Fireball (Faint Intergalactic Redshifted Emission Balloon) is a NASA/CNES balloon-borne experiment to study the faint diffuse circumgalactic medium from the line emissions in the ultraviolet (200 nm) above 37 km flight altitude. Fireball relies on a Multi Object Spectrograph (MOS) that takes full advantage of the new high QE, low noise 13 μm pixels UV EMCCD. The MOS is fed by a 1 meter diameter parabola with an extended field (1000 arcmin<sup>2</sup>) using a highly aspherized two mirror corrector. All the optical train is working at F/2.5 to maintain a high signal to noise ratio. The spectrograph (R~ 2200 and 1.5 arcsec FWHM) is based on two identical Schmidt systems acting as collimator and camera sharing a 2400 g/mm aspherized reflective Schmidt grating. This grating is manufactured from active optics methods by double replication technique of a metal deformable matrix whose active clear aperture is built-in to a rigid elliptical contour. The payload and gondola are presently under integration at LAM. We will present the alignment procedure and the as-built optic performances of the Fireball instrument.
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 at, 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.
The FIREBall-2 (Faint Intergalactic Redshifted Emission Balloon-2) is a balloon-borne ultraviolet spectro-imaging mission optimized for the study of faint diffuse emission around galaxies. A key optical component of the new spectrograph design is the high throughput cost-effective holographic 2400 ℓ =mm, 110x130mm aspherized reflective grating used in the range 200 - 208nm, near 28°deviation angle. In order to anticipate the efficiency in flight conditions, we have developed a PCGrate model for the FIREBall grating calibrated on linearly polarized measurements at 12° deviation angle in the range 240-350nm of a 50x50mm replica of the same master selected for the flight grating. This model predicts an efficiency within [64:7; 64:9]±0:7% (S polarization) and [38:3; 45]±2:2% (P-polarization) for the baseline aluminum coated grating with an Al<sub>2</sub>O<sub>3</sub> natural oxidation layer and within [63:5; 65] ±1% (S-polarization) and [51:3; 54:8] ±2:8% (P-polarization) for an aluminum plus a 70nm MgF<sub>2</sub> coating, in the range 200 - 208nm and for a 28°deviation angle. The model also shows there is room for significant improvements at shorter wavelengths, of interest for future deep UV spectroscopic missions.
Fireball is a NASA/CNES balloon-borne experiment to study the faint diffuse circumgalactic emission in the ultraviolet around 200 nm. The field of view of the 1 meter diameter parabola is enlarged using a two-mirror field corrector providing 1000 arcmin<sup>2</sup> at the slit mask. The 0.1 nm resolution Multi Object Spectrograph is based on two identical Schmidt systems sharing a reflective aspherical grating. The aspherization of the grating is achieved using a double replication technique of a metallic deformable matrix. We will present the F/2.5 spectrograph design and the deformable matrix process to obtain the Schmidt grating with elliptical contours.