The above stated difference in price assumes both telescopes are suited for photography of objects in space

The above stated difference in price assumes both telescopes are suited for photography of objects in space (called astrophotography). A lot of people enjoy doing nothing else but viewing directly through their telescopes and couldn’t care less about taking pictures. For these people there is a subtype of the Newtonian reflector specially invented for them. This relatively new subtype is called a Dobsonian reflector, invented by John Dobson of the San Francisco Sidewalk Astronomers. An eight inch Dobsonian reflector may cost as little a one tenth of the price of a four inch apochromatic refractor! Despite this fact, please keep in mind that I am not saying that reflectors are always better than refractors. Remember large reflectors are for deep sky objects. Refractors are for planetary detail when price is no object. Also, you should realize that you can still get a good view of a deep sky object with a refractor, though it may not be as bright as in a larger reflector. You may get a good view of a planet through a reflector, but the secondary mirror will limit its sharpness. Though I hate to keep throwing exceptions at you, I feel obligated to say that recent advancements in computer technology now allow Dobsonians (with slight modifications) to be serious astrophotography telescopes.

Now is a good time to look at other fundamental types of reflector telescopes. These types in their simplest form are usually only found in large professional observatories. Since many modern telescopes for amateurs are actually a combination of these basic types, you need to understand them before we continue. Figure 4 contains diagrams of three types with each type being shown in its simplest form.

At the top of Figure 4 you see the diagram of a Cassegrainian reflector. There are two main differences between it and the Newtonian type. First note that the secondary mirror is NOT at a 45 degree angle with respect to the main tube and is convex, not flat like a Newtonian’s secondary mirror. Also, the Cassegrainian’s eyepiece is located at the rear of the telescope’s main tube, not on the side. Of course, the reason why the secondary mirror is not slanted is so that it will reflect the light toward the rear of the telescope where the eyepiece is located. The convex shape of the secondary mirror lengthens the cone of light it reflects so that this mirror can be placed farther away from the objective than it otherwise would be. This extra distance is important because the farther away the secondary mirror is from the objective, the smaller the secondary can be. Were the secondary flat, its diameter would need to be at least half of the diameter of the objective in order to concentrate the maximum amount of the light at the rear of the telescope! Such a large secondary would block a large fraction of the incoming light from reaching the objective, thereby making any image viewed much dimmer. What’s worse, the diffraction resulting from the secondary mirror’s huge size would drastically decrease image sharpness.