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

TL;DR

This paper presents a novel multi-phase optical alignment approach for contamination-sensitive Far-Ultraviolet spectrographs in the Aspera SmallSat mission, utilizing advanced metrology tools to meet tight tolerances.

Contribution

It introduces a multi-phase alignment method combining laser scanning, interferometry, and holography for precise optical alignment in a compact, contamination-sensitive FUV spectrograph.

Findings

Successful implementation of the alignment process in a cleanroom environment.

Achieved optical alignment tolerances suitable for the mission's requirements.

Enhanced alignment accuracy using a combination of metrology techniques.

Abstract

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 detector. Each spectrograph channel consists of an off-axis parabola primary mirror and a toroidal diffraction grating optimized for the 1013-1057 Angstroms bandpass. Despite the simple configuration, the optical alignment/integration process for Aspera is challenging due to tight optical alignment 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 a Zygo interferometer and a custom computer-generated hologram. The detector focus requires iterative in-vacuum alignment using a Vacuum UV collimator. The alignment is done in a controlled cleanroom facility at the University of Arizona.