COHSI uses a hardware OH suppression technique because it uses relatively small arrays (2, 2562 arrays) which simply do not have enough pixels to capture a large number of spectra at R=3000 and have full J+H spectral coverage. With the advent of the new 10242 arrays this restriction is removed and it is possible to do the OH suppression in software (digital suppression) by simply capturing the spectra at high spectral resolution and ignoring the small percentage of pixels hit by OH lines. This is how our next generation spectrograph CIRPASS will work.
We have completed a design study in to CIRPASS and our conclusions thus far are that a fibre-fed echelle spectrograph is the best way forward particularly for an 8-m class telescope. However, to ensure that scattered light and ghost images do not substantially increase the background above that of the sky it will be necessary to employ a hardware mask as well.
Details of our work are given in the CIRPASS design study reports Part A Part B
Optimal aperture size study Sorry figures not included yet