Spectral interferometric wavefront sensing: a solution for petalometry at Subaru/SCExAO

TL;DR

This paper introduces a multi-wavelength interferometric petalometer for Subaru/SCExAO that enhances the measurement of differential piston aberrations caused by pupil fragmentation, improving high contrast imaging performance.

Contribution

It proposes a novel multi-spectral interferometric petalometer design using existing instrument modes to accurately measure pupil petal aberrations.

Findings

Effective disambiguation of phase wrapping errors.

Successful implementation with the VAMPIRES instrument.

Enhanced differential piston measurement accuracy.

Abstract

The petaling effect, induced by pupil fragmentation from the telescope spider, drastically affects the performance of high contrast instruments by inducing core splitting on the PSF. Differential piston/tip/tilt aberrations within each optically separated fragment of the pupil are poorly measured by commonly used Adaptive Optics (AO) systems. We here pursue a design of dedicated low-order wavefront sensor -- or petalometers -- to complement the main AO. Interferometric devices sense differential aberrations between fragments with optimal sensitivity; their weakness though is their limitation to wrapped phase measurements. We show that by combining multiple spectral channels, we increase the capture range for petaling aberrations beyond several microns, enough to disambiguate one-wave wrapping errors made by the main AO system. We propose here to implement a petalometer from the multi-wavelength imaging mode of the VAMPIRES visible-light instrument, deployed on SCExAO at the Subaru Telescope. The interferometric measurements obtained in four spectral channels through a 7 hole non-redundant mask allow us to effiiently reconstruct diffierential piston between pupil petals.