The reflector telescope uses a mirror to gather and focus light. All
celestial objects (including those in our solar system) are so far away
that all of the light rays coming from them reach the Earth as parallel
rays. Because the light rays are parallel to each other, the reflector
telescope's mirror has a parabolic shape.
The parabolic-shaped mirror focuses the parallel lights rays to a single
point. All modern research telescopes and large amateur ones are of the
reflector type because of its advantages over the refractor telescope.
There are many advantages to using a reflector telescope as opposed to a
refractor telescope. Reflector telescopes do not suffer from chromatic
aberration because all wavelengths will reflect off the mirror in the
same way. Support for the objective mirror is all along the back side so
they can be made very BIG!
Reflector telescopes are cheaper to make than refractors of the same
size. Because light is reflecting off the objective, rather than passing
through it, only one side of the reflector telescope's objective needs to
Even with these advantages, there are some disadvantages to a reflector
telescope. First, it is easy to get the optics out of alignment. A
reflector telescope's tube is open to the outside and the optics need
frequent cleaning. Often a secondary mirror is used to redirect the light
into a more convenient viewing spot. The secondary mirror and its
supports can produce diffraction effects: bright objects have spikes (the
“Christmas star effect'').
In both the reflector and refractor telescopes, the focus is before the
eyepiece, so the image in astronomical telescopes is upside down.
Telescopes used to look at things on the Earth's surface use another lens
to re-invert the image right-side up.
Most reflector telescopes will use a smaller secondary mirror in front of
the large primary mirror to reflect the light to a more convenient
viewing spot. Isaac Newton used a flat secondary mirror at a 45° angle to
reflect the light to an eyepiece at the side of the telescope tube near
the top. Such an arrangement, called a Newtonian design is used by many
Many reflector telescope use another light path design called the
Cassegrain design to reflect the light back through a hole in the primary
mirror, so that detectors or the eyepiece can be conveniently placed
behind the telescope. Most of the large telescopes used for research,
including the Hubble Space Telescope, are of this design.