The UV multi-object slit-spectrograph FIREBall-2 simulator

The FIREBall-2 Instrument Model (FIREBallIMO) is a piece of software simulating the optical behaviour of the Multi-Object Two-Curved Schmidt Slit Spectograph of FIREBall-2 (Faint Intergalactic Redshifted Emission BALLoon), a balloon-borne telescope (40 km in alt.) designed to perform a direct detection of the faint Circum Galactic Medium (CGM) in emission around low-z galaxies. The spectrograph has been optimized to operate in a narrow UV band [195-225] nanometers, the so-called atmospheric sweet spot, where the sky background presents no emission lines and can be considered approximately flat, a value of 500 continnum units, seen through an optical transmission of 50% at an atmospheric pressure of 3 millibars. This paper gives an overview of the software current modular architecture after a year of productive effort (in terms of parametric model space definition, associated data cubes generation and digital processing) starting from the instrument initial optical model designed under Zemax software to the final 2D-detected image. A special emphasis is put on the design of a cython-wrapped driver able to retrieve dense ray-sampled PSFs out of the Zemax box efficiently. The optical mappings and distortions from the sky to the spectrograph's entrance slit plane and from the sky to the detection plane are presented, as well as some end-to-end simulations leading to Signal-to-Noise Ratio estimates computed on artificial point-like or extended test sources.