40l/mm and 150l/mm grating efficiencies
Use the following plot to decide which order to use when observing with
the 40l/mm grating in CGS4. Use 1st order beyond 2.5 microns.
The data are from Richardson Grating Laboratory for 1st and 2nd order.
3rd order is estimated.
The above plot shows only the grating efficiencies themselves. It does
not include atmospheric transmission and the throughput of the CGS4 filters.
For example, if one wished to observe a feature a 1.35 microns while setting
the central wavelength for the middle of the H band (around 1.6-1.7 microns),
you would expect essentially no throughput at 1.35 microns. This is because
i) the atmospheric transmission itself is only about 20% at this wavelength
and ii) the B2 filter, used for H and K band work, cuts off at about 1.4
For those wishing to observe features that occur close to or at the
cut-off wavelengths for the near-infrared bands, please contact your support
scientist or Paul Hirst beforehand for advice on the most appropriate