The Talbot effect's impact on the high contrast imaging modes of METIS

Abstract

The ELT, Europe's Extremely Large Telescope, with its 39m main mirror will be the largest optical/infrared telescope in the world, able to work at the diffraction limit. METIS is one of its first light instruments with powerful imaging and spectroscopic capabilities in the thermal wavelengths. It contains several high contrast imaging (HCI) modes, which allow it to detect and characterize exoplanets amongst others. The HCI performance is highly dependent on pupil stabilization mechanisms and a closed loop compensation of non-common path aberrations degrading the wavefront error of the instrument.The Talbot effect is a near-field effect on collimated light, where spatial frequencies of the wavefront are re-imaged periodically along the optical path. The periodicity is known as the Talbot length, which is a function of the wavelength and the wavefront's spatial frequencies with the latter being a result of the wavefront errors caused by the surface form errors of optical elements. The aberrations oscillate from amplitude to phase, in the spatial scale of one Talbot length, which can have an impact on the performance of the HCI modes. We evaluate the impact of the Talbot effect with respect to the METIS phase aberration budget by assuming representative power spectral density profile for the surface form error of each optical surface. We propagate the errors to the subsequent pupil plane and finally investigate the resulting point spread function profile. Simulations are fed back into the HCI error budget and if necessary, the specifications regarding instrument surface form are adjusted.

The High-contrast End-to-End Performance Simulator (HEEPS): influence of ELT/METIS instrumental effects

“…In addition to creating phase errors in the wavefront, aberrations inside the instrument can also lead to amplitude aberrations, due to phase aberrations being converted into amplitude aberrations upon Fresnel propagation on a distance equal to the Talbot length. A thorough description of how amplitude aberrations are simulated inside METIS is given in Boné et al 2020 17 . In addition to the pure static aberrations described in there, we include here the effect of chromatic beam wander.…”

“…Observing close to meridian crossing for our ADI sequences will therefore be key to maximize performance, as we have no way to compensate for amplitude aberrations within METIS. The only lever that we can act upon is to overspecify the quality of the optics located close to focal planes, which have the largest contribution to amplitude errors 17 .…”