The reflector telescope, which uses mirrors instead of
Whatever the telescope, it’s most important spec is its
aperture: the diameter of its main, light-gathering lens or mirror. (This lens
or mirror is called the telescope's objective.) The bigger the aperture, the
sharper and brighter the view will be.
To understand how telescopes work,
let's ask the following question. Why can't you see an object that is far away?
For example, why can't you read the writing on a dime when it is 150 feet (55
meters) away with your naked eyes? The answer to this question is simple: the
object does not take up much space on your eye's screen (retina). If you
want to think about it in digital camera terms, at 150 feet the writing on the dime does not cover enough pixels
on your retinal sensor for you to read the writing. If you had a "bigger
eye," you could collect more light from the object and create a brighter
image, and then you could magnify part of that image so it stretches out over
more pixels on your retina. Two pieces in a telescope make this possible:
The objective lens (in refractors) or primary
mirror (in reflectors) collects lots of light from a distant object
and brings that light, or image, to a point or focus.
An eyepiece lens takes the bright light from the
focus of the objective lens or primary mirror and "spreads it
out" (magnifies it) to take up a large portion of the retina. This is
the same principle that a magnifying glass (lens) uses; it takes a small
image on the paper and spreads it out over the retina of your eye so that
it looks big.
When you combine the objective lens
or primary mirror with the eyepiece, you have a telescope. Again, the basic
idea is to collect lots of light to form a bright image inside the telescope,
and then use something like a magnifying glass to magnify (enlarge) that bright
image so that it takes up a lot of space on your retina.