Also the ratio of H alpha/H beta emission in nebulae for example is useful for measuring the amount of interstellar extinction eg as described here
https://web.williams.edu/Astronomy/research/PN/nebulae/exercise2.php
In fact there are a number of interesting measurements you can make based on measuring the relative intensity of various lines in planetary nebulae using just a low resolution spectrograph, as described in Francois Teyssier’s low resolution spectroscopy observers guide
http://www.astronomie-amateur.fr/Documents%20Spectro/SpectroscopieBasseResolution_En.pdf
and detailed on his website here
http://www.astronomie-amateur.fr/feuilles/Spectroscopie/NGC2392.html
Robin