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Why is bigger aperture Better?

By Brian Tung.
Larger aperture means brighter images.  It also means you can get more 
effective magnification.  Why is this so?

The first part is easy (well, easier).  A telescope can be divided into
two basic parts: the objective--typically a large lens at the front of
the telescope, or a large mirror at its rear--and the eyepiece.  The
purpose of the objective is to take light from a distant object (say, a
galaxy), and to focus that light into an image located inside the
telescope, near the eyepiece.

Your eyepiece then acts as a magnifying glass, allowing you to see that
image enlarged, just as though you were observing an actual miniature
galaxy inside the telescope.  The bigger the telescope, the more light
is collected by the telescope, and therefore, the brighter the image
magnified by the eyepiece (assuming equal magnifications).

The second part--that is, why bigger apertures permit more power--is
not as easy to explain.  The exact details are outside the scope of
this post (you can ask me, if you like), but in essence, the fact that
light is a wave, instead of a stream of rays, creates some irreducible
blurring in the little image of the galaxy within the telescope.  And
the larger the objective, the less blurring there is.

So, when you magnify the image with an eyepiece, the aperture largely
determines whether what you've magnified is blurring, or detail within
the object.  The image formed by a 5-inch scope is simply more blurred
by the wave nature of light than the image formed by a 10-inch scope.
You know that when you take a blurry photo of something, looking at it
through a magnifying glass won't make it clearer, and in the same way,
no amount of magnification with an eyepiece will make the image in the
5-inch scope as clear and detailed as that in the 10-inch scope.  There
is simply more detail in the image of the 10-inch scope, and that is
why it can take more magnification.  This particular effect of light is
called diffraction, and when a scope is good enough that diffraction
limits the magnification you can use, and not some intrinsic defect in
the telescope, that scope is called diffraction limited.

What's more, the amount of blurring can be expressed as an angular
size, telling you how close two things can be in the sky before they
are blurred together, more or less.  We're used to expressing things
in linear size, so that the Moon is about 3,475 km across, but we can
also say how big things are in terms of angles.  From the horizon to
the zenith (the point overhead) is a right angle--90 degrees--and
about 180 Full Moons, stacked top to bottom, would fit in that in that
space.  So the Moon has an angular width of 90/180, or 0.5 degrees.

Well, it turns out that the blurring caused by the wave nature of light
in even a 5-inch scope is tremendously small: only about 1/4,000 of a
degree.  But that caused in a 10-inch scope is even smaller--half as
much, or 1/8,000 of a degree.  Details are visible in the 10-inch scope
that have only half the angular width of those in the 5-inch scope.  So
not only can the 10-inch scope take more magnification, it can take
just about *twice as much* magnification.  And if you had a 15-inch
scope, it could take three times as much magnification, and so on.

So, why not make humongous telescopes?  Obviously, bulk is one reason,
but another reason is that the wave nature of light and the optical
quality of the scope aren't the only things to produce blurriness in
the image.  Another factor is the atmosphere.  The atmosphere's blurring
can also be expressed in terms of angles, and it turns out that that
blurring is usually at least 1/4,000 of a degree, also--sometimes better
but sometimes worse.  Thus, there's a certain point at which larger
aperture just doesn't help you get additional detail, so there's not
much point to adding additional aperture without doing something about
the atmosphere.

One thing to do is to get above the atmosphere altogether--that's what
Hubble did.  (That also allows Hubble to take pictures in ultraviolet,
which the atmosphere is almost opaque to.)  Another thing to do is to
apply adaptive optics, which deform the optics: normally the wrong thing
to do, but in this case done carefully so as to cancel out the blurring
caused by the atmosphere.  Some large observatory telescopes do this.

Anyway, that's why larger apertures produce brighter images and take
higher magnification.

Brian Tung <>
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