The telescope was invented in Holland in the 17th century. In its most basic design, it is formed by a objective and an eyepiece. The objective collects more light than our naked eye and, for this reason, if we look at the stars without and with a telescope, in the same area of observation we see more stars with this instrument
The focus is the point where the rays that come from far away converge. Then, the focus of the objective and the eyepiece match at the same point. Doing this in that way, the image comes out much bigger than we would see if we only did it with our eyes directly. The result is a much larger image.
Normally, when a telescope is acquired, it is usually equipped with different eyepieces. The manufacturer always informs of the focus of the objective and the eyepieces. To calculate the total increase is only necessary to divide the focus of the objective by the focus of the eyepieces.
The image is inverted from what it is like in reality, but since it is used to seeing stars it is not important. Those who want to use it also for ground observation have to adapt an image investor.
There are two main categories of telescopes:
a- Refractors are forme by lenses. The main problem is that lenses of more than 10 cm are not usually manufactured due to cost problems.
b- The reflectors are made up of mirrors. These can be designed in large diameters.