Apart from what others have said, you'll run into problems with chromatic lens aberrations the further your "key" wavelength is outside of the visible spectrum

Apart from what others have said, you'll run into problems with chromatic lens aberrations the further your "key" wavelength is outside of the visible spectrum so your two images might start to mismatch too much. I know in IR still photography you're supposed to add a focus offset marked on your lens to adjust for that. A regular achromatic lens quickly fails outside of the wavelength range it was designed for[1]. [1] https://en.wikipedia.org/wiki/Chromatic_aberration#Minimization https://en.wikipedia.org/wiki/Chromatic_aberration#Minimizat...