The rest is light transport, e.g. lensing via index of refraction: https://courses.lumenlearning.com/boundless-physics/chapter/...

Here's a cool article on a piece of an old spectrometer: https://utsic.utoronto.ca/spectroscopy-beyond-the-visible-sp...

More information from a distributor of optics: https://www.knightoptical.com/stock/default/windows-and-diff...