AAO - VPH grating page. The DCG layer is usually sandwiched between two glass substrates.
Transmissive operation allows shorter pupil relief in spectrograph optics.
Use in littrow configuration eliminates beam dilation. Very robust construction Glass substrates can be antireflection coated. The grating is approximately 80 mm square. The picture shows the LDSS VPH grating in the optical mount that locates inside the LDSS instrument. A plot of efficiency of the grism is given below.