Adaptive optics performance of a simulated coronagraph instrument on a large, segmented space telescope in steady state

Abstract

Directly imaging Earth-like exoplanets ("exoEarths") with a coronagraph instrument on a space telescope requires a stable wavefront with optical path differences limited to tens of picometers RMS during exposure times of a few hours. Although the structural dynamics of a segmented mirror can be directly stabilized with telescope metrology, another possibility is to use a closed-loop wavefront sensing and control system in the coronagraph instrument that operates during the science exposures to actively correct the wavefront and relax the constraints on the stability of the telescope. We present simulations of the temporal filtering provided using the example of LUVOIR-A, a 15-m segmented telescope concept. Assuming steady-state aberrations based on a finite-element model of the telescope structure, we (1) optimize the system to minimize the wavefront residuals, (2) use an end-to-end numerical propagation model to estimate the residual starlight intensity at the science detector, and (3) predict the number of exoEarth candidates detected during the mission. We show that telescope dynamic errors of 100 pm RMS can be reduced down to 30 pm RMS with a magnitude 0 star, improving the contrast performance by a factor of 15. In scenarios where vibration frequencies are too fast for a system that uses natural guide stars, laser sources can increase the flux at the wavefront sensor to increase the servo-loop frequency and mitigate the high temporal frequency wavefront errors. For example, an external laser with an effective magnitude of −4 allows the wavefront from a telescope with 100 pm RMS dynamic errors and strong vibrations as fast as 16 Hz to be stabilized with residual errors of 10 pm RMS thereby increasing the number of detected planets by at least a factor of 4.