Also called a grating for short, a diffraction grating is by far the most common dispersive element in spectrographs these days. As might be guessed, diffraction gratings work through diffraction. The gratings can be transmissive or reflective, but the principles are the same for both types. Gratings have grooves on one surface that act as the slits for diffraction. This causes the wavelengths to interfere such that each wavelength constructively interferes at specific angles according to the diffraction equation:
where a is the groove spacing, θm is the angle of diffraction for a given value of m, θi s the incident angle, m is an integer called the grating order, and λ is the wavelength. The equation is additive for reflective gratings and subtractive for transmissive gratings. This is represented schematically for the reflective case:
Note that, for m=0, there is no dispersion and the incident beam is just reflected (refracted for transmissive gratings). The shape of the groove doesn’t actually matter, just the spacing. This has led to different types of gratings with different groove shapes. StellarNet uses ruled (blazed) and holographic gratings. Ruled gratings have a triangular pattern, like vinyl siding on a house. They are highly efficient, but suffer from noise like stray light. Holographic gratings have a sinusoidal surface. They don’t suffer as much from noise, but they are also not as efficient. StellarNet spectrometers uses gratings that have been optimized for each device and wavelength range. View each spectrometer’s standard model tab to see which diffraction grating is used in each pre-configured spectrometer model.
