It doesn't gather as much light as ground based telescopes, but the background (for example, from airglow) is also much lower, so the SNR is pretty good.
That's one of the reasons for building in the Atacama desert. Also, the adaptive optics technology has pretty much removed the majority of the negatives from the atmosphere.
What's SNR?
An abbreviation for “signal-to-noise ratio” that imo should have been StNR or simply S/N. (Thus I sympathize with not parsing it immediately — it is widely used, though, so it is good to know.)