Development of Scientific Ultraviolet Grating for Spectrum Survey

Objective Chinese Space Survey Telescope obtains large field of view, wideband slitless spectrum by adopting near focal plane mosaic gratings instead of by switching gratings which are used by other space projects. The spectral resolution is greater than 200, with spectral coverage ranging from 250 to 1000 nm. Gratings with high diffraction efficiency and good spectrum imaging performance are critical for scientific performance. Domestic ultraviolet transmission diffraction gratings are rarely used in astronomy. To meet the scientific requirements in the U-band, holographic ion beam etching is used to create ultraviolet gratings with wavelengths ranging from 250 to 425 nm. The gratings' various performances are evaluated. Methods Grating groove parameters are designed by taking into account both first-order diffraction efficiency, suitable zeroth-order diffraction efficiency, and peak efficiency wavelength. Grating fabrication parameters and groove profile are optimised using numerical simulation programmes while scientific performance is taken into account. Diffraction efficiency uniformity is achieved across a wide range of incident angles caused by non-telecentric optics and polarised light from various scientific objects. Sensitivity parameters are integrated into acceptable parameter domains to guide the fabrication of ultraviolet gratings. The groove parameters can be well controlled after a few iterations, and ultraviolet gratings are successfully developed. Two metrics are compared based on two groupsof grating groove characteristic parameters, groove depth and the ratio of top angle projection, and the left and right bottom angles. The first metric is found to be suitable for describing peak diffraction efficiency and peak wavelength position, while the second metric is found to be suitable for describing average diffraction efficiency. Diffraction efficiency has a peak wavelength position that is roughly linear to the first metric. Results and Discussions The grating performance of ultraviolet wavelength has been greatly improved after years of effort. After several experiments the grating groove parameters and shape are precisely controlled, yielding a diffraction grating with a groove density of 333 line/mm, blaze angle of 12.51 degrees, and peak efficiency wavelength of 315 nm. The grating performs admirably. Peak diffraction efficiency is 72.3%, with an average efficiency of 65.1%. Peak wavelength position can also be controlled. The groove shape and diffraction wavefront of the grating are controlled at a high level, and the diffraction wavefront RMS reaches 0.06 lambda and PV value reaches 0.33 lambda. The application of the grating can obtain good spectrum imaging performance. Conclusions The performance of astronomical ultraviolet gratings is optimised in groove parameters to achieve the best average efficiency under a variable incident angle and controlled polarisation sensitivity operating conditions. After a few turns of fabrication and evaluation good grating performance is achieved. This will be a contribution to the application of sky spectrum surveys.