August 15, 2008

In Today's One Minute Astronomer...

Newtonian Reflectors

The telescope with the most light-collecting power for your money is the venerable Newtonian reflector, invented by Sir Isaac Newton himself in 1689. If you love to see the wisps of faint nebulae and the spiral structure of distant galaxies, then this may be the scope for you.

The Basics

• The idea behind a Newtonian reflector is simple. Light falls on a curved primary mirror held at the bottom of a long tube. The mirror focuses the light back to a point at the top of the tube where a small flat secondary mirror directs the light towards an eyepiece at the side of the telescope. This way, the observer’s head won’t get in the way of the starlight falling on the mirror.

• Because it’s simple, a Newtonian is the least expensive telescope per inch of aperture. A quality 12” telescope on an altazimuth mount sells for less than $1000 .

• The main practical drawback with a Newtonian is size and weight. That 12” Newtonian is more than 5 feet long and weighs 80 lbs. Hard to handle in an apartment elevator. And you might find it taxing to wrestle into your backyard night after night.

A replica of Newton's original reflector

A Deeper Look

• There are no additional lenses or mirrors to correct the image in a Newtonian, so you may detect an 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.

• Newtonians also need frequent adjustment, or collimation, of the secondary mirror. This simple procedure takes a few extra minutes at the beginning of each observing session once you get the hang of it.

• And keep this in mind: to get the most out of a large reflector on faint deep-sky objects, you need dark sky. In a big city, the brightness of the background sky may exceed the surface brightness of the objects you want to see. In that case, the biggest telescope in the world won’t help you.

Good To Know

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 altazimuth mount. 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.

Personal View

One day, when I live under darker skies and have lots more time, I'm getting myself an 18-inch Dobsonian and savoring the views of the deep-sky fuzzies I now struggle in vain to see. Let's hope by that time I'm still young enough to wheel such a scope out to the back yard!

--- Highly Recommended ---

A great scope for kids and casual stargazers, this easy-to-use Dobsonian has no complicated control knobs or adjustments. See bands on Jupiter, craters and mountains on the moon, as well as hundreds of star clusters, galaxies, and nebulae. Includes two eyepieces and Starry Night software. Sky and Telescope calls it ".... a new standard for the beginner's telescope". Learn More.

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