Analysis and Development of a Low-Order Wavefront Sensor for Exoplanet Detection Applications

Abstract

In light of the recent high-profile planet-finding missions such as Kepler, the hunt for exoplanets has become an area of increased interest and research. In order to locate an Earth-mass planet in the habitable zone of a Sun-like star, 1010 contrast must be achieved. Various means exist for achieving this contrast; however, this assumes that the light collected by the telescope is perfect. In practice, the wavefronts collected tend not to be perfect but slightly aberrated. These aberrations decrease the achievable contrast of the imaging system. Therefore, in addition to striving to find ways to increase the contrast of images, it is necessary to actively investigate manners in which the wavefront aberrations can be sensed and corrected. This thesis explores two methods of low-order wavefront sensing (LOWFS). The first method was proposed by Olivier Guyon in 2009 and consists of reimaging the core of the focal plane electric field to a defocused sensor. This thesis demonstrates that a defocused sensor plane is required such that the even-order aberrations can be sensed, only odd aberrations can be sensed using a focused sensor. A computational analysis showed that the focused sensor was highly effective at predicting odd aberrations in an ideal environment and in the presence of noise. The defocused sensor can only predict a maximum of 8 of the 12 aberrations considered. In the presence of noise, the efficacy of the Guyon’s defocused sensor deteriorated even more showing that it is not an effective method for sensing low-order aberrations. The second LOWFS method explored was a novel technique which utilized interferometry to extract the aberrations. The sensing equations were developed and a one-dimensional simulation performed. This simulation was able to retrieve all aberrations in the wavefront with relative accuracy, including two which were very similar to each other. This method is still in its development; however, it shows great promise for the effective estimation of low-order aberrations.