How Telescopes Work
Telescopes can work in two basic
ways, refraction or reflection.
Refraction
Refracting
telescopes use two convex lenses to magnify an
image. Light rays coming from an object (for
example a planet) that is considered to be at a
distance of infinity, come through the first lens
(the Objective lens). The light gets refracted, or
bent, and converge at a focal point to create a
real, diminished, inverted image. This image
becomes the new object for the next lens to focus
on. The light rays continue past the focus and hit
the second lens (the Eyepiece lens) and get bent a
second time to create a virtual image that is
magnified much larger than the first image, and
inverted.
Reflecting telescopes use a curved mirror as
the Objective instead of a lens to focus
incoming light from an object. The light
reflects to a second mirror in one of two
types of orientations:
1) To a plane mirror set at
45º which reflects to an Eyepiece lens
that bends the light to the Eyepiece lens.
2) To a convex mirror that
reflects the image back towards a slit in the
middle of the Objective mirror to
the
Eyepiece lens.
"For an astronomical telescope to produce
bright images of distant stars, the objective lens
must be large to allow in as much light as
possible. Indeed, the diameter of the objective
lens (and hence its "light-gathering power") is an
important parameter for an astronomical telescope,
which is why the largest ones are specified by
giving the objective diameter (Physics Sixth
Edition Giancoli)." As long as a telescope has
light coming into it's Objective lens or mirror it
is able to see an image. The brighter the light
the sharper the picture will be, no matter the
distance between the object and the telescope.
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