  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).
