The closest I've ever been to the moon is through a telescope.

In spring, everything comes back to life and the earth is covered with green. In summer, in the green pond, you can always see lotus flowers standing delicately on the water surface, creating the beautiful scene of "the lotus leaves are endless and green, and the lotus flowers are red in the sun". In autumn, the weather is clear and the fields are waving with golden wheat waves. In winter, everything is covered with silver. Nature shows its beauty to people in different ways in different seasons.

1. How do people see objects?

We can see this colorful world thanks to our human visual organ, the eye. To understand how the human eye forms images, we need to first understand the structure of the human eye. From the outside, our eyes consist of two parts: the white of the eye and the eyeball. The outermost part of the black eyeball is a thin, transparent cornea. Inside the cornea is a transparent liquid called aqueous humor. Behind the aqueous humor is an elastic, adjustable lens. Behind the lens is a transparent jelly called vitreous body. They are all permeable to light. They are wrapped in three layers of membranes. The innermost layer is called the retina, which contains many photoreceptor cells that can sense light stimulation. The middle layer is called the choroid, which contains many pigments. Its function is to keep the inside of the eyeball dark (like a darkroom in a camera) to prevent other light from leaking in and affecting vision. The outermost layer is called the sclera, which is the white of the eye we see. It contains many blood vessels and nerves and has the function of protecting and maintaining the shape of the eyeball [1]. When light from an object passes through the cornea, aqueous humor, lens, and vitreous body, it is refracted and focused on the retina to form an inverted image. The photoreceptor cells on the retina are stimulated by the light and generate impulses, which are transmitted to the brain by the optic nerve to form vision, which is what we usually call seeing things.

Figure 1 Anatomy of the eyeball

The image formed by the eyeball is an inverted image, which eventually becomes an upright image under the regulation of our brain.

Figure 2 Human eye imaging

The human eye can see objects. Theoretically, the human eye can see objects from 0 to infinity by adjusting the curvature of the lens through muscles. However, the ability of the human eye to distinguish objects is measured by viewing angle and brightness. In other words, the light of the object needs to reach the human eye at a certain angle, and the human eye can see it. At the same time, it must also have a certain brightness so that the human eye can perceive it. In summary, the larger the size and brightness of an object, the farther it can be seen; conversely, the smaller the brightness and size of an object, the closer it can be seen.

The angle of the same object to the eye will decrease as the distance from the eye increases. For example, in daily life, we will observe the license plate of a vehicle on the road. As the vehicle drives away, the license plate will change from being clear at first to only being able to see the outline, and finally becoming a blur.

Figure 3 Human eye imaging at different distances

If you want to see scenery thousands of meters away and explore the sea of ​​stars in the universe while sitting at home, you need the help of this era's clairvoyance - the telescope.

2. The emergence of the telescope

A telescope is a visual optical instrument used to observe distant objects. It can magnify the very small angle of distant scenes by a certain factor, making them have a larger angle in the image space, making objects that were originally impossible to see or distinguish with the naked eye become clear and discernible.

The history of the telescope can be traced back to the 17th century. There are many different versions of the invention of the telescope. One of them is described as follows: In 1608, two children were playing with a few lenses in front of the shop of optician Hans Lippershey in Middelburg, the Netherlands. They saw the weather vane on the church in the distance through the front and back lenses, and they were very excited. Hans Lippershey was very curious, so he also picked up the two lenses to observe the weather vane in the distance, and found that the weather vane in the distance was much magnified. Lippershey ran back to the shop and put the two lenses in a tube. After many experiments, Hans Lippershey invented the telescope[2].

Figure 4 The world's first telescope

Since its invention 400 years ago, the invention and development of the telescope has continuously “shortened” the distance between objects. With the help of the telescope, humans have redefined our position in the universe and realized that not only is the Earth not the center of the universe, but the Sun is also not the center of the Milky Way. A huge galaxy like the Milky Way, with a diameter of 100,000 light years and more than 100 billion stars, is just a drop in the ocean of the vast universe. In a sense, the development of the telescope is the development of modern astronomy [3]. Interestingly, we often see this kind of scientific equipment in many places. Whether you believe it or not, the telescope has always been one of the most important scientific instruments.

3. Classification of telescopes

After introducing the development history of telescopes and their significance to mankind, let’s talk about the classification of telescopes.

There are three general types of telescopes.[4]

1. Refracting Telescope

A telescope that uses a lens as an objective lens is called a refracting telescope. Refracting telescopes can be divided into two types: the one that uses a concave lens as an eyepiece is called a Galilean telescope; the one that uses a convex lens as an eyepiece is called a Keplerian telescope. The advantages of the Galilean telescope are that it has a very simple structure, less light energy loss, a short lens tube, is lightweight, and the image of the scene is an upright image. The disadvantages are that it has a small magnification and a narrow field of view, so it is generally only used as an opera glass and a toy telescope; the advantage of the Keplerian telescope is that it has a large field of view, but the image obtained is inverted, so it is necessary to add a prism group or a lens group behind the objective lens to rotate the image so that the eye observes an upright image. General refracting telescopes all use the Keplerian structure. Since the image quality of refracting telescopes is better than that of reflecting telescopes, the field of view is large, it is easy to use and easy to maintain, small and medium-sized astronomical telescopes and many special instruments mostly use the refracting system, but large refracting telescopes are much more difficult to manufacture than reflecting telescopes, because it is very difficult to manufacture large-aperture high-quality lenses, and there is a problem of glass absorbing light, so large-aperture telescopes all use the reflecting type.

Figure 5 Structure and imaging principle of Galileo telescope

Figure 6 Kepler telescope structure and imaging principle

2. Reflecting Telescope

A telescope that uses a concave reflector as a crop lens is called a reflecting telescope. Reflecting telescopes can be divided into several types, such as Newtonian telescopes and Cassegrain telescopes.

Figure 7 Newtonian telescope

Figure 8 Cassegrain telescope

The main advantage of a reflecting telescope is that there is no chromatic aberration. When the objective lens adopts a parabola, spherical aberration can also be eliminated. However, in order to reduce the influence of other aberrations, the usable field of view is smaller. In terms of its processing difficulty and manufacturing process, because only one surface of the primary mirror needs to be processed, this greatly reduces the manufacturing cost and difficulty. Therefore, all optical telescopes with an aperture greater than 1.34 meters are currently reflecting telescopes. A larger-aperture reflecting telescope can obtain a prime focus system (or Newton system), a Cassegrain system, and a folded-axis system by changing different secondary mirrors. In this way, a telescope can obtain several different relative apertures and fields of view. In terms of application, reflecting telescopes are mainly used for work in astrophysics.

3. Catadioptric telescope

The catadioptric telescope is based on a spherical reflector, and then adds a refractive element for correcting aberration. This not only avoids the difficult processing of large aspherical mirrors, but also obtains good imaging quality. The more famous one is the Schmidt telescope invented by German optician Schmidt in 1931. Its structure is to place a Schmidt aspherical lens at the center of the spherical reflector, so that the central part of the light beam is slightly converged, while the outer part is slightly divergent, which just corrects the spherical aberration and coma.

Figure 9 Schmidt telescope

Another well-known catadioptric telescope is the Maksutov telescope made by the former Soviet optician Maksutov in 1941. Its structure is to add a negative meniscus lens in front of the spherical reflector. By selecting the appropriate parameters and position of the meniscus lens, spherical aberration and coma can be corrected at the same time. In a catadioptric telescope, the image is formed by the reflector, and the refractor is used to correct the aberration. Its characteristics are large relative aperture, strong light power, wide field of view, and excellent image quality. In terms of application, the catadioptric telescope is suitable for sky survey photography and observation of nebulae, comets, meteors and other celestial bodies. If a small visual telescope adopts a catadioptric Cassegrain system, the lens tube can be made very short.

Figure 1 0 Maksutov Telescope

4. Positive Image System

What is seen through the Kepler telescope is the inverted image of the object. In order to facilitate observation, an erecting prism system must be added to the system to flip the inverted image. The systems that can be used to flip the inverted image include the Roof Prism system (also known as the Schmidt-Biehan roof prism system) and the Porro Prism system (also known as the Porro prism system). The principles and applications of the two systems are similar. The only difference is that the image after the inversion of the Behan prism system is still on the same axis, while the image after the inversion of the Porro prism is on a different axis from the image before the inversion, so the system structure is not tight enough.

Let's first look at the structures of these two positive image systems:

The Behan prism system consists of two glass prisms separated by an air gap. Multiple total reflections cause the image to flip vertically, and the "roof" of the second prism flips the image sideways, resulting in a 180° rotation of the image. The picture shows a Leica telescope, which uses the Behan prism system.

Figure 11. Behan prism system

The Porro prism system is an isosceles right-angle prism made of glass blocks, with the end plane facing the right angle. In use, light enters from the largest rectangular surface of the prism, undergoes two total reflections on the inclined surface, and then penetrates the original incident plane and exits. Because the light only enters and exits in a normal state, the prism does not have a dispersion effect. However, the image passing through the Porro prism will be flipped 180° and will move in the direction of the original entry, that is, the direction of travel has also changed 180°.

Figure 12 Porro prism system

5. Using a multi-axis cage structure to build a telescope

The telescopes we see in our daily lives are packaged and fixed in specific casings. The structure and the direction of the light path cannot be seen, which often gives people a sense of mystery. But when its mysterious veil is revealed, you will sigh that the telescope is just made up of a few lenses.

The Kepler telescope system built with a multi-axis cage structure allows you to intuitively see the internal structure of the telescope. On the premise of understanding the imaging principle of the telescope and related optical knowledge, you can use multi-axis cage optomechanical parts and optical elements to build a DIY telescope like building blocks.

The Behan prism system is used as an erecting system and is placed in a Kepler telescope system constructed with a multi-axis cage structure. The optical elements in the system are coaxial and the structure is simple and compact. In teaching and popular science, the optical phenomena can be observed intuitively while the role of the Behan prism system can also be demonstrated. Scenes at different distances can be observed by moving the objective lens or eyepiece.

Figure 13 Kepler telescope system using RayCage multi-axis cage structure to build a Behan prism system

Although the Porro prism system will lead to a non-compact system structure, it is widely used in most telescopes due to its advantages such as simple structure, low cost and good optical effect. Based on the advantage of the multi-axis cage structure that can be used with multiple optical axes, a unique prism clamp is designed to use the Porro prism system in a microscope built with a multi-axis cage structure to achieve the effect of inverting the image.

Figure 14: Kepler telescope system using RayCage multi-axis cage structure to build a Porro prism system

The invention of the telescope has changed the way we observe the universe, and this change has lasted for more than 400 years. But in terms of using telescopes, humans themselves have also undergone some changes. We must realize that before we understand what the telescope presents, it is not only an extension of vision, but also an extension of ideas. This article only briefly introduces the simple knowledge of telescopes. Interested readers can refer to relevant materials to learn more about telescopes. Here, Ruiguang Cage Company only uses a multi-axis cage structure to build a simple Kepler telescope. If you are interested in Galileo telescopes and reflecting telescopes, you can also build your own DIY telescope to "embrace" the stars and the moon!

References

1. Davson, Hugh and Perkins, Edward S.. “Human Eye.” Encyclopedia Britannica, 1 Dec. 2021, https://www.britannica.com/science/human-eye.

2. Hua Qingfu. See farther, see more clearly: telescope and microscope[J]. Today's Middle School Students, 2019(35):20-22.

3. Su Dingqiang. Telescope and astronomy: 400 years of review and outlook[J]. Physics, 2008(12):836-843.

4. Xia Xiongping. The invention and evolution of the optical telescope[J]. Invention and Innovation (Student Edition), 2007(04):12-13.

Creator: Chen Tingxiao, Ruiguang Kaiqi (Zhenjiang) Optoelectronics Technology Co., Ltd. , China Instrument and Meter Society Youth Optical Science (Zhenjiang) Theme Popular Science Education Base

(Note: This article is a contribution to the popular science creation collection activity "Instruments and Instrumentation Technology Workers Around Us" launched by the Chinese Instrument Society)