Integrated Coronagraph Design and Wavefront Control using Two Deformable Mirrors
N. Kasdin/Princeton University
A coronagraph for achieving high-contrast is one of the two key technologies being studied for exoplanet imaging (the other being an external starshade). While many different types of coronagraphs have been proposed, all operate in the same basic architecture: using masks, stops, and mirrors to modify the beam path and thus produce high contrast in the final image. In particular, current TDEMs are studying the performance of pupil mapping and bandlimited Lyot coronagraphs. All the systems under test and consideration also have been designed using the same principal: design a perfect coronagraph to achieve the desired contrast, inner working angle, and throughput and then add deformable mirrors to correct the wavefront. In reality, the coronagraph and wavefront control system are inseparable; both must work together to produce sufficient starlight suppression. In this proposal we present our methodology for co-designing both and propose to advance the use of two deformable mirrors for wavefront amplitude control to past TRL 4.
All coronagraphs work on the same principal: they modify the amplitude of the electric field across the exit pupil in order to create a final high-contrast image. This may be done through apodization, phase plates, remapping, or phase and amplitude adjustment at intermediate image planes. Nevertheless, in every case, high contrast can only be achieved through a change in amplitude. Unfortunately, phase and amplitude errors in the telescope limit the achievable contrast, often to no better than a contrast of 10^-5. All practical systems must then include the capability for both amplitude and phase control of the wavefront; most system designs include multiple deformable mirrors in sequence. This implies that whatever coronagraph is chosen, it need only be designed to achieve contrast to the level at which amplitude errors dominate. The remaining contrast will be accomplished via the wavefront control system with the final inner working angle determined by the capability of the deformable mirrors. This suggests that designing for high-contrast requires viewing the coronagraph as a system including the deformable mirrors. It also suggests that the differences among the different coronagraphs are minimal and the choice should be based on cost, sensitivity, and the ease and effectiveness of the integrated wavefront control.
It has been known for almost a decade that amplitude control will be required for any final starlight suppression system based on coronagraphy. It has also been generally accepted that this will be accomplished using multiple deformable mirrors. The pioneering work in this area was done at Princeton and our laboratory remains the only facility with two operational deformable mirrors in series. We also have modified our control algorithms for both amplitude and phase control using two deformable mirrors and are producing broadband dark holes on both sides of the image plane. For this TDEM we propose upgrading our current DMs to improve performance and refining our algorithms, including adaptive approaches to enhance the DM models. We will modify our shaped pupil coronagraphs to optimize the integrated design, perform development tests at the Princeton laboratory to verify the algorithms, and then move to the High Contrast Imaging Laboratory at JPL for final verification.
Strategic Astrophysics Technology