrafocal image |
 curvature signal at high resolution |
 curvature signal binned into 60 subapertures |
Simulation parameters:0.66 arcsec seeing (at 500 nm), sensing wavelength = 700 nm (monochromatic), infrared image wavelength = 2.2 µm, 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. |
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