Stuart Shaklan/Jet Propulsion Laboratory
This proposal advances the state of high-contrast imaging optical modeling by satisfying specific, board-approved milestones of the Exoplanet Exploration Program technology plan. This work makes important progress toward our understanding of the effects that limit the performance of stellar coronagraphs by validating analytical models against experimental results.
Laboratory coronagraphs have achieved better than 10-9 contrast (defined as the ratio of scattered light to PSF core light) at the 4th Airy ring in 10% broadband light. This remarkable result is due in no small part to optical models that identify the likely limiting sources of error, e.g. coronagraph mask position sensitivity, deformable mirror controllability, broad-band optical propagation effects, and edge diffraction effects, to name a few. Typically, current models can predict the individual error contributions to contrast degradation, or contrast sensitivity, to a level of a few x 10-9 to 10-8. But as we attempt to push toward 10-10 contrast, as required to image earth-like exoplanets, the model inaccuracies limits their effectiveness. Current models predict that the performance should exceed what has been achieved in the laboratory; we have not yet identified the limiting factors. Model validation at deeper levels is required. As recognized by the framers of the TPF-C Technology Plan when the milestones were first conceived, validated models are critical to moving from current levels of performance to the much more challenging levels required for exoearth detection.
Specifically, we are proposing to carry out the seven modeling validation tests defined in the 2010 document "Exoplanet Exploration Coronagraph Technology, Technology Milestone #3A White Paper, Coronagraph Starlight Suppression Model Validation" (S. Shaklan, JPL Document D-64582). These tests measure the sensitivity of image plane contrast to: source position, beams shear, mask focus position, bandwidth, beam diameter vs. optics diameter, dead deformable mirror (DM) actuators, and the DM actuator density. Our plan calls for approximately six months of testing on the JPL High Contrast Testbed (HCIT): 2 months in air, and two 2-month cycles in vacuum. Slight modifications to the testbed, namely the addition of a translating optic and an iris diaphragm, will be required. These additions will have no negative impact on testbed operation and performance.
We are proposing to perform the Milestone 3A tests using a band-limited coronagraph (BLC). We have chosen this approach because the BLC has achieved the best broad-band contrast exceeding 10-9, significantly better than any other approach to date. But our model validation activity is applicable in large part to all coronagraphs; the DM, beam walk, and edge sensitivity tests are configuration-independent. The defocus, source position, and bandwidth tests will have sensitivities unique to the BLC, but by validating these models, we will gain confidence in our prediction of the sensitivities to these effects in other coronagraphs. The knowledge gained by validating contrast sensitivity will also contribute to flowing down coronagraph instrument error budgets, the very cornerstone of transferring performance demonstrated on ground testbeds to flight-like designs for on-orbit predictions.
Strategic Astrophysics Technology