system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I keep seeing posts, “I pointed my telescope here for 20 hours to give you this image.” Why does it have to be 20 hours of pointing? What’s happening exactly?
gpt
Simple version, there are 2 kinds of light a telescope camera gets: signal, and noise.

Signal is what you want; it is light that came from the object itself. Noise is what you don't want; light from the atmosphere, from interstellar dust, cosmic rays hitting the detector, etc.

What you can see in a picture depends on how these two things compare: this is usually called the "signal-to-noise ratio", or SNR. A high SNR means that you can see a lot of detail, while a low SNR may even mean that you can't see the thing at all. A high SNR is like listening to a flute play in a silent concert hall. A low SNR is like listening to a flute play on the side of a busy highway.

When a chunk of light hits the detector, it doesn't know which it is, which means that it can't really figure out which is which. There is, however, one difference between the two that we can use: Signal increases with time, noise increases as the square root of time.

For example: imagine you have two identical telescopes pointed at the same object. Telescope A spends 1 minute pointed at that object, telescope B spends 4 minutes pointed at that object. Telescope B will have four times as much signal, but only twice as much noise.

This means that as you spend more time pointing at a single target, the SNR increases, which lets you see more and better detail.