Construction
The first telescope
As we said, you can build this first simple instrument without difficulty while allowing you to learn the structure and operation of telescopes in general. Although it is simple, it can reveal the craters of the Moon and the satellites of Jupiter. It is also very useful for demonstrating lens aberrations. You really need to build this telescope as a necessary step towards understanding the solutions employed in the second and improved telescope model. In the section "From Lenses to Optical Instruments", you saw how a telescope works; here we simply remind you that the objective lens produces an image of the object observed, and this image is magnified by the eyepiece.
We show our first telescope, which is made using easy-to-find materials. The components of this instruments are:
- ring to secure the eyepiece lens from behind
- ring for centering the eyepiece lens
- eyepiece: lens with focal length of 20-50 mm. You can buy one in an optical or photographic shop, or you can get one free by using the lens of a disposable camera.
- ring to secure the eyepiece lens from the front
- a cardboard tube for the eyepiece. You can use the tube from a roll of plastic food wrap or paper towels. You can also use short sections of this tube to make rings 1, 2, and 4 which you need to hold the eyepiece lens in place
- coupling between the eyepiece tube and the main tube. This is a hollow cylinder with an outer diameter that fits snugly into the end of the main tube and an inner diameter that provides a snug but movable fit to the outside of the eyepiece tube. You can make the coupling using several plywood disks glued together or using a polystyrene cylinder with a hole bored through it. If you use polystyrene, you will need to add an opaque covering at each end.
- main tube. Use a cardboard or plastic tube about as long as the focal length of the objective lens and with an outside diameter of 50-60 mm about. Suitable sources include map mailing tubes and core tubes for carpets, drawing paper, or wrapping paper.
- objective lens. You can use a common eyeglass lens with a focal length of 500-1000 mm. You can buy it in a optical shop. Ask the optician to reduce the lens diameter in order to fit it precisely into the tube cap
- diaphragm. Cut it from a black card, then open a hole of about 15 mm in diameter in the center of the disk
- Cap of the tube. If you buy a tube for drawing sheets, you should have a cap which will be useful for retaining the objective and the diaphragm. Otherwise, you can made it with a disk of cardboard. Make a series of radial cuts around the edge of the disk to make a set of tabs, Moisten the tabs; then place the tube cap on one end of the principal tube and bend the tabs around the outside of the tube. Glue the tabs together where they overlap, but be careful not to glue the cap to the principal tube yet. When the glue is dry, slip the cap off and cut in the cap a hole a few mm less in diameter than the outside diameter of principal tube.
The distance between objective and eyepiece lenses must be equal to the sum of their focal lengths. The eyepiece tube must stick out a few centimeters so you can move it to focus the telescope. Make the length of the principal tube short enough to allow you to grip the protruding part of the eyepiece tube with your fingers as you adjust the focus.
The eyepiece tube must slide smoothly in its channel, but it should not be loose enough to fall out if you hold the telescope vertically. Paint the inside of the tubes with black opaque paint (matte finish) or India ink . Secure the cap of the main tube to keep it from pulling away from the tube.
How to use the first telescope
Do not use the diaphragm at first, but leave the objective at the greatest aperture. Point the instrument towards a distant object. Move forward and backward the eyepiece tube until the image is as distinct as possible. You will soon realize that the image is of poor quality and is never distinct. This simple objective lens has many defects that produce the poor-quality image.
You can reduce some aberrations by decreasing the lens aperture. This is why we use a diaphragm on the objective. It is a round disk of black stiff paper with a 15-mm-diameter hole in the center. This diaphragm, placed in front the objective, reduces both the effects of the lens defects and the brightness of the image. As a consequence, you can only observe objects brightly illuminated by the Sun. To minimize the chromatic aberrations, you will need to replace the spectacle lens you used as the objective for your first telescope with an achromatic lens, as we shall see below.
The telescope support
As soon as you use your telescope, you will see that you cannot hold it steadily enough in your hands to maintain a stable image. You will need to build a support to help you to point your instrument and keep it steady. You can mount this support on a photographic tripod by means of a 1/4 W threaded hole.
Lens aberrations
The first telescope will give you a good feel for lens aberrations. In this simple instrument, chromatic aberration is the most conspicuous. The aberrations can be greatly reduced by means of careful lens design. As it is not possible to limit all kinds of aberrations using only a single lens, objectives and eyepieces are created using multiple lenses. By selecting different types of glass for the various lenses and using appropriate surface curvatures and distances between lenses, it is possible to control in a satisfactory manner the aberration of the system. In general, the success of an objective or an eyepiece in correcting aberrations depends on the number of lenses used to make it.
For the second telescope, shown, we use an achromatic objective, made up of two lenses of different shapes, one converging and the other diverging. Sometimes they are glued together by means of Canada Balsam or a synthetic resin (cemented doublet), other times they are kept separated (air-spaced doublet). These two lenses have different indices of refraction, one high (Flint glass), and the other low (Crown glass). Hence, the chromatic aberrations of the two lenses act in opposite senses, and tend to cancel each other out, thus producing a much more distinct image than a single lens could achieve.
Usually, these objectives are constructed to reduce other types of aberration as well. Obviously, achromatic objectives vary in quality. In some of them, it is still possible to perceive a residual chromatic aberration, or the images they produce are well focused in the center only, or they produce a pincushion or barrel distortion. describes the main optical aberrations.
Producing an upright image
With the first telescope you built, images were inverted, and in the section "From Lenses to Optical Instruments" we explained why. But astronomers don't really care whether they see star images "straight up" or "upside down." In fact, with the exception of the Sun, all stars are so distant that not even with the most powerful telescopeshas anyone ever seen their disks. They appear to us always as points of light, and to see a point of light upright or overturned does not make any difference. However, many people would like to use their telescopes for terrestrial observations, in which case "right side up" does make a difference.
Several different methods allows you to erecting images without significantly degrading their quality. Then we show the main erecting systems. These optical devices are sold with a case and tubes for connecting them with the eyepieces and the focussing systems.
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