It makes sense that each frame would suffer from signal-to-noise problems and be degraded, but given that this image was created over 2 hours -- is there really

It makes sense that each frame would suffer from signal-to-noise problems and be degraded, but given that this image was created over 2 hours -- is there really any loss in quality if sampled correctly? I could honestly believe there are a bunch of complexities beyond this, but: is this already having to be done _anyway_?