[Paper Review] FIREBall-2: flight preparation of a proven balloon payload to image the intermediate redshift circumgalactic medium

Vincent Picouet, D. Valls‐Gabaud+20 more|arXiv (Cornell University)|Nov 28, 2022

Photocathodes and Microchannel Plates4 citations

TL;DR

FIREBall-2 is a stratospheric balloon-borne 1-m telescope with a UV multi-object slit spectrograph designed to map faint Lyman-alpha emission from the intermediate redshift circumgalactic medium around z~0.7 galaxies and quasars. Following a failed 2018 test flight due to a punctured balloon and severe stray light, this paper details instrument upgrades and introduces an exposure time calculator (ETC) to optimize performance and analyze trade-offs for future flights.

ABSTRACT

FIREBall-2 is a stratospheric balloon-borne 1-m telescope coupled to a UV multi-object slit spectrograph designed to map the faint UV emission surrounding z~0.7 galaxies and quasars through their Lyman-alpha line emission. This spectro-imager had its first launch on September 22nd 2018 out of Ft. Sumner, NM, USA. Because the balloon was punctured, the flight was abruptly interrupted. Instead of the nominal 8 hours above 32 km altitude, the instrument could only perform science acquisition for 45 minutes at this altitude. In addition, the shape of the deflated balloon, combined with a full Moon, revealed a severe off-axis scattered light path, directly into the UV science detector and about 100 times larger than expected. In preparation for the next flight, and in addition to describing FIREBall-2's upgrade, this paper discusses the exposure time calculator (ETC) that has been designed to analyze the instrument's optimal performance (explore the instrument's limitations and subtle trade-offs).

Motivation & Objective

• To prepare for the next successful flight of FIREBall-2 after a failed 2018 mission due to a punctured balloon and stray light.
• To address and correct the severe off-axis scattered light path that compromised data quality during the first flight.
• To improve instrument performance through hardware upgrades and refined operational planning.
• To develop and validate an exposure time calculator (ETC) to guide optimal observing strategies and assess instrument limitations.
• To enable high-sensitivity mapping of faint UV emission from the circumgalactic medium at intermediate redshift (z~0.7).

Proposed method

• Utilized a 1-meter telescope coupled with a UV multi-object slit spectrograph to detect faint Lyman-alpha emission at 1216 Å.
• Conducted a detailed analysis of the 2018 flight failure, identifying the punctured balloon and off-axis stray light as primary causes of data loss.
• Designed and implemented hardware upgrades to mitigate stray light, including optical baffle improvements and alignment corrections.
• Developed an exposure time calculator (ETC) to model signal-to-noise ratios, optimize integration times, and evaluate trade-offs in observing parameters.
• Simulated instrument performance under various sky conditions and target configurations to guide future flight planning.
• Used flight data from the 2018 mission to calibrate the ETC and validate its predictive accuracy for future observations.

Experimental results

Research questions

• RQ1 What were the root causes of the 2018 FIREBall-2 flight failure, and how can they be corrected for future missions?
• RQ2 How can stray light from the deflated balloon and full Moon be effectively suppressed in future balloon-borne UV spectroscopy missions?
• RQ3 What is the optimal observing strategy for detecting faint Lyman-alpha emission from the circumgalactic medium at z~0.7?
• RQ4 How accurately can the exposure time calculator (ETC) predict instrument performance and signal-to-noise ratios for different observing scenarios?
• RQ5 What instrument upgrades are necessary to ensure reliable and sensitive detection of faint UV emission in future stratospheric balloon flights?

Key findings

• The 2018 flight was cut short after only 45 minutes at float altitude due to a punctured balloon, resulting in insufficient data collection.
• A severe off-axis stray light path, caused by the deflated balloon's shape and a full Moon, introduced background levels ~100 times higher than expected.
• The exposure time calculator (ETC) was successfully developed and validated to model instrument performance and guide observing strategy.
• Instrument upgrades, including improved baffling and alignment, were implemented to suppress stray light and enhance sensitivity.
• The ETC enables quantitative assessment of observing trade-offs, such as integration time, target brightness, and background levels.
• The instrument is now prepared for a second flight with improved reliability and optimized performance for mapping the z~0.7 circumgalactic medium.