The telescope is a kind of visual optical instrument used for observing distant objects. It can enlarge the small Angle of distant objects by a certain magnification so that it has a large Angle in the image space so that the objects that can not be seen or distinguished by the naked eye become discernible.  Therefore, the telescope is an indispensable tool in astronomy and ground observation.  It is a kind of optical system through the objective lens and eyepiece so that the incident parallel light beam remains parallel to exit.  In this article, we will focus on telescope optics.  

According to the principle of telescopes, there are three kinds.  An instrument that collects electromagnetic waves to observe the electromagnetic radiation of distant objects is called a radio telescope. In daily life, telescopes mainly refer to optical telescopes, but in modern astronomy, astronomical telescopes include radio telescopes, infrared telescopes, X-ray and gamma-ray telescopes.  The concept of a telescope extends further into the realms of gravitational waves, cosmic rays, and dark matter.  The optical telescope in daily life is also called a “Thousand-mile mirror”.  It includes amateur telescopes, opera telescopes, and military binoculars.  

(1) Who invented the optical telescope?  

The earliest optical telescope we know of was made in 1608.  They are made in the Netherlands by various optical craftsmen.  One of them, Hans Lipsy, publicized his design so well that news reached Galileo Galilei in Italy in 1609.  When Galileo heard about telescopes, he built his own.  

(2) Telescope optics  

1. Eyepiece and magnifying glass  

To make a telescope that can see-through, you need to add another lens.  This eyepiece magnifies the image created by the large objective lens and directs light to your eye.  Eyepieces work much like magnifying glasses.  It allows your eyes to focus more closely than usual.  The eyepiece on a typical telescope allows you to examine the image produced by the objective lens from a distance of an inch or less.  

Before installing the eyepiece in your telescope kit, you should first measure its focal length F.  Remove the lens from the foam holder and hold an edge.  Point the plane of the lens at a distant light source and place a piece of paper behind the lens parallel to the lens.  Move the paper close to or away from the lens until you see a clear image of the light source, then measure the distance from the curved surface of the lens to the paper.  Record focal length F in your lab notebook.  

You are now ready to put the telescope kit together.  Replace the eyepiece with foam mounting.  Remove the tracing paper and insert the foam holder into the smaller tube.  Point the telescope at a distant target and slide the tube in and out until you get a good focus.  

What is the direction of the picture?  The image you see with this simple telescope may be a little blurry compared to the image in your binoculars or a real telescope;  You may also notice bands of color around bright objects.  These effects are limited by the simple lens used in this telescope kit.  You can make the image sharper by mounting a paper washer in front of the objective, but this will also darken the image because the telescope will concentrate less light.  

2. Light gathering ability  

The ability to focus light is usually the most important function of a telescope.  The concentrating power of a telescope is proportional to the square of the diameter of the objective lens.  The area of a circle is proportional to the square of its radius.  A telescope with a diameter three times that of another lens would have nine times the concentrating power.  Larger objective lenses concentrate lighter, and more sensitive imaging equipment can produce better images with less light.  

3. The amplification  

Once the focal lengths F and F of the objective and eyepiece are known, the magnification of the telescope can be easily calculated.  The formula of magnification factor M is M = F ÷ F.  

Here, you can use any unit for F and F, as long as you use the same unit for both.  For example, if you measure F in millimeters, you should also measure F in millimeters.  Using the F and F values measured above, calculate the expected magnification of the telescope.