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PRAXIS is a second generation instrument that follows on from GNOSIS, whichwas the first instrument using fibre Bragg gratings for OH backgroundsuppression. The Bragg gratings reflect the NIR OH lines while beingtransparent to light between the lines. This gives a much higher signal-noiseratio at low resolution but also at higher resolutions by removing thescattered wings of the OH lines. The specifications call for high throughputand very low thermal and detector noise so that PRAXIS will remain sky noiselimited. The optical train is made of fore-optics, an IFU, a fibre bundle, theBragg grating unit, a second fibre bundle and a spectrograph. GNOSIS used thepre-existing IRIS2 spectrograph while PRAXIS will use a new spectrographspecifically designed for the fibre Bragg grating OH suppression and optimisedfor 1470 nm to 1700 nm (it can also be used in the 1090 nm to 1260 nm band bychanging the grating and refocussing). This results in a significantly highertransmission due to high efficiency coatings, a VPH grating at low incidentangle and low absorption glasses. The detector noise will also be lower.Throughout the PRAXIS design special care was taken at every step along theoptical path to reduce thermal emission or stop it leaking into the system.This made the spectrograph design challenging because practical constraintsrequired that the detector and the spectrograph enclosures be physicallyseparate by air at ambient temperature. At present, the instrument uses theGNOSIS fibre Bragg grating OH suppression unit. We intend to soon use a new OHsuppression unit based on multicore fibre Bragg gratings which will allowincreased field of view per fibre. Theoretical calculations show that the gainin interline sky background signal-noise ratio over GNOSIS may very well be ashigh as 9 with the GNOSIS OH suppression unit and 17 with the multicore fibreOH suppression unit.

PRAXIS: low thermal emission high efficiency OH suppressed fibre spectrograph