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Some owl species possess 300 times the sensitivity of the human eye in dark conditions. If you use Pogson's ratio i.e. 5th root of 100, this sensitivity difference means whereas man sees celestial objects down to magnitude 6, owls see objects down to magnitude 15. Add to this the ability of many animals' eyes to allow UV light to reach the retina, and the ability of some to see to a limited extent in infrared, then you have a sky that is full of galaxies, quasars etc etc etc that we humans can only see with help. Human eyes are sensitive to UV but we don't get to see it because of the absorption properties of the tissues and substances between lens and retina.
Richard
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