How can astronomers zero in on one single star for spectral analysis yet be sure they are not receiving light pollution from other stars in the field of view?
Before the spectrum is taken, we are usually able to image the star field and we can also image the position of the slit, which lets in the light. We can then rotate the slit so that it is optimally placed, and we can control the width of the slit so that we only get the light from the star. Depending on the instrument, it is alright to get other stars’ light in the slit as long as the other sources of light are lengthwise along the slit. The spectrograph can usually resolve in one dimension, we then just pick out the source that we want to analyze.