FIREBall-2: The Faint Intergalactic Medium Redshifted Emission Balloon Telescope

Abstract

The Faint Intergalactic Medium Redshifted Emission Balloon (FIREBall) is a mission designed to observe faint emission from the circumgalactic medium of moderate-redshift (z ∼ 0.7) galaxies for the first time. FIREBall observes a component of galaxies that plays a key role in how galaxies form and evolve, likely contains a significant amount of baryons, and has only recently been observed at higher redshifts in the visible. Here we report on the 2018 flight of the FIREBall-2 Balloon telescope, which occurred on 2018 September 22 from Fort Sumner, New Mexico. The flight was the culmination of a complete redesign of the spectrograph from the original FIREBall fiber-fed integral field unit to a wide-field multiobject spectrograph. The flight was terminated early owing to a hole in the balloon, and our original science objectives were not achieved. The overall sensitivity of the instrument and telescope was 90,000 LU, due primarily to increased noise from stray light. We discuss the design of the FIREBall-2 spectrograph, including modifications from the original FIREBall payload, and provide an overview of the performance of all systems. We were able to successfully flight-test a new pointing control system, a UV-optimized, delta-doped, and coated electron multiplying CCD, and an aspheric grating. The FIREBall-2 team is rebuilding the payload for another flight attempt in the fall of 2021, delayed from 2020 as a result of COVID-19.

End-to-end ground calibration and in-flight performance of the FIREBall-2 instrument

“…Despite that, the pointing performance stayed well within the technical specification. 8 The in-flight raw guider error signal is found to be 0.7″ RMS per axis (Fig. 8), an upper limit to the guidance error, as it includes the guider signal noise and the field rotation guiding error of ∼0.3 00 RMS.…”

“…The FIREBall-2 gondola has a height of ∼5 m and a weight of ∼2.3 metric tons (including the 500-kg ballast). 8 This payload is designed to reach an altitude of 35 km attached to a stratospheric balloon filled with 1.1 million cubic meters of helium. Because the atmosphere absorbs most of the UV light coming from the cosmos, the FIREBall spectrograph has been optimized to operate in the narrow atmospheric transmission window around 2000 Å, where there is a dip in the atmospheric UV absorption above 35 km.…”

“…20 Aside from these considerations, almost all subsystems performed nominally. Some flight anomalies are detailed in the work by Hamden et al 8 as well as the four fields observed. In order to focus on the instrument performance, we will only detail the reduction and analysis of the first field (DEEP 2: RA ¼ 253.06, DEC ¼ 34.97, θ ¼ 20 deg) when the instrument was at the nominal altitude.…”

“…The sharp drop-off in scattered light on the bottom fifth of the image makes the subtraction difficult, yielding a residual along the edge of the scattered region. All images have been smoothed with a Gaussian kernel with sigma of 1.5 and a radius of 3 pixels.The emission from the brightest star, Bright Star 1, is visible in the last two images as a horizontal line roughly in the middle 8.…”

“…3 2018 Flight and Data Processing3.1 FlightFIREBAll-2 was launched on a 1.1-million-cubic-meters balloon from the Fort Sumner Columbia Scientific Balloon Facility on September 22 at 10:20 AM local MDT. The flight sequence is detailed in the work by Hamden et al8 …”