Newtonian Reflectors

Reflecting telescopes (or reflectors) collect light using a curved mirror at the rear of the main tube rather than a lens at the front end.  Isaac Newton invented the first reflecting telescope in the late 17th century.  He used a second small diagonal mirror to direct light out the side of the telescope to an eyepiece. His immensely practical design, now called the Newtonian reflector, is the main type of purely reflecting telescope in use today by amateur astronomers.  Newtonian telescopes have made a comeback in the past two decades with the introduction of the Dobsonian telescope, which is a Newtonian reflector mounted on a simple but sturdy mount.

For a given aperture, Newtonian reflectors are generally the least expensive telescope because, unlike the lenses of a refractor, only one surface of a mirror needs careful figuring and polishing. And since no light passes through a mirror, less expensive glass is used. Some homemade reflectors even use glass from the discarded portholes of a ship!

The layout of a Newtonian reflector.  The front of the tube is open; the mirror sits at the back of the tube.  A smaller secondary mirror reflects light out the side to an eyepiece.

The layout of a Newtonian reflector. The front of the tube is open; the mirror sits at the back of the tube. A smaller secondary mirror reflects light out the side to an eyepiece.

Newtonians are a great value.  A top-of-the line 4-inch refractor costs as much or more as 12-inch Newtonian reflector, yet the reflector has 9 times the light gathering capability. Newtonian reflectors occasionally require adjustment of their optical alignment– especially if the scope gets bumped.  The adjustment is not complicated, but it takes a little practice.  Newtonians also tend to be bulky, with tube lengths of 4-5 feet or more.  Newtonians also have a type of optical aberration called “coma”. This makes stars appear wedge-shaped at the edge of the field of view. But it’s not a big deal if the focal ratio is bigger than f/6 or so.  Another advantage of Newtonians: because mirrors reflect all colors of visible light exactly the same, reflecting telescopes have no chromatic aberration.

A Dobsonian version of a Newtonian reflector

Newtonians came back into style in a big way in the 1980’s when telescope makers commercialized a design by the former monk and astronomy popularizer John Dobson.

The “Dobsonian” telescope is just an inexpensive large-aperture Newtonian on a simple alta-zimuth mount that points left-right (in azimuth) and up-down (in altitude).  Dobsonians can’t be beat for purely visual deep-space observing.  With huge mirrors (up to 36” in diameter) these new-age Newtonians are called “light buckets” because of their immense light-collecting ability.

If you’re interested in strictly visual observing, and you want the most aperture (and light-gathering capability) for your money, a Dobsonian telescope is well worth your consideration. An 8-inch (200 mm) f/6 Dobsonian with a focal length of 1200 mm is an excellent workhorse telescope, even for beginners. For most observers, it is not too long or heavy to move around, and still has enough aperture to show thousands of deep-sky objects.  Yet a quality Dobsonian scope of this size can cost less than $500.

Newtonian/Dobsonian Pros

  • Largest aperture per dollar
  • Easy to use when mounted in a simple Dobsonian configuration
  • No chromatic aberration

Newtonian/Dobsonian Cons

  • Large, long tubes
  • Require occasional minor alignment
  • Some aberration (coma) at short focal ratios

Newtonians are best for

  • Observing Moon, planets, and fainter deep-sky objects
  • Budget-conscious observers who want a lot of telescope and don’t mind lugging it around