Types of Telescopes

quick look at it. One popular and inexpensive non-tracking scope is a dobsonian reflector. A dobsonian reflector is just a newtonian reflector with a large primary mirror. It looks like a cannon. It has an inexpensive alt-azimuth mount, so tracking is not feasible.

Reflectors come with different mounts, depending on whether you want to track a celestial body, or just take a

Part "d" shows a coude reflector. The secondary mirror is essentially the same as the one in the cassegrain reflector, but a third mirror is used to reflect the light from the secondary mirror out the side of the scope, just like a newtonian reflector.

Part "c" shows a cassegrain reflector. The secondary mirror reflects light from the primary mirror out the bottom of the scope. The secondary mirror has a curvature that corrects for problems inherent in the primary mirror.

Part "b" of the image shows a reflector without a secondary mirror. This is called a prime focus reflector, and the astronomer has to sit at the prime focus to see the image formed there. No one does this anymore.

Newton is said to have invented the first reflector, and his design is shown in part "a" of the figure to the left. A small secondary mirror reflects the light from the primary mirror out the side of the newtonian reflector.

Reflectors use mirrors to form images. The largest or primary mirror in a reflector collects light from celestial bodies, and secondary mirrors correct and/or redirect that light to an eyepiece for viewing. Generally speaking, the larger the primary mirror, the better the scope. Since primary mirrors are parabolic or spherical in shape, they are called out by their side-to-side lengths, or diameters. A primary mirror's light collecting ability (and quality) increases with the mirror's total area, or equivalently, the square of the mirror's diameter.

Refractors use lenses to form images. Because lenses are difficult to properly shape, and their weights can distort their shapes, they are no longer used by professional astronomers. They are however popular as first time telescopes for the general public.