Any optical spectrograph must contain some device, like a prism or diffraction grating, that disperses light.?

Answer 1

There are three classes of spectrograph.
1. Dispersive instruments are built around a prism or a grating. The resolution and throughput are the limiting factors for dispersive instruments.

2. Non-dispersive or filter wheel instruments have a set of filters. This is the simplest and least expensive design, but is limited to a small number of wavelength bands and relatively poor spectral resolution. The throughput is very good.

3. Fourier transform instruments are built around an interferometer and provide the best spectral resolution and throughput, but are the most expensive option.