N.B. Both 740 l/mm and 3300 l/mm VPHGs have more
strongly-peaked blaze functions than the SRGs, however, the
VPHG diffraction efficiencies are still greater than the
SRG efficiency at the edge of the Bench band-pass.
Predicted performance with the STA1 CCD:
The figure below shows a direct comparison of the 740 l/mm
VPH grating with the 790 l/mm surface ruled grating from the
KPNO Mayall 4m RC psectrograph collection. The two gratings were
both used with the Bench spectrograph to obtain dome flats and
comparison lamp data. The plots show the flux ratios of the 740
l/mm VPH to the 90 l/mm surface ruled grating in orders 1 and 2.
For each of the wavelngth regions measured, the VPH grating has
to be rotated to the optimum angle of incidence and diffraction,
indicated as 17°, 19°, etc.
The plots demonstrate that the VPH grating is has much higher
throughput, yet gives comparable spectral resolution.