Validation of strategies for coupling exoplanet PSFs into single-mode fibres for high-dispersion coronagraphy

Abstract

On large ground-based telescopes, the combination of extreme adaptive optics (ExAO) and coronagraphy with high-dispersion spectroscopy (HDS), sometimes referred to as high-dispersion coronagraphy (HDC), is starting to emerge as a powerful technique for the direct characterisation of giant exoplanets. The high spectral resolution not only brings a major gain in terms of accessible spectral features, but also enables a better separation of the stellar and planetary signals. Ongoing projects such as Keck/KPIC, Subaru/REACH, and VLT/HiRISE base their observing strategy on the use of a few science fibres, one of which is dedicated to sampling the planet's signal, while the others sample the residual starlight in the speckle field. The main challenge in this approach is to blindly centre the planet's point spread function (PSF) accurately on the science fibre, with an accuracy of less than 0.1 λ/D to maximise the coupling efficiency. In the context of the HiRISE project, three possible centring strategies are foreseen, either based on retro-injecting calibration fibres to localise the position of the science fibre or based on a dedicated centring fibre. We implemented these three approaches, and we compared their centring accuracy using an upgraded setup of the MITHiC high-contrast imaging testbed, which is similar to the setup that will be adopted in HiRISE. Our results demonstrate that reaching a specification accuracy of 0.1 λ/D is extremely challenging regardless of the chosen centring strategy. It requires a high level of accuracy at every step of the centring procedure, which can be reached with very stable instruments. We studied the contributors to the centring error in the case of MITHiC and we propose a quantification for some of the most impacting terms.

Experimental validation of exoplanet centring strategies for high dispersion coronagraphy

“…The panels labelled 1-5 show the steps followed to place the planet's PSF into the science fibre. Reproduced with permission from El Morsy et al 27 as described for the first strategy (Fig. 2, panel 4), we finally move the planet's PSF into the science fibre (Fig.…”

“…The yellow inset shows the home-build setup used to retro-inject light into the four feedback fibres. 27 The static aberration measured by the ZELDA wavefront sensing are estimated at 7 nm RMS in 15 days if the testbed remains in the configuration where the panels are closed. This error term remains not significant for the tests performed on MiTHIC due to the tests timescale.…”

“…• C-S fibre distance: Another important step is to be able to accurately switch between the C and the S. To fulfil this requirement, an accurate calibration of the C-S distance is necessary and the results 27 demonstrates that for 100 measurements to calibrate the offset between the two fibres, 99% of the points are within the requirement of 0.1 λ/D. We also noticed that the standard deviation estimation in tip-tilt is 0.02 λ/D, which is approximately equal to the PSF's jitter uncertainty.…”

“…The resulting interpolation function allow to obtain the tip-tilt command to place the stellar PSF at any position in the camera's field of view (FoV). El Morsy et al 27 computed the residual of the interpolation function as measured on the tracking camera and demonstrated that the residual are quasi-Gaussian with a standard deviation of 0.03 λ/D, which is below 0.1 λ/D and noticed that 97.61% of the residuals fall within ±0.1 λ/D. The stability of the calibration grid used to build the interpolation function relies on the bench stability.…”

“…The contrast of a factor 3.48 is modest but does not impact the performance results on-sky because the system is blind to Reproduced with permission from El Morsy et al 27 the planet's signal during the centring procedure. However, the modest contrast in the laboratory is essential to check the final accuracy.…”