The FIREBall-2 UV sample grating efficiency at 200-208nm

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 2400l/mm, 110x130mm aspherized reflective grating used in the range 200 - 208nm, near 28 degrees 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 degrees 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 Al2O3 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 MgF2 coating, in the range 200 - 208nm and for a 28 degrees deviation angle. The model also shows there is room for significant improvements at shorter wavelengths, of interest for future deep UV spectroscopic missions.