reflecting telescopes

1. Reflecting
telescopes

2. A reflecting telescope is a telescope
that uses a single or a combination of
curved mirrors that reflect light and
form an image. The reflecting
telescope was invented in the 17th
century by Isaac Newton as an
alternative to the refracting telescope
which, at that time, was a design that
suffered from severe chromatic
aberration.
DEFINITION

3. Almost all of the major telescopes used
in astronomy research are reflectors.
Reflecting telescopes come in many
design variations and may employ extra
optical elements to improve image
quality or place the image in a
mechanically advantageous position.
VARIATIONS

4. HISTORY
In late 1668 Isaac Newton built his first
reflecting telescope. He chose an alloy of
tin and copper as the most suitable
material for his objective mirror. He
chose a spherical shape for his mirror
instead of a parabola to simplify
construction. He found that the telescope
worked without colour distortion and that
he could see the four Galilean moons of
Jupiter and the crescent phase of the
planet Venus with it.

5. DESIGNS
The Gregorian telescope,
described by Scottish
astronomer and
mathematician James
Gregory in his 1663 book
Optica Promota. This one
produces an upright image,
useful for terrestrial
observations.

6. DESIGNS
The Newtonian
telescope is one of the
simplest and least
expensive designs for a
given size of primary,
and is popular with
amateur telescope
makers as a home-build
project.

7. DESIGNS
The cassegrain telescope was
first published in a 1672
design attributed to Laurent
Cassegrain. It has a parabolic
primary mirror, and a
hyperbolic secondary mirror
that reflects the light back
down through a hole in the
primary. The folding and
diverging effect of the
secondary mirror creates a
telescope with a long focal
length while having a short
tube length.

8. Technicalconsiderations
A curved primary mirror is the reflector telescope's basic
optical element that creates an image at the focal plane.
The distance from the mirror to the focal plane is called
the focal length. Film or a digital sensor may be located
here to record the image, or a secondary mirror may be
added to modify the optical characteristics and/or redirect
the light to film, digital sensors, or an eyepiece for visual
observation.

9. The primary mirror in most modern
telescopes is composed of a solid glass
cylinder whose front surface has been
ground to a spherical or parabolic shape. A
thin layer of aluminum is vacuum
deposited onto the mirror, forming a highly
reflective first surface mirror.
Technicalconsiderations

10. Some telescopes use primary mirrors which are made
differently. Molten glass is rotated to make its surface
paraboloidal, and is kept rotating while it cools and solidifies.
The resulting mirror shape approximates a desired paraboloid
shape that requires minimal grinding and polishing to reach the
exact figure needed.
Technicalconsiderations

11. Useinastronomicalresearch
Reflectors work in
a wider spectrum
of light since
certain wavelengths
are absorbed when
passing through
glass elements.
Nearly all large research-grade astronomical telescopes
are reflectors. There are several reasons for this:
Light of different wavelengths travels through a medium other
than vacuum at different speeds. This causes chromatic
aberration. Reducing this to acceptable levels usually involves a
combination of two or three aperture sized lenses The cost of
such systems therefore scales significantly with aperture size. An
image obtained from a mirror does not suffer from chromatic
aberration to begin with, and the cost of the mirror scales much
more modestly with its size.

12. Reflector vs.Refractortelescopes
Prosandconsofeach
Reflector:

13. REFractor: