Model validation and tolerancing of scalar vortex masks in the High Contrast Imaging Testbed (HCIT) facility

Abstract

The Habitable Worlds Observatory (HWO) mission will require coronagraphs capable of suppressing starlight at the $\sim 10^{-10}$ contrast level to directly image exo-Earths. High contrast achromatic coronagraphic masks are the missing critical component to achieving this. Vortex coronagraphs, particularly scalar vortex designs with an achromatic focal plane mask, offer key advantages. While all vortex coronagraph varieties provide high throughput, a small inner working angle, and rejection of low-order aberrations, the scalar approach enables dual-polarization observation in a single optical path. This simplifies instrument design and increases transmission by maintaining light from the planet in two orthogonal polarization states. In this work we test scalar vortex masks and investigate their contrast limitations. We perform phase metrology to assess the mask defects and manufacturing deviations and use it to refine the coronagraphic model used for electric field conjugation (EFC) algorithms and end-to-end simulations. We also measure the impact of model-mismatch with EFC by varying model parameters including clocking angle, and central wavelength in laboratory demonstrations. Finally, we validate our scalar vortex models against experimental results from the High Contrast Imaging Testbed (HCIT) facility at JPL by finding good agreement between lab and simulated performance. This ultimately helps to benchmark simulated contrast predictions for future scalar vortex coronagraph designs for HWO.

Figures (6)

• Figure 1: (left) Phase measurement of sawtooth with central phase dimple SVC prototype mask taken with the digital holographic microscope. (center) Best fit SVC model to DHM measurement. (right) Residual difference between measurement and model.
• Figure 2: (a) Simulated broadband contrast of a sawtooth with Roddier dimple SVC with varying sizes of central defect hole or opaque spot. (b) Simulated central hole and and offset opaque spot. (c) Simulated narrowband contrast with various offsets of a r = 10 micron opaque spot covering a r = 6 micron central defect.
• Figure 3: Sensitivity of broadband EFC performance to focal plane mask clocking mismatch. (a) Final contrast vs. modeled clocking angle, showing a clear optimum near 27°. (b) Contrast vs. EFC iteration for selected model angles, showing degraded convergence for mismatched clocking.
• Figure 4: Effect of central wavelength mismatch on broadband EFC performance. (a) Final contrast vs. modeled central wavelength, with optimal performance at 760 nm. (b) Contrast vs. EFC iteration for varying central wavelength $\lambda_0$, showing consistent convergence below 1.5e-7.
• Figure 5: Side-by-side comparison of narrowband dark holes dug from 3-10 $\lambda/D$ on the In-Air Coronagraph Testbed at JPL with model-based EFC (left) and model-free iEFC (right) with a Lyot stop misalignment not matched in the model.