Optical alignment of contamination-sensitive Far-Ultraviolet spectrographs for Aspera SmallSat mission

Aspera is a NASA Astrophysics Pioneers SmallSat mission designed to study diffuse OVI emission from the warm-hot phase gas in the halos of nearby galaxies. Its payload consists of two identical Rowland Circle type long-slit spectrographs, sharing a single MicroChannel plate (MCP) detector. Each spectrograph channel, consisting of an off-axis parabola (OAP) primary mirror and a toroidal diffraction grating optimized for the 103-104 nm bandpass. Despite the simple configuration, the optical challenging alignment/integration process for Aspera is challenging due to tight optical tolerances, driven by the compact form factor, and the contamination sensitivity of the Far-Ultraviolet optics and detectors. In this paper, we discuss implementing a novel multi-phase approach to meet these requirements using state of the art optical metrology tools. For coarsely positioning the optics we use a blue-laser 3D scanner while the fine-alignment is done with Zygo interferometer and a custom computer generated hologram. The detector focus requires iterative in-vacuum alignment using Vacuum UV collimator. The alignment is done in a controller cleanroom facility at the University of Arizona.