iNGENET - La comunidad de la ingeniería mexicana

1. Raising the problem
For everyone who loves science, especially astronomy, the telescope is naturally one of the most desirable scientific instruments. On a clear night, facing the vast universe, how fascinating the colorful celestial bodies are! At this time, everyone hopes to have a small astronomical telescope to see the orbiting mountains on the moon, the profit and loss of Venus, the halo of Saturn, the moons of Jupiter, the polar cap on Mars, and the great nebula of Andromeda and Orion. Nebula and so on. Such a telescope is best done by yourself. In today’s highly developed science and technology, whether it is from a theoretical or technical perspective, every enthusiast who is interested in astronomical observations can make astronomical telescopes by themselves.
2. Research purpose
Through self-made astronomical telescopes, understand and master the basic optical knowledge of telescopes, and learn to self-made simple and small Kepler refracting telescopes. Cultivate your own hands-on ability. And once you master this technology, you can take the initiative to give play to your expertise in the field of astronomy science.
3. Research content
There are many types of astronomical telescopes. However, considering the production technology, economic conditions and use characteristics, it is more suitable for amateur astronomers, such as simple small Galilean refracting telescopes, simple small Kepler refracting telescopes, etc. The telescope we researched and manufactured is a Kepler-type refracting telescope.
1. Basic optical knowledge of self-made telescope
1) The imaging principle of optical components
The optical elements used in Kepler’s refracting telescopes are mainly convex lenses. In order to facilitate the discussion, we must first grasp several definitions:
Vertex: The center of the mirror, called the vertex of the mirror.
Center of curvature: The intersection point C of the normals of the sphere is called the center of curvature of the mirror.
Radius of curvature: The distance from the center of curvature C of the mirror surface to the mirror surface is called the radius of curvature.
Main plane and main point: Sometimes in order to simplify the imaging method of the optical system, two special planes perpendicular to the main optical axis are set up in the optical system, that is, if the light enters the optical system, it intersects the first plane MN The distance from the main optical axis h to the point M, then, when the light exits the optical system, the point M’intersecting the second plane M’N’ still has a distance from the main optical axis. These two planes are called the first principal plane and the second principal plane. The intersection of the first principal plane and the main optical axis is called the first principal point; the intersection of the second principal plane and the main optical axis is called the second principal point. Figure 1 shows two points N and N’. In this way, the distance from the principal point to the first and second focal points F and F’of the optical system is the first focal length and the second focal length of the optical system. For a thin lens, the two main planes coincide.
Node: When a group of parallel light beams with a certain inclination angle u to the main optical axis are incident, the outgoing light beam will be concentrated at a point B’on the image focal plane; and the light beam emitted from the optical system must be able to find a light P’ B’, parallel to a certain incident light PB. At the same time, the PB and P’B’ rays must respectively intersect at points P and P’at the same distance from the main optical axis on the first and second principal planes. The intersection points K and K’of the PB and P’B’ rays and the main optical axis are called nodes, as shown in Figure 2. In the same way, any incident light passing through node K must have a parallel conjugate exit light passing through K’. For a two-sided conjugate thin lens, if the medium on both sides of the lens is the same, then the two principal points and two nodes are coincident with the center of the lens. This is the optical center of the lens. Therefore, in general drawing and analyzing thin lens imaging, for convenience, the principal plane of the lens can be used to represent the thin lens. The light passing through the optical center will not change its direction after it exits the lens.
When a beam of incident light parallel to the main optical axis passes through the lens and is refracted by the convex lens, it will generally be concentrated at the second focus F'(real focus). The light passing through the optical center O, after passing through the lens, its advancing direction does not change. The place B’where the two light rays converge is the position where the object is imaged, as shown in Figure 3.
In the telescopes of astronomy enthusiasts, the use of thin lenses to make popular small telescopes is simple and can initially meet the requirements of astronomy enthusiasts for the quality of telescopes. Therefore, it is very convenient to use the optical imaging diagram method of thin lenses to help solve the design problems of astronomical telescopes.