Quote Originally Posted by Kirk Keyes View Post
If a diffraction grating is used in the design of a recording spectrograph, then care does have to be taken to eliminate 2nd order diffractions so that they do not show up when trying to measure a 1st order diffration. The second order pattern will be at 1/2 the wavelength of the 1st order, and when trying to record wide ranges of wavelengths, it can be a big problem.

It's not an issue if a prism is used.

It's not an issue either if a thick (holographic) grating is used that largely suppresses higher diffraction orders.