Telescope (Refractor and reflector)
A telescope is an optical device which makes far objects
appear closer: it samples a small area of view, a field, and then
magnifies it so that distant objects appear larger.
Parallel light rays entering the telescope are focussed to a
single point (called the focus or focal point) and then magnified
with a very powerful set of lenses (called an eyepiece) to give
enlarged views of distant objects. The eyepiece acts in the
reverse direction to the telescope lens, taking the focussed rays
and sending them to the eye as parallel rays.
There are two basic types of telescopes, refractors and
reflectors, whether the magnification is based upon refraction or
reflection (see optical laws).
The refractor telescope uses a lens to gather and focus light.
The first telescopes built were refractors. Today, excellent
small refractors are still made for amateur telescopes, but the
disadvantages of this kind of instrument have stopped the
construction of very large refractors for use in astronomical
Here is a drawing of the path light follows inside a refractor:
Refractor telescopes present many advantages:
- after the initial alignment, the optical system of a
refractor telescope is more resistant to misalignment
than the reflector telescopes.
- the glass surface inside the tube is sealed from the
atmosphere so it rarely needs cleaning.
- since the tube is closed off from the outside, air
currents and other effects are eliminated. This means
that the images are steadier and sharper than those from
a reflector telescope of the same size.
Refractor also suffer from a series of disavantages:
- All refractors suffer from an effect called chromatic
aberration (see technical card about aberration) that
rainbow of colors around the image. One way to reduce
this problem is to use multiple compensating lenses to
counteract the effects of this kind of aberration.
Another way, uses a very long objective focal length
(which is the
distance between the focus and the objective) to minimize
the effect. This is why the early refracting telescopes
made very long.
- Another problem is the shape of the lens. First of all,
it is technically difficult to make a glass lens with no
inside it, with a perfect curvature on both sides of the
lens. Second problem is the thickness of the lens: in
fact, how well
the light passes through the objective decreases as the
thickness of the lens increases. But at the same time,
objective can only be supported at the ends, the glass
lens might sag under its own weight...
Refractor telescopes (as well as reflector ones ) can
suffer from a defect called spherical abberation so that
not all of
the light is focussed to the same point...
Because of these disadvantages, the largest refractor
telescope built is the one at Yerkes Observatory, with an
objective 1.02 meters across at one end of a 19.2 meter tube.
The reflector telescope uses a parabolic-shaped mirror to gather
and focus light. All modern research telescopes and large amateur
ones are of the reflector type... (click
here to know more about the different types of reflector
Here is a drawing of how light behaves in a reflector telescope:
The advantages of reflector telescopes can be resumed as
- Reflector telescopes do not suffer from chromatic
aberration since all wavelengths will reflect off the
mirror in the same way (click here
to know more about chromatic aberration).
- Mirrors for reflector telescopes are easier to build:
first, only one side of the reflector telescope's
objective needs to be perfect. Second, mirrors can be
made very big, since their support is on the back side of
the telescope. For these reasons, reflector telescopes
are cheaper to make than refractors of the same size.
Despite the advantages of the reflector telescope, astronomers
must accept some minor problems: