Test of a multilayer-coated EUV grating for I-IV order spectroscopic measurements of the solar corona

METIS, the Multi Element Telescope for Imaging and Spectroscopy, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission. The original METIS proposal included four optical paths, for observations in: 1) linearly polarized visible-light (590-650 nm), 2) narrow-band ultraviolet HI Lyman-alpha (121.6 nm), 3) narrow-band extreme-ultraviolet HeII Lyman-alpha (30.4 nm), 4) spectrographic mode for the HI Lyman-alpha and He II Lyman-alpha in corona. The design, coating performances, and test activities of the grating for the spectroscopic path are here described. The grating is optimized to work at near normal incidence and to diffract the 121.6 nm radiation at the first order and the 30.4 nm at its 4th order, consequently the two spectroscopic channels are overlapped on the focal plane. The grating is spherical with variable-line-spaced rulings, 1800 gr/mm central density. The selection of the spectroscopic channel to be acquired, either the 121.6 nm or the 30.4 nm, is made by a suitable filter wheel. The grating is multilayer-coated, to have high efficiency in both the spectral channels. In this paper we describe the tests made on a prototype with flat surface and constant groove spacing. The measures have been carried out at the BEAR beamline at the ELETTRA Synchrotron in Trieste (Italy). The grating was initially coated by gold and successively by a Mo-Si multilayer optimized at 30.4 nm. The efficiencies at the first and fourth order (121.6 and 30.4 nm) have been measured before and after the multilayer deposition. The quality of the multilayer deposition has been tested by atomic force microscope measurements on the grating surface and by reflectivity measurements performed on a test reference mirror. The experimental data are compared with numerical simulations accounting for the coating roughness and the smoothening effect on the blaze profile after the multilayer deposition. To our knowledge, this is the first time that such a grating configuration is proposed.