High diffraction efficiency varied-line-space concave gratings for the Lyman ultraviolet explorer

Area of Science:

• Astronomy and Astrophysics
• Optical Engineering
• Spectroscopy

Background:

• The Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer (CAFE) project requires sensitive detection of the warm-hot circumgalactic medium.
• Detecting OVI emission lines at 103.2 nm and 103.8 nm necessitates specialized optical components.

Purpose of the Study:

• To design and fabricate a reflection varied-line-space concave grating (VLSCG) for the CAFE spectrograph.
• To achieve high diffraction efficiency in the LUV range and a compact F/3.6 aperture ratio.

Main Methods:

• Holographic lithography and ion beam etching were employed for grating fabrication.
• An additional lens was integrated into the holographic exposing system to create varied line spacing.
• Oblique ion beam bombardment was used to create triangular groove profiles, enhancing diffraction efficiency.

Main Results:

• Successfully fabricated VLSCGs with a central line density of 3300 lines/mm.
• Achieved groove efficiencies of 51% at 106.4 nm and 31% at 127.4 nm.
• Measured point source image extent of 0.68 mm with corrected aberrations in a simulated spectrograph optical path.

Conclusions:

• The developed VLSCG meets the demanding optical requirements for the CAFE project.
• The fabrication techniques enable efficient LUV diffraction and aberration correction for compact spectrographs.