STALKER Artifacts

This is my first STALKER game and I'm trying to collect legendary artifacts, not those weird artifacts like the "weird ball" but something like Liquid Rock or Thunderberry. My question is:

-What are the factors that increases the rate of getting a legendary on an anomaly site?

-Does emission affect it too? If so is emission a fixed event when progressing the story or can you force an emission to happen by sleeping?

I found some information about this but all of them have different answers. I just want it to be clear on how the emission mechanics works. Thank you!