Echelle and holographic gratings compared for scattering and spectral resolution

An echelle grating with 316 grooves/mm and a 63 degrees blaze angle is compared with a 3600-groove/mm holographic grating. The scattered light and resolution of the 10- x 20-cm gratings are photoelectricaly evaluated in a 3-m spectrometer optimized to eliminate coma and baffled to minimize instrumental scattered light. Both gratings are ghost-free, but the holographic grating has substantially lower scattered light for test lines near 4000 A and yields improved stray light rejection within absorption lines. It also exhibits slightly higher resolution. The throughput of the spectrometer with the holographic grating is about 5 times higher than the equivalent combination of spectrometer, echelle grating, and predisperser.