### Definition of the astronomical telescope

Astronomical telescopes or astronomical binoculars are the optical instruments used to help the eye see celestial objects such as stars, planets, satellites, etc. Although the size of celestial objects is huge the distance of celestial objects is so far that when observed using the eyes, celestial bodies appear small. Astronomical telescopes function to enlarge the image of celestial objects so that they can be seen using the eyes.

**Types of astronomical telescopes**

There are several types of astronomical telescopes including refraction telescopes and reflecting telescopes. The refraction telescope, also known as the Keplerian telescope, consists of two convex lenses placed at both ends of the tube. Convex lenses that are more distant from the eye of the observer are called the objective lens, while the convex lenses that are closer to the eye are called the ocular lens. For the image of the object can see the objective lens surface must be substantial. Making large-diameter lenses is difficult, so the size of the refracting telescope is not very large. Huge telescopes are usually reflective telescopes. Reflection telescopes use concave mirrors as objective mirrors.

This article focuses on the astronomical refraction telescope or the Keplerian telescope.

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**How the astronomy telescope works**

The refraction astronomy telescope consists of two convex lenses. The convex lens that has a longer distance from the eye of the observer is called the objective lens, while the convex lens that is closer to the observer’s eye is called ocular lens or eye lens. The location of the object is very far causing the angle formed between the object with an objective lens is very small, therefore the objective lens functions to bring the image closer to the ocular lens so that the angle becomes more substantial. The ocular lens function to increase the angle so that the size of the image formed on the retina becomes larger.

The distance between objects with the objective lens is very far and is considered infinite. As explained in the article about the convex lens, if the object distance is very far or infinite, the image is right at the focal point of the objective lens. The image is real, inverted and smaller. Because the image is at the focal point of the objective lens, the distance between the image and the objective lens (di_{ob}) equals the focal length of the objective lens (F_{ob}).

The image produced by an objective lens is real, so it is considered as an object by the ocular lens. If the eyes are accommodated minimum (eye focused at far point), the image formed by the ocular lens is at infinite. As explained in the topic of the convex lens, so that the image formed by the ocular lens is infinite, the real image produced by the objective lens and considered as an object by the ocular lens, in addition to being in the focal point of the objective lens, must also be at the focal point of the ocular lens. So it can be concluded that the focal points of both the objective lens and the first focal point of the ocular lens coincide with each other, as shown in the figure. When the focal length of the ocular lens (f_{ok}) is equal to the distance of the real image from the ocular lens (do_{ok}), the final image formed by the ocular lens is virtual, inverted, enlarged and infinite. The infinite image distance does not mean image size is infinite.

Based on the figure and explanation above, it can be concluded that the distance between the objective lens and the ocular lens of an astronomical telescope = the length of an astronomical telescope = the focal length of the objective lens (f_{ob}) + focal length of the ocular lens (f_{ok}).

If the eye has a maximum accommodation, then the image formed by the ocular lens is at the near point of the normal eye or about 25 cm in front of the ocular lens, where the image distance is smaller than the focal length of the ocular lens (f_{o}). The image distance formed by the ocular lens is smaller than the focal length of the ocular lens (f_{ok}) so that the image is virtual and inverted. A complete explanation can be learned on the topic of the equation of the astronomical telescope.

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**The total magnification of the astronomical telescopes**

Is the size of the image of an object seen through the astronomical telescope greater when observed by eyes with the minimum accommodation or maximum accommodation? Which should be used as an objective and ocular lens, a convex lens that has a large focal length or a convex lens that has a small focal length? These questions can be answered after you learn the equation of the astronomical telescope.