Telescopes >> Eyes

Before the early 1600s, scientists' interest in the stars could only go so far because they were bound by the limits of their eyesight.  The invention of the telescope provided important advances in the field of observational astronomy for many reasons, but two stand out above the rest.

1. Telescopes can collect more light than the naked eye and can thus see a more magnified image.
2. The telescope has the ability to focus on one specific portion of the field of view with a higher resolution than the human eye.

The human eye is incredible, but its size severely limits the amount of light and information it can take in.  Telescopes give humans a way to have 3-feet-wide eyes  without attracting too much unwanted attention on the street.  That 3-feet-wide eye can collect a lot of light which translates into a better, closer-appearing image.  Light enters the telescope through the primary mirror (for reflector telescopes) or the objective lens (refractors) and gets focused to an eyepiece at the end of the scope.  (More complex telescopes have more reflection steps between the first lens/ mirror and the eyepiece.)

Telescopes also put the human eye to shame when it comes to focusing on a particular point in the field of vision without letting the image get really blurry.  When human eyes look at the night sky and try to focus on a single object, the image often gets blurry because of excess light coming from other stars.  Telescopes can "squint" more effectively than the human eye can, finding the perfect balance between letting enough light in and keeping enough out.

In summation, telescopes are just better than the naked eye at producing clear, magnified images of celestial objects.  They collect more information, focus it more effectively, and can even collect information outside of the visual spectrum.  As great as the human eye is at observing things in our immediate environment, it really can't hold a candle to the telescope.