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No, the most important and difficult things for a telescope to do are:
What matters in making the dim bright is the area of the opening of the
telescope. All the light that goes into the telescope goes into your
eyes. So it's like having giant eyes. You can see things that are dimmer
because more light is put together into what you see.
So the most important thing in deciding how good a telescope is, is its
aperture, the size of its opening. People talk about the diameter of
the telescope, for example. the telescope at Mount Palomar has a diameter
of 200 inches (that's about 5 meters.) A crappy telescope, like the one
Galileo used, might have an aperture of only say 2 inches diameter.
What's important is the area (because that's how much light gets let
in), so the Palomar telescope's area is pi (100 inches)2 (the
area of a circle is pi times the RADIUS squared). A telescope more like
what Galileo used had an area of pi (1 inch)2, so that Palomar's
telescope collects 10,000 times as much light as Galileo's.
The world's biggest telescopes (in terms of light-gathering ability) are the
Keck telescopes in Hawaii. They have apertures of 10 meters, so they
collect 4 times as much light as the Palomar telescope. Here is a list of the
world's biggest telescopes, both existing and planned.
Larger telescopes also have better resolution. Why should that be? It
turns out that this is a result of the wave nature of light! If you imagine
a water wave coming in to a narrow bay, then it spreads out in all
directions. If instead it goes through a very broad area, it won't spread
out as much. In the same way, light going through a narrow hole spreads out
in all directions, but light going through a wide hole continues in the
direction it came in. This spreading out of light is called
diffraction.
One way to think about this from a physics point of view is
called "Heisenberg's Uncertainty Principle." This is a principle of quantum
mechanics, and says basically if you know where something is, very
accurately, then you can't be accurate about where it is going. A small
hole gives you accurate knowledge of where the light is, but as a result, it
spreads the light out in all directions to give you uncertain knowledge of
where it's
going.
The equation relating the opening size of a telescope and the resolution
is
This tells you the best possible resolution you can get (the
smaller the resolution the better--because it means you can see
smaller details.) Resolution can be limited in other ways too. For
example, by distortion in the atmosphere (this is what makes stars
"twinkle.")
The human eye has a resolution that's limited by the size of its opening
too. Using for the size of a pupil about 0.5 cm or 5x10-3 m, and
for the wavelength of light about 500 nm or 5x10-7 m, the
equation gives us a resolution of 2.5x105x10-4=25 arc
seconds, or about half an arc minute. The textbook gives 1 arc minute, so
perhaps the human eye's resolution is not as good as theoretically possible
given its opening size.
So once you have a telescope, you can make the light pass through a
spectrograph. That gives a spectrum of the light from whatever you're
looking at. A spectrograph is basically a prism or diffraction grating
(a material with lines close together that gives a similar
effect to a prism)
A refracting telescope uses lenses and usually takes up a
lot of space and is hard to build.
How a refracting telescope works. Light is brought to a
focus and then magnified by another lens. The entire length of the tube
is used for this focussing.
How does a telescope work? Well, think about a magnifying glass. A
magnifying glass is fine, but it won't work past a certain distance from
the object you're magnifying. That distance is its focal length.
Look at an object too far away, and it will be blurry in a magnifying
glass. But if you put a piece of paper at the right place behind the
magnifying glass, it show an image in focus. For an object infinitely far
away, the image is formed at a distance behind the lens equal to the focal
length. The magnification is the ratio of the focal lengths. So
to get a higher magnification all you have to do is to use a shorter focal
length eyepiece!
Sir Isaac Newton invented the reflecting telescope. His
design, now called the Newtonian telescope, has the light bouncing off a
mirror, back through the telescope to another mirror, and then through the
side of the tube. It surprises some people that the mirror in the way
only causes the light to dim instead of casting a shadow on the final
image.
There are several different kids of reflecting telescopes in addition to
the Newtonian design. One of the more popular is the Casegrain telescope,
in which the secondary mirror reflects the light from the main (primary)
mirror back through a hole in it.
Here's a picture of a Casegrain telescope (these are usually pretty
compact, and are more portable):
The world's largest
telescopes, both operational and planned
The Hubble Space Telescope science
page
Optical
interferometry a technique that can increase the resolution of a
telescope by combining views from different locations
Telescopes: What they do
Many people (not professional astronomers!) think that the main value of a
telescope is in its magnification. But this isn't so! If you took a
photograph through a telescope, you could keep zooming in on the photo
forever, magnifying it more and more, no matter what kind of a telescope you
had. And in fact, a telescope has at least two parts, one to bring in
light, and the other, the eyepiece, brings the light into focus and
magnifies the image. High magnification eyepieces are cheap and easy to
make, compared with the rest of a telescope.
The Hubble Space Telescope has an
aperture of 2.4 meters, so it doesn't collect as much light as even Palomar,
let alone Keck. However, its resolution is limited only by
diffraction, not the Earth's atmosphere, which limits most telescopes
to about 0.5 arc seconds resolution. Hubble's is 0.05 arc seconds.
Reflecting and Refracting
Telescopes
There are two basic types of telescopes. Refracting telescopes use
lenses only. They're the classic kind you're more likely to see in
movies--their shape is long and tapered. Reflecting telescopes use
mirrors instead. Because the light bounces back through the same space it
came in before reaching a focus, reflecting telescopes don't have to be as
long. Also, it's easier to hold up a large mirror than it is to hold up a
large lens. A lens can only be held from its edges, but a mirror can be
held from behind (if you hold a lens from behind, the light can't shine
through!) A lens has to be pure all the way through, but a mirror only
has to be polished on its surface. Lenses can have trouble focussing
different colors at the same time. For all these reasons, reflecting
telescopes are the type favored by professional astronomers. 
