The Newtonian Reflector Telescope

This Uniquely-look telescope first developed by another pioneer in science, Sir Isaac Newton. The design of this telescope is fairly simple. Other than the lenses in the eyepiece, there is only one surface that has a curve in it (the primary mirror). Light enters the top of the telescope and travels to the bottom of the tube, where it is reflected off of the primary mirror, and focused back up the tube. Before reaching the focal point, the light is reflected off the secondarymirror, which is set at a 45 degree angle in the center of the tube, near the top. The light then leaves the scope through the side of the tube, where the focuser is placed.

The design is so simple that, prior to the arrival of large-telescope manufacturers, many amateur astronomers built their own reflectors, grinding the necessary curve into the glass of the primary mirror. Some still do.

Advantages

1. The single largest advantage of the Newtonian reflector is cost. Because of the simplicity of its design, it is by far the least expensive type of telescope, per inch of aperture. For the price of some 4 inch apochromatic refractors, you could buy a 20 inch reflector. Of course, a larger aperture means brighter, clearer images; and the brighter the images, the more dim objects you

can see.

2. Reflectors do not suffer from chromatic aberration, since the light is reflected off a single surface, instead of being refracted through a lens.

3. In the sizes available to the beginner, the focuser is usually placed at a comfortable viewing position near the top of the scope (unlike larger models).

4. Because one end of the tube is open, Newtonian reflectors cool down more quickly than some other types of telescope.

Disadvantages

1. Reflectors have their own type of aberration: coma. Stars near the edge of the field of view seem to have tails, like little comets, or commas. The problem is worse in large scopes (f/5 or larger).

2. Reflectors have a central obstruction. The secondary mirror blocks some of the incoming light, which lowers the contrast.

3. Diffraction spikes: the secondary mirrors in Newtonian reflectors are held in place by a spider, which is simply a structure with three or four thin vanes that stretch from the edge of the tube toward the middle, where the mirror is attached. This structure interferes with the view and causes diffraction spikes, small rays that appear on brighter stars in the pattern of the spider’s vanes.

4. Reflectors need some cooling time. Because the light entering a reflector travels down and then back up the tube, it is more affected by tube currents. Time is needed for the temperature inside and outside of the tube to equalize. The larger the scope, the longer it takes; partially because the main mirror itself becomes so thick in bigger reflectors that it needs considerable time to cool.

5. Reflectors need to be collimated. Themirrors need to be periodically lined up with each other and with the focuser; otherwise, images will be distorted. To do so is a fairly simple process, but Newtonian reflectors need it more often than other designs.

6. The coatings on the mirrors are not permanent. Though, with care, they should last for many years, mirror coatings will probably need eventual replacement. There are companies that do this for a reasonably small price.

7. Because the focuser sticks out at a 90 degree angle to the direction of targets, Newtonian reflectors are somewhat difficult to aim.