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FAQ Newtonian Reflectors  The first reflecting telescope, built in
1668, is attributed to Sir Isaac Newton. The idea was to replace refracting
lenses with a reflecting surface placed on a mirror. This allowed for use of
material that was not necessarily transparent... different materials could be
used (Pyrex is one of the more common materials used today) that were easier to
manufacture in early times. The first reflecting telescopes were made with metal
mirrors, because they were easier to make than glass. It wasn't until the 1800's
that the first silver-on-glass mirror was utilized. Nowadays, silver reflective
coatings have mostly been replaced with aluminum coatings... these coatings do
not tarnish or oxidize as quickly as silver, but need to be applied in a
chamber. A Newtonian reflector's light gathering source (a concave mirror) is
set into the "bottom" of the tube, and a small, flat secondary mirror directs
the light into the eyepiece, which sits in the focuser on the upper end of the
tube. Remember that Newtonian reflectors do not make good spotting scopes, as
the "picture" you will see will be upside-down or sitting on it's "shoulder".
Newtonian Reflector Pro's & Con's: Pro's:
Con's:
NOTE: Collimation and cleaning of a Newtonian reflector is not difficult, and for most people who want a large-apertured scope, it is a small price to pay for the low cost and high visual yield that a Newtonian reflector provides. TOP There are two basic types of mounts for the Newtonian reflector: Dobsonian  This type telescope is basically a
Newtonian reflector sitting on an altazimuth ("alt-az") mount. It is named after
John Dobson of the San Francisco Sidewalk Astronomers, and actually, he prefers
that this type telescope not be named after him, but be called a "Sidewalk
Telescope". The term "Dobsonian" has prevailed in spite of Mr. Dobson's wishes.
The alt-az mount is very basic. It sits on a "lazy-susan" type base, so that the
telescope and it's rocker box can rotate 360 degrees. Thanks to John Dobson,
amateur astronomers have access to a very inexpensive, large aperture Newtonian.
This is not a good telescope design for astrophotography, since the mount has no
way of "locking down", but it is very good for visual work, and really teaches a
beginner the sky by finding objects yourself and "star-hopping" to them. That is
how I started ten years ago... with a 10.1" Coulter Odyssey Dobsonian (which
sadly, by the way, is out of business). I recommend a "Dob" to those who have an
explorer's spirit, and want to get to know the sky as well as they do their own
back yard. Equatoria The equatorial mount for the Newtonian reflector is basically the same as it is for the refractor, with a few exceptions. Because of the increased weight of a Newtonian reflector (especially 6" and up), the mount needs to be heavier to achieve the increase in stability that a larger scope demands. Some equatorially-mounted Newtonians come standard with a right-ascension (east to west movements) motor, and may even be battery-operated for cordless use out in the field. Instead of a "normal" looking tripod, a "pier" mount may take it's place... kind of a big piece of pipe with three legs attached at the very bottom for support. TOP |
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