spectrographs will use Volume Phase Holographic (VPH) gratings. A VPH grating
consists of a layer of dichromated gelatin (DCG) deposited onto a
glass substrate. The lines of the grating are formed in the DCG by exposure
to light and then chemical processing to form a hologram. Ruling densities
between 100 and 6000 lines per mm can be achieved. Further information
about VPH gratings can be found on the AAO
- VPH www page.
An example of the efficiency of
a VPH grating compared with a conventional AAO diffraction grating can
be seen in the figure below.
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| Comparison of the efficiency of a 1516 lines per mm VPH grating and a conventional 1200 lines per mm AAO diffraction grating. Note that the VPH efficiency curve includes reflection losses (~10%) which can be avoided with suitable antireflection coatings on the grating substrates. |