Understanding Telescopes – What’s A Newtonian Reflector?


The first reflecting telescope, built in 1668, is attributed to Sir Isaac Newton. The idea was to replace refracting lenses with a strong 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:


  • Best price per inch of aperture of all the designs
  • Optical design is free of chromatic aberration
  • Tends to have a wider field of view because of “faster” focal lengths (f4.5-8)
  • The eyepiece is comfortably placed for most observing
  • Since the mirror is at the bottom of the tube, dew forming on the optics is rarely a problem


  • Not a good daytime use telescope, as images are upside down
  • In fast focal ratios (f-5 or faster) stars on the very edge of the field will look like little comets… this is called “coma”
  • With large aperture Newtonian reflectors, a ladder or step-stool may be needed to view objects directly overhead
  • The optics require collimation (aligning) more often than refractors or Schmidt-Cassegrains/Maksutov’s
  • The open-tube design means dust, etc. can get on the mirrors–keep them capped when not in use
  • Large scopes need “cool-down” time to produce best images
  • Most vulnerable design as far as tube currents, thermal effects from ground, etc.

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.

There are two basic types of mounts for the Newtonian reflector:


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 . 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.


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.

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