Building a telescope can be a very complex process, but many people who successfully build one find it to be a rewarding experience. A home builder can usually cheaply obtain most of the non-optical components, and the optical components can be purchased in kits or online. Most home builders who do not have degrees in physics or engineering will probably prefer to build a telescope from specific plans, a number of which are available for free; commercial plans are also available online and through astronomy magazines.
The most critical portion of any telescope is the lenses. Traditionally, anyone wishing to build a telescope would have to grind their own lenses from glass, or hire an optics specialist for that purpose. These days, appropriate lenses or kits can be ordered online or through uncatalogued. Kits can include just the lenses and focal information, the lenses and prisms, or all the components necessary to build a telescope.
In a basic telescope, there are two lenses. The objective lens is the large lens found at the outer end of the telescope, which produces an image of whatever the instrument is pointed at. The second lens, or the eye lens, focuses and magnifies that first image into something that the human eye can easily interpret. Although it is relatively simple to construct a telescope with just two lenses and a casing, the image produced by this type of telescope will be upside down and backward.
In order to build a telescope which has an image that is correctly oriented, a builder will need to reverse the image with two reflective surfaces mounted at 90 degree angles from one another. The most common reflective surfaces used are mirrors and prisms. Mirrors are usually cheaper, but prisms are easier to mount securely and keep clean. It is also possible to use a specially cut tetrahedral piece to flip the image both ways inside a single prism, simplifying the building process and reducing the number of mount points.
The telescope body may be the last consideration in the design process, but the first step necessary to build a telescope. For a home telescope, a simple cardboard shipping tube mounted on a tripod or a other movable base is usually sufficient. The length of the body depends on the focal distance of the objective lens.
Ambitious builders sometimes add additional instrumentation to their telescopes. Perhaps the most useful of these is a positioning system consisting of a compass and a level marking degrees. This simple system provides an easy way to reliably record the position of the telescope or to find specific objects using coordinate information. Other add-ons include camera mount points and, for larger telescopes, electric motors.
