Key Technologies
The Roman Coronagraph Instrument will premiere in space the key technologies needed by future missions to image and characterize rocky planets in the habitable zones of nearby stars. By demonstrating these tools in a system with end-to-end, scientific observing operations, NASA will reduce the cost and risk of a potential future flagship mission.
In particular, the Coronagraph Instrument will demonstrate five key technologies for future missions: ultra-precise wavefront sensing and control, large-format deformable mirrors (DMs), high-contrast coronagraphic masks, ultra-low-noise photon-counting electron-multiplying charge-coupled devices (EMCCDs), and data post-processing.
Wavefront Control
The baseline Roman Coronagraph design includes four active optics to control the wavefront: a fast steering mirror (FSM), a flat focusing mirror (FCM), and two deformable mirrors (DM 1 and DM 2) with 48x48 actuators each. High-order wavefront control is implemented by the Electric Field Conjugation (EFC) method. The EFC loop operates on science focal plane data by measuring the interaction of aberrated on-axis starlight with a sequence of DM actuator probes. Pointing, focus, and low-order wavefront drifts are sensed by the Low-Order Wavefront Sensing and Control (LOWFS/C) subsystem using the Zernike phase-contrast technique on starlight rejected from the occulting mask. Corrections to Zernike modes Z5—Z11 are applied to DM 1. The FSM control loop corrects line-of-sight pointing jitter to below 0.95 milliarcsec.
Deformable Mirrors (DMs)
Deformable mirrors are a key enabling subsystem inside any coronagraph with active wavefront control. The baseline Roman Coronagraph design includes four active optics to control the wavefront: a fast steering mirror (FSM), a flat focusing mirror (FCM), and two deformable mirrors (DM 1 and DM 2) with 48x48 actuators each.
Coronagraphic Masks
The Coronagraph Instrument will feature two masks: Hybrid Lyot and Shaped Pupil. The masks are paired with specific filters to create observing modes. The masks are designed to operate at a specific wavelength/filter, so not all mask and filter combinations are valid. Combinations other than the supported ones may not be commissioned during the Tech Demo Phase.
Hybrid Lyot Coronagraph (HLC)
The HLC provides full 360 degree high contrast field of view. The Focal plan occulting mask is a circular, r = 2.8 λc /D partially transmissive nickel disk overlaid with PMGI dielectric layer with a radially and azimuthally varying thickness profile. The HLC design incorporates a numerically optimized, static actuator pattern applied to both deformable mirrors. The Lyot Stop is an annular mask that blocks the telescope pupil edges and struts.
Shaped Pupil Coronagraph (SPC)
The shaped pupil apodizer is a reflective mask on a silicon substrate with aluminum regions for reflection and black silicon regions for absorption. The hard-edged occulting mask has either a bowtie-shaped opening for characterization (spectroscopy) mode or an annual aperture for debris disk imaging. The SPC Spectroscopy designed in 2017 produces a 2 x 65 degree bowtie dark zone from 3.0 – 9.1 λc /D over a 15% bandpass. The SPC Wide Field of View design produces a 360 degree dark zone from 5.9 – 20.1 λc /D in a 10% bandpass.
Electron-Multiplying Charge-Coupled Device (EMCCD)
Electron Multiplying CCD (EMCCD) technology is advantageous for a coronagraph application. Programmable gain provides wide dynamic range suitable for bright scenes expected during acquisition and coronagraph configuration, while photon counting capability can be used for faint light observations with zero read noise.
EMCCD detectors are baselined for direct imaging, spectroscopy and wavefront sensing applications in CGI. Subarray readout suitable for a wavefront sensor application enables 1000 frame-sec -1 operation to accommodate tip-tilt sensing.
Work at JPL has focused on low flux characterization with radiation damaged sensors. JPL has invested in modifications to the commercial version of the EMCCD that are expected to improve margins against radiation damage in a flight environment.
JPL’s EMCCD test lab has measured a low flux threshold of 0.002 c- psf-1-sec-1, equivalent to a 32.4 magnitude star through a 2.4m telescope at 500 nm with 10% bandwidth. Devices irradiated to 5 years equivalent life at L2 meet coronagraph technology requirements.
Post-Processing
Direct detection and characterization of mature giant or sub-Neptune exoplanets in the visible will require space- based instruments optimized for high-contrast imaging with contrasts of 10-9. The coronagraph instrument will reach raw contrasts of approximately 10-8 or better using state-of-the-art starlight suppression and wavefront control techniques. A ten-fold contrast improvement is therefore required using post-processing techniques in order to detect 10-9 planets from speckles. Post-processing techniques that are successful on both ground-based and space-based instruments need to be validated at such high contrast levels. Investigations on algorithms for CGI data post-processing have encompassed both end-to-end data simulations and analysis of laboratory testbed data. Reference differential imaging (RDI) trials have probed a range of wavefront stability and noise scenarios. Simulations with spacecraft rolls have also enabled tests of angular differential imaging (ADI).