  Fibre Bragg grating (FBG) OH suppression is capable of greatly reducing the
bright sky background seen by near infrared spectrographs. By filtering out the
airglow emission lines at high resolution before the light enters the
spectrograph this technique prevents scattering from the emission lines into
interline regions, thereby reducing the background at all wavelengths. In order
to take full advantage of this sky background reduction the spectrograph must
have very low instrumental backgrounds so that it remains sky noise limited.
Both simulations and real world experience with the prototype GNOSIS system
show that existing spectrographs, designed for higher sky background levels,
will be unable to fully exploit the sky background reduction. We therefore
propose PRAXIS, a spectrograph optimised specifically for this purpose. The
PRAXIS concept is a fibre fed, fully cryogenic, fixed format spectrograph for
the J and H-bands. Dark current will be minimised by using the best of the
latest generation of NIR detectors while thermal backgrounds will be reduced by
the use of a cryogenic fibre slit. Optimised spectral formats and the use of
high throughput volume phase holographic gratings will further enhance
sensitivity. Our proposal is for a modular system, incorporating exchangeable
fore-optics units, integral field units and OH suppression units, to allow
PRAXIS to operate as a visitor instrument on any large telescope and enable new
developments in FBG OH suppression to be incorporated as they become available.
As a high performance fibre fed spectrograph PRAXIS could also serve as a
testbed for other astrophotonic technologies.
