The extreme ultraviolet imager of solar orbiter: optical design and alignment scheme

Abstract

The Extreme Ultraviolet Imager (EUI) is one of the remote sensing instruments on-board the Solar Orbiter mission. It will provide dual-band full-Sun images of the solar corona in the extreme ultraviolet (17.1 nm and 30.4 nm), and highresolution images of the solar disk in both extreme ultraviolet (17.1 nm) and vacuum ultraviolet (Lyman-alpha 121.6 nm).The EUI optical design takes heritage of previous similar instruments. The Full Sun Imager (FSI) channel is a singlemirror Herschel design telescope. The two High Resolution Imager (HRI) channels are based on a two-mirror optical refractive scheme, one Ritchey-Chretien and one Gregory optical design for the EUV and the Lyman-alpha channels, respectively.The spectral performances of the EUI channels are obtained thanks to dedicated mirror multilayer coatings and specific band-pass filters. The FSI channel uses a dual-band mirror coating combined with aluminum and zirconium band-pass filters. The HRI channels use optimized band-pass selection mirror coatings combined with aluminum band-pass filters and narrow band interference filters for Lyman-alpha.The optical performances result from accurate mirror manufacturing tolerances and from a two-step alignment procedure. The primary mirrors are first co-aligned. The HRI secondary mirrors and focal planes positions are then adjusted to have an optimum interferometric cavity in each of these two channels. For that purpose a dedicated alignment test setup has been prepared, composed of a dummy focal plane assembly representing the detector position.Before the alignment on the flight optical bench, the overall alignment method has been validated on the Structural and Thermal Model, on a dummy bench using flight spare optics, then on the Qualification Model to be used for the system verification test and qualifications.

Focusing diffraction optics for orbital telescopes

“…The Lyman-alpha line of hydrogen (λ = 121.6 nm) is the most intense EUV line of the solar spectrum; it is sensitive to variations of atmospheric structures forming at temperatures typical of the lower chromosphere/upper transition region (about 30000 K) [70]. An Al/MgF 2 /B 4 C multilayer coating and an Al/MgF 2 coating, which provides a reflection coefficient in excess of 75%, were used for the Lyman-alpha line in [71] and [70,72], respectively.…”

Analyses of tabulated optical constants for thin films in the EUV range and application to solar physics multilayer coatings

“…We mainly used data from filters and mirrors which have been developed for the Extreme Ultraviolet Imager (EUI) 9 onboard the Solar Orbiter mission (ESA mission to be launched in 2020). The EUI instrument includes two EUV telescopes: the Full Sun Imager (FSI) and the High Resolution Imager (HRI).…”

“…Magnesium has been used in several filters and multilayer mirrors in EUV optics for space applications 9,19,28 . Mg-based multilayers present superior reflective performance.…”

The EUI flight instrument of Solar Orbiter: from optical alignment to end-to-end calibration

“…The mirror positions and orientations have been adjusted by shimming of the mirror mounts. The foreseen 'front-side' interferometric alignment method [10] uses a transmission flat on the interferometer to illuminate the instrument through its entrance aperture with a collimated beam. The obtained interferograms however showed unexpected parasitic patterns and aberration which made these measurements useless.…”

“…The OBS unit includes three channels, a Full Sun Imager (FSI) and two High Resolution Imagers (HRI). The optical design of these channels, all three of them with an off-axis mirror system, has been previously described [7] [10] . Positioning and interferometric alignment of their respective mirrors were required during the assembly activities.…”