non linear

Robin Leadbeater

All spectrographs are non linear to at least some degree and have to be individually calibrated in wavelength. Mike’s spectrograph is fundamentally different to mine for example. His uses a prism and the dispersion is due to refraction whereas mine (and Hugh’s) uses a diffraction grating and the dispersion is due to diffraction. As a result the relationship between wavelength and distance along the spectrum are completely different and in fact run in opposite directions !  (with diffraction the short wavelengths are least deflected whereas in refraction the short wavelengths are refracted the most)