Not an astrochemist here, but a molecular biologist with a general interest in science… As best I understand it, since different objects move at different "speeds" across the sky (that is objects will pass in front of one another and then diverge form our apparent view point) you can actually use the differences between the background light and the object you’re measuring if you sample at different times to make accurate spectroscopy measurements. I might actually assume that if light from a star were to pass through a cloud of gas it might actually aid in identified chemicals where there was weak, but important, absorption peaks and more photons would be required to identify the more subtle parts of the spectrum. I’m assuming that every field in science has ways of dealing with noise. If anyone with a real background in astrophysics or astrochemistry would care to comment, I’d really like to know how you guys do it (spare no technical details and talk shop like you would to others in your field.)… There’s nothing like running your first real NMR spec after a real world synthesis and seeing a shit tonne of noise and peaks you didn’t expect.