Toward Sub-Picometer Spectroscopy from Space: A Spaceborne Vacuum Ultraviolet Fourier Transform Spectrometer for Exospheric Hot Hydrogen
Dr. Ed Thiemann
Abstract: Fourier transform spectroscopy is a powerful technique that yields unparalleled resolution at visible and infrared (IR) wavelengths. A typical Fourier Transform Spectrometers (FTS) is based on a Michelson interferometer, whose transmissive beam splitter requirement has limited FTSs to wavelengths above ~140 nm. Recently, a new all-reflective FTS was developed at the SOLEIL Synchrotron. It uses wavefront division interferometry to measure down to 40 nm with a resolving power of 10⁶ (i.e. 40 femtometers at 40 nm). It has been used routinely for over 15 years and has had a significant impact on Vacuum Ultraviolet (VUV) spectroscopy.
Through NASA’s Heliophysics Technology and Instrument Development for Science (HTIDeS) program, Dr. Ed Thiemann has been leading the development of a spaceborne version of the SOLEIL VUV FTS. It is intended to ultimately measure suprathermal, or ‘hot’, hydrogen in Earth’s exosphere through absorption of the solar Lyman-α line. The instrument is designed for resolving powers approaching 10⁶ on a 0.5 m optical bench. The significance becomes apparent when considered in the context of SOHO/SUMER, the most powerful solar VUV spectrograph to ever fly, which achieved resolving powers of 3x104 with a ~3 m optical bench.
The complete instrument prototype has now been assembled and is undergoing testing and optimization. It includes a scan mechanism with 25 mm of travel, a piezo-driven scan stabilizer, a detector module and a laser displacement measuring interferometer. A helium-neon (HeNe) reference channel follows the optical path of the VUV science channel, and corrects the stabilized scan phase error of approximately 10 nm RMS to below 1 nm. Simulations indicate that a frequency stabilized reference channel can also reduce 100 nm of encoder error to 0.18 nm, which would allow a coarse, ruggedized encoder to be used for flight. The first spectra, measured at 633 nm and 638 nm, show a resolving power of approximately 150,000, which meets the science requirement. This seminar presents the novel measurement approach of the SOLEIL FTS and the current progress of the NASA spaceborne VUV FTS. It also covers the path from laboratory verification at Lyman-α to sounding-rocket flights and the eventual Earth’s Coronal Hot Hydrogen Observatory (ECHHO) orbital mission.
To participate in this event virtually via Zoom, go to https://uiowa.zoom.us/j/91406289317?pwd=EAusKTQl2eUzXeC7JvR5FndXOvBB1o.1&from=addon.