First, the Earth's atmosphere blocks out a broad range of the electromagnetic spectrum , allowing a narrow band of visible light to reach the surface. Second, the Earth's atmosphere blurs the light it lets through. The blurring is caused by varying density and continual motion of air. By orbiting above the Earth's atmosphere, the Hubble can get clearer images. In fact, even though HST has a mirror 15 times smaller than large Earth-bound telescopes , it can still resolve detail almost 100 times finer.
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