Post-Detection Wavefront Distortion Compensation

When dealing with a relatively bright astronomical object it is possible to obtain nearly diffraction limited images despite turbulence effects, by post detection processing of a nonstatistical nature, i.e., unlike the Knox-Thompson/Labyrie speckle imaging methods. The key to this concept lies in forming a set of narrow spectral band short exposure images of the target object while at the same time passing the balance of the white light from the target through a short exposure wavefront sensor. The wavefront sensor is of the Hartmann type and uses an array of lensletts to form a corresponding array of short exposure images of the target. Wavefront tilt on each lenslett, which manifests itself in the recorded array of images as a shift in the position of the image, can then be assembled into an estimate of the total wavefront distortion at the time of the short exposure. This allows the OTF (n.b. not just the MTF but rather the OTF) for the short exposure narrow spectral band image to be calculated, so that the recorded short exposure narrow band image can be compensated for the effect of wavefront distortionpost detection compensation. It is calculated that for a target of magnitude rn = 7.5 or brighter, the wavefront sensor data will have a sufficient signal-to-noise ratio. To overcome a signal deficiency problem in the post detection compensated image a number of such images would have to be added. To determine the number that have to be added, we have to multiply the number of such narrow band short exposures that would have to be combined if there were no turbulence induced wavefront distortion by the factor (D/ro)2.