Lyman-alpha Filter Prototype to Enable Astronomical Photometry in the Lyman Ultraviolet

TL;DR

This paper presents a prototype Lyman-alpha filter using a hydrogen absorption cell designed to enable ultraviolet space-based photometry by blocking geocoronal emission, with initial testing results and potential scientific applications.

Contribution

Development of a hydrogen absorption cell filter prototype for Lyman-ultraviolet space telescopes, enabling improved astronomical observations in the far ultraviolet.

Findings

Prototype cell transmits down to 1150 Å

Pressure control via getter module demonstrated

Testing shows effective Lyman-α photon absorption

Abstract

Observations of astronomical objects in the far ultraviolet (FUV wavelengths span 900-1800{\AA}) from earth's orbit has been impeded due to bright Lyman-{\alpha} geocoronal emission. The Johns Hopkins Rocket Group is developing a hydrogen absorption cell that would act as a narrow band Lyman-{\alpha} rejection filter to enable space-based photometric observation in bandpasses that span over the Lyman ultraviolet region shortward of the geocoronal line. While this technology has been applied to various planetary missions with single element photomultiplier detectors it has yet to be used on near earth orbiting satellites with a multi-element detector. We are working to develop a cell that could be easily incorporated into future Lyman ultraviolet missions. The prototype cell is a low-pressure (~ few torr) chamber sealed between a pair of MgF2 windows allowing transmission down to 1150 {\AA}. It is filled with molecular hydrogen that is converted to its neutral atomic form in the presence of a hot tungsten filament, which allows for the absorption of the Lyman-{\alpha} photons. Molecular hydrogen is stored in a fully saturated non-evaporable getter module (St707TM), which allows the cell pressure to be increased under a modest application of heat (a 20 degree rise from room temperature has produced a rise in pressure from 0.6 to 10 torr). Testing is now underway using a vacuum ultraviolet monochromator to characterize the cell optical depth to Lyman-{\alpha} photons as functions of pressure and tungsten filament current. We will present these results, along with a discussion of enabled science in broadband photometric applications.