A Point-Spread Function for the Extreme Ultraviolet High-Resolution Imager on board Solar Orbiter

TL;DR

This paper characterizes the point-spread function of Solar Orbiter's EUV imager, enabling image correction for diffraction and scattering, which significantly improves image quality and scientific analysis.

Contribution

The authors provide the first detailed PSF model for HRIEUV, quantifying diffraction and scattering effects to enhance image correction techniques.

Findings

26% of light is diffracted mainly by the entrance filter mesh.

42% of light is diffusely scattered by mirror microroughness.

Correcting for PSF effects increases bright structure intensity by up to 40%.

Abstract

We present the point-spread function (PSF) of the Extreme Ultraviolet High-Resolution Imager (HRIEUV) onboard Solar Orbiter, which observes the Sun at 174 Angstrom. This PSF provides a quantitative description of light diffracted by the mesh and mounting supporting the entrance filter, light diffracted by the mesh supporting the filter-wheel filter, as well as light that is diffusely scattered by the microroughness of the mirrors. Deconvolution with this PSF corrects the images for instrumental scattered light, substantially improving image quality and photometric accuracy. First, we determine the diffraction component of the PSF from mechanical drawings of the instrument. We find that 26% of the incoming light is diffracted, predominantly by the entrance-filter mounting and mesh. Second, we fit the diffuse scattered light using partial image occultations during the 2023-Jan-03 Mercury transit. We find that the diffuse scattered light is well described by a softened power law, which scatters 42% of light over the detector. Combined, 57% of the incoming light is redistributed over the detector by diffraction and scattering. Correcting for these effects markedly enhances the dynamic range and contrast of the observations. The intensity in bright structures intensifies by up to 40% and the intensity in dark structures decreases by up to 85 %. All images features become much clearer, facilitating a more precise scientific analysis of HRIEUV observations.