  PRAXIS is a second generation instrument that follows on from GNOSIS, which
was the first instrument using fibre Bragg gratings for OH background
suppression. The Bragg gratings reflect the NIR OH lines while being
transparent to light between the lines. This gives a much higher signal-noise
ratio at low resolution but also at higher resolutions by removing the
scattered wings of the OH lines. The specifications call for high throughput
and very low thermal and detector noise so that PRAXIS will remain sky noise
limited. The optical train is made of fore-optics, an IFU, a fibre bundle, the
Bragg grating unit, a second fibre bundle and a spectrograph. GNOSIS used the
pre-existing IRIS2 spectrograph while PRAXIS will use a new spectrograph
specifically designed for the fibre Bragg grating OH suppression and optimised
for 1470 nm to 1700 nm (it can also be used in the 1090 nm to 1260 nm band by
changing the grating and refocussing). This results in a significantly higher
transmission due to high efficiency coatings, a VPH grating at low incident
angle and low absorption glasses. The detector noise will also be lower.
Throughout the PRAXIS design special care was taken at every step along the
optical path to reduce thermal emission or stop it leaking into the system.
This made the spectrograph design challenging because practical constraints
required that the detector and the spectrograph enclosures be physically
separate by air at ambient temperature. At present, the instrument uses the
GNOSIS fibre Bragg grating OH suppression unit. We intend to soon use a new OH
suppression unit based on multicore fibre Bragg gratings which will allow
increased field of view per fibre. Theoretical calculations show that the gain
in interline sky background signal-noise ratio over GNOSIS may very well be as
high as 9 with the GNOSIS OH suppression unit and 17 with the multicore fibre
OH suppression unit.
