Optical alignment of the SPICE EUV imaging spectrometer

Abstract

SPICE is a high resolution imaging spectrometer operating at extreme ultraviolet wavelengths, 70.4 - 79.0 nm and 97.3 - 104.9 nm. It is a facility instrument on the ESA Solar Orbiter mission. SPICE will address the key science goals of Solar Orbiter by providing the quantitative knowledge of the physical state and composition of the plasmas in the solar atmosphere, in particular investigating the source regions of outflows and ejection processes which link the solar surface and corona to the heliosphere. By observing the intensities of selected spectral lines and line profiles, SPICE will derive temperature, density, flow and composition information for the plasmas in the temperature range from 10,000 K to 10MK. The optical components of the instrument consist of an off axis parabolic mirror mounted on a mechanism with a scan range of 8 arc minutes. This allows the rastering of an image of the spectrometer slit, which is interchangeable defining the instrument resolution, on the sky. A concave toroidal variable line space grating disperses, magnifies, and re-images incident radiation onto a pair of photocathode coated microchannel plate image intensifiers, coupled to active pixel sensors. For the instrument to meet the scientific and engineering objectives these components must be tightly aligned with each other and the mechanical interface to the spacecraft. This alignment must be maintained throughout the environmental exposure of the instrument to vibration and thermal cycling seen during launch, and as the spacecraft orbits around the sun. The built alignment is achieved through a mixture of dimensional metrology, autocollimation, interferometry and imaging tests. This paper shall discuss the requirements and the methods of optical alignment.