Adaptive optics operation with two wavefront sensors in a coronagraph for exoplanet observations

Abstract

A stellar coronagraph system for direct observations of extra solar planets is under development by combining unbalanced nulling interferometer (UNI), adaptive optics, and a focal plane coronagraph^{1, 2, 3, 4, 5}. It can reach a high contrast as using lambda/10000 precision optics by lambda/1000 quality ones. However, a sufficiently high contrast has yet to be obtained for the experiment. It is thought that the remaining speckle noise at the final coronagraph focal plane detector is produced by a “non-common path error” of lambda/100 level, which is a wavefront error of differences between the coronagraph and a wavefront sensor (WFS) of adaptive optics, even when the WFS indicates lambda/1000 conversion. The non-common path error can be removed by the focal plane sensing method of wavefront correction by wavefront sensing at the final focal plane detector, although it has an issue of operation for very faint targets because of a slow feedback loop. In the present paper, we describe how our coronagraph system becomes practically higher contrast by upgrading the control method of adaptive optics with the WFS assisted by a focal plane wavefront sensing. Then, we control a wavefront error by two feedback loops, the first of which uses a WFS to make fast control for telescope optics deformation and the second of which uses a focal plane detector to compensate for the non-common path error with slow control. We show experiment results of the coronagraph system performance with both wavefront sensing methods.