Anatomy of the HST
Like any telescope, the HST has a long tube that is open at one end to let in light. It has mirrors to gather and bring the light to a focus where its "eyes" are located. The HST has several types of "eyes" in the form of various instruments. Just as insects can see ultraviolet light or we humans can see visible light, Hubble must also be able to see the various types of light raining down from the heavens.
Specifically, Hubble is a Cassegrain reflector telescope. That just means that light enters the device through the opening and bounces off the primary mirror to a secondary mirror. The secondary mirror in turn reflects the light through a hole in the center of the primary mirror to a focal point behind the primary mirror. If you drew the path of the incoming light, it would like the letter "W," except with three downward humps instead of two.
At the focal point, smaller, half-reflective, half-transparent mirrors distribute the incoming light to the various scientific instruments. (We'll talk more about those instruments in the next section.) As you might have guessed, these aren't just ordinary mirrors that you might gaze in to admire your reflection.
HST's mirrors are made of glass and coated with layers of pure aluminum (three-millionths of an inch thick) and magnesium fluoride (one-millionth of an inch thick) to make them reflect visible, infrared and ultraviolet light. The primary mirror is 7.9 feet (2.4 meters) in diameter, and the secondary mirror is 1.0 feet (0.3 meters) in diameter.
Next we'll talk about what Hubble does with all that light after it hits the telescope's mirrors.