Testbed demonstration of high-contrast coronagraph imaging in search for Earth-like exoplanets

Abstract

Direct imaging of an Earth-like exoplanet requires starlight suppression with a contrast ratio on the order of 1 × 10 −10 at small angular separations of 100 milliarcseconds or less in visible light with more than 50 nm bandwidth. To our knowledge, the technology needed to achieve the contrast and stability has not been demonstrated as of January 2019. The science requirements for near future National Aeronautics and Space Administration (NASA) missions such as James Webb Space Telescope (JWST)'s Near Infrared Camera (NIRCam) coronagraph and Wide-Field InfraRed Survey Telescope (WFIRST) Coronagraph Instrument (CGI) are at least 10 times short. To investigate and guide the technology to reach this capability, we built a high contrast coronagraph testbed at NASA's Jet Propulsion Laboratory (JPL). Titled the Decadal Survey Testbed (DST), this state-of-art testbed is based on the accumulated experience of JPL's High Constrast Imaging Testbed (HCIT) team. Currently, the DST hosts a Hybrid Lyot Coronagraph (HLC) with an unobscured, circular pupil. The DST also has two deformable mirrors and is equipped with the Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to sense and correct the dynamic wavefront disturbances. In this paper, we present up-to-date progress of the testbed demonstration. As of January 2019, we repeatedly obtain convergence below 4 × 10 −10 mean contrast with 10% broadband light centered at 550 nm in a 360 degrees dark hole with a working angle between 3 λ/D and 8 λ/D. We show the key elements used in the testbed and the performance results with associated analysis.

Wavefront sensing and control in space-based coronagraph instruments using Zernike’s phase-contrast method

“…For instance, as mentioned above, the DM electronics that we used for all of the experiments presented here exhibited significant noise and a few actuators were randomly changing by up to 1 nm in surface height. While we know from experience that uncommanded motions can be solved by tracking down faults in the DM electronics and that DM stability on the order of a picometer is possible, 39 having the capability to stabilize the DM in closed loop can help relax the long-term DM stability requirements. To demonstrate this capability, we set up an experiment (see Fig.…”

“…The DST has demonstrated two-sided dark holes with raw contrast of ∼10 −10 using two DMs and a Lyot coronagraph. 39 We installed and tested the ZWFS within the DST coronagraph instrument in a configuration that is similar to the HabEx and LUVOIR design. Figure 1 shows a schematic of the DST with the WFS beam path highlighted in green.…”

“…We carried out the following work with two experimental Boston Micromachines 2K MEMS DMs 40 with 50 actuators across the active area (47.5 illuminated). This particular set of DMs was relatively low grade compared to those used in previous studies 39 and had several actuators that were nonresponsive or exhibited anomalous behavior. Parallel efforts are underway to install higher quality MEMS DMs on DST and improve their control electronics.…”

“…In past experiments, 39 the FPM was a Lyot coronagraph mask that blocked the core of the PSF and allowed off-axis light to continue to the remainder of the coronagraph instrument and science camera. In this work, we replaced the coronagraph FPM with a reflective ZWFS mask with a circular depression (or "dimple") in a 100-nm-thick aluminum coating that applied a phase shift, θ ¼ 4πh Z ∕λ 0 , to the core of the PSF upon reflection, where h Z is the depth of the dimple (see Fig.…”

High-contrast integral field spectrograph (HCIFS): multi-spectral wavefront control and reduced-dimensional system identification

“…1, mainly consists of a coronagraph and an adaptive optics (AO) system: the coronagraph [4][5][6][7][8] suppresses starlight that hides the planet signals, and the AO system [9][10][11] corrects the residual starlight speckles caused by Earth's atmospheric turbulence or the telescope's optical aberrations. Typically, a high-contrast imaging instrument can achieve 10 −3 contrast within the region of interest in ground-based telescopes [12][13][14][15] and is predicted to allow 10 −9 − 10 −10 contrast in future space telescopes [16][17][18]. To study the chemical composition of a planet's atmosphere, a high-contrast instrument needs to operate in a wide bandwidth to obtain the planet's absorption spectrum.…”