The FIREBall-2 UV balloon telescope: 2018 flight and improvements for 2020

Abstract

The Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall-2, FB-2) is designed to discover and map faint UV emission from the circumgalactic medium around low redshift galaxies (z similar to 0.3 (C IV); z similar to 0.7 (Lyff); z similar to 1.0 (O VI)). FIREBall-2's first launch, on September 22nd 2018 out of Ft. Sumner, NM, was abruptly cut short due to a hole that developed in the balloon. FIREBall-2 was unable to observe above its minimum require altitude (25 km; nominal: 32 km) for its shortest required time (2 hours; nominal: 8+ hours). The shape of the deflated balloon, as well as a concurrent full moon close to our observed target field, revealed a severe, off-axis scattered light path directly to the UV science detector. Additional damage to FB-2 added complications to the ongoing effort to prepare FB-2 for a quick re-floght. Upon landing, several mirrors in the optical chain, including the two large telescope mirrors, were damaged, resulting in chunks of material broken off the sides and reflecting surfaces. The magnifying optical element, called the focal corrector, was discovered to be misaligned beyond tolerance after the 2018 flight, with one of its two mirrors damaged from the landing impact. We describe the steps taken thus far to mitigate the damage to the optics, as well as procedures and results from the ongoing efforts to re-align the focal corrector and spectrograph optics. We report the throughput of the spectrograph before and after the 2018 flight and plans for improving it. Finally, we describe several methods by which we address the scattered light issues seen from FIREBall-2's 2018 campaign and present the current status of FB-2 to fly during the summer campaign in Palestine, TX in 2020.