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NEWTONIAN REFLECTORS
The first reflecting telescope was supposed to have been built by Sir Isaac Newton. The idea was to replace the refracting lenses with a reflecting surface placed on a mirror, allowing for the use of materials that were not transparent.
The first reflecting telescopes were made with metal mirrors. In the 1800's the first silver or glass mirror was made. Nowadays, silver reflective coatings have been replaced with aluminum coatings, which do not tarnish or oxidize as quickly as silver, but need to be applied in a chamber. Newtonian reflectors do not make good spotting scopes, for the picture the viewer sees will be upside-down, or sitting on its shoulder.
Newtonian Pros and
Cons:
Pros: The Newtonian represents the best price per inch of aperture of all the designs. The optical design is free of chromatic aberration. It tends to have a wider field of view because of faster focal lengths. The eyepiece is comfortably placed for most observing. Since the mirror is at the bottom of the tube, there is rarely a problem of dew forming on the optics.
Cons:
The Newtonian is not a good telescope for daytime use,
as images are upside down. In fast focal rations, stars
on the very edge of the field
will look like small comets,
a phenomenon called "coma." With a large aperture,
Newtonian reflectors require a ladder or stepstool to
view objects directly overhead. The optics require
aligning more often than refractors.
The open tube design means that dust and bugs can get on the mirrors. Large scopes need cool down time to produce the best images. This is the most vulnerable design when it comes to tube currents and thermal effects from the ground.
Note: Collimation and cleaning of a Newtonian reflector is not difficult, and for most people who want a large aperture scope, it is a small price to pay for the low cost and high visual yield that a Newtonian reflector provides
.
There are two types of mounts for Newtonian reflectors:
Dobsonian Mount
A Dobsonian telescope is basically a Newtonian
reflector sitting on an altazimuth mount. It sits on a
lazy-susan type base, so the telescope and its rocker
box can rotate 360 degrees. This is not a good design
for photography, since the mount has no way of locking
down, but it is very good for visual work, and is a good
telescope for beginners.
Equatorial Mount
The equatorial mount for the Newtonian reflector is
basically the same as it is for the refractor, with a
few differences. Due to the increased
weight of the Newtonian reflector, the mount needs to be
heavier to achieve the increase in stability that a
larger scope requires. Some equatorial Newtonians come
with a right-ascension motor, and may even be battery
operated for use out in the field.
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