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February 13, 2018

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Earth-like planets imaging and spectroscopy requires the ability of imaging objects 10^10 times fainter than their stars at very close angular separations. The solution found to reach such goals is to combine a coronagraph and wavefront sensing and wavefront control tools. However, the telescope and the coronagraph design phases have to address stability studies, particularly in presence of segmented telescopes such as LUVOIR, to understand the impact of segment-level errors on coronagraphic PSF quality.

The PASTIS model provides a fast way to compute the contrast in the dark hole generated by a coronagraph, and affected by local errors on the segments. In this post, we introduce our very first results using this model and compare them with the outputs of a traditional end-to-end simulation. 

For this study, we use a 36-segment pupil, combined with an apodized Lyot coronagraph (APLC) providing a contrast better than 10^10 between 4 and 9 lambda/D. An example of local aberrations on this pupil are shown in Fig. 1.

Local_ab

Fig. 1. Local aberrations applied on the pupil chosen for the study.

First of all, PASTIS provides the contrast in the dark hole, as a function of the aberrations on the different segments. In the following figure, we compare the mean contrasts in the dark hole computed from the images of the end-to-end simulation and from PASTIS.

output_contrast

Fig. 2. Plot of the output contrasts computed by the end-to-end simulation and from the matrix-based analytical model for piston aberrations from 1pm to 10nm rms on the segments. For each rms value, we select 250 random phases and compute the mean, minimum, and maximum contrasts over the 250 output contrasts.

On these curves, there is a 3% error on the computation of contrast between the PASTIS computation (continuous lines) and the end-to-end simulation computation (dotted lines). However, the PASTIS values have been 10^7 times faster to compute than the end-to-end simulation values.

Secondly, PASTIS can also provide a good estimation of the morphology of the speckle in the dark hole, depending on the aberrations applied on the different segments.

PASTISvsE2E
Fig. 3. Comparison of PSFs when local aberrations are applied on the pupil segments. First column of each table: type of Zernike polynomial being applied. Second column: phase applied on the pupil. Third column: deteriorated PSFs (in the dark hole only, which explains the donut shape) issued from the end-to-end simulation. Fourth column: deteriorated PSFs issued from PASTIS

To conclude, PASTIS is adaptable to any segmented pupils, such as the ELTs, TMT, JWST, LUVOIR, or even to non-hexagonal-segment pupils such as the GMT.

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