Guide
Telescopes – Optics and Mechanics
Basics on the structure, optics, and functionality of modern telescopes – clearly explained.
The Telescope as an Observational Instrument
To get an idea of what really matters when observing the night sky, we should devote a few lines to the purpose of a telescope. If you believe advertising, a telescope is a magnifying device that shows the sky at up to 600x magnification or more, revealing colorful nebulae in stunning detail. In practice, however, you will quickly realize that this is not the case. On the contrary: many objects appear most impressive at relatively low magnifications. While magnification is important, it is not the decisive factor in determining the performance of your instrument. The ability to gather light (light-gathering power) and the image contrast of the optics are key characteristics that distinguish a good telescope. There are several types of telescopes, all with their own advantages and disadvantages. Unfortunately, there is no all-in-one solution that meets every need.
Let us therefore begin with a brief overview of the different telescope designs. Broadly speaking, telescopes are divided into refractors and reflectors.
Refractors (lens telescopes) typically consist of an objective lens made up of two elements separated by an air gap (achromatic lenses). The objective collects incoming light and focuses it at the focal point. An eyepiece placed at the focal point magnifies the image. The distance between the objective and the focal point is called the focal length.
In reflectors (mirror telescopes), the role of the objective is taken over by a concave (parabolic) primary mirror. This mirror is located at the rear of the tube. It also collects incoming light and focuses it at the focal point. Between the focal point—where the eyepiece is placed—and the primary mirror, there is a secondary mirror that redirects the concentrated light toward the eyepiece. There are essentially two main types of reflecting systems.
In a Newtonian reflector, the light is redirected sideways at a 45° angle. The eyepiece is usually located near the top of the tube, and the observer looks into the telescope from the side.
In Cassegrain telescopes, the primary mirror has a hole in the center. The secondary mirror is mounted centrally in the optical path at the front end of the tube and reflects the image back through the hole in the primary mirror toward the rear of the tube, where the eyepiece is located. As with refractors, the eyepiece is positioned at the back of the telescope.
Both designs have their strengths and weaknesses. The decision to purchase a telescope ultimately depends on the astronomer’s intended use and budget.
Refractors generally offer superior image quality compared to reflectors of the same size. They are less sensitive to misalignment and therefore require very little maintenance. These characteristics make refractors an ideal choice for beginners.
Optics
In principle, astronomical telescopes are divided into two optical concepts: reflector telescopes (mirror telescopes) and refractor telescopes (lens telescopes). Both designs have different characteristics and areas of application.
The optics of a telescope play a key role in determining how much light is collected and how detailed a celestial object can be displayed. The most important factors are the aperture (diameter of the lens or mirror) and the quality of the optical components.
The larger the aperture, the more light the telescope can gather. This is especially important for observing faint objects such as nebulae, star clusters, or galaxies. At the same time, the optical quality affects the sharpness and contrast of the image.
In addition to light-gathering capability, image quality is also crucial. Optical imperfections such as chromatic aberration or distortions can negatively impact the observing experience. High-quality systems are therefore designed to minimize these effects as much as possible.
Ultimately, choosing the right optical concept depends on which objects you prefer to observe and what requirements you have in terms of portability, maintenance, and ease of use.
Refractor (Lens Telescope)
A refractor telescope consists of an objective lens and an eyepiece. It is important that the objective lens is achromatic in design (double lens with an air gap). The diameter of the objective lens is crucial for the light-gathering power of your telescope.
Advantages of a refractor telescope compared to a reflector telescope:
a) no loss of light due to obstruction, as occurs in reflectors
b) very high image sharpness
Reflector (Mirror Telescope)
The most common type of reflector telescope is named after its inventor, Sir Isaac Newton (*1643, †1727). Newtonian reflectors use a concave mirror that is spherical—or, in higher-quality versions, parabolic—whose surface is coated with aluminum. The light rays are reflected by the primary mirror and, shortly before reaching the focal point, are redirected by a flat secondary mirror tilted at 45° by 90°, so that the focal point lies outside the main tube, where the eyepiece is then placed.
Which is better: a refractor or a reflector telescope?
This question is difficult to answer, especially for amateur use. As a general rule of thumb, a reflector telescope of a given aperture is slightly inferior to a refractor of the same aperture in terms of image quality and resolution.
Which is better: a refractor or a reflector telescope?
There is no clear answer to this question, especially for amateur use. As a rule of thumb, you can assume that a reflector telescope of the same aperture may be slightly inferior to a refractor in terms of image quality and resolution.
Mechanics
A telescope consists of a variety of mechanical components that differ not only in their design and functionality but also in how they are operated. As mentioned earlier, a stable tripod and a solid mount are essential for a satisfying observation of the night sky. There are two main types of mounts, which are explained in more detail below.
Alt-azimuth Mount
In an alt-azimuth mount, the telescope body (optical tube) is mounted in a fork. You can move the telescope horizontally and vertically. This type of mount is especially suitable for beginners, as celestial objects can be aligned quickly and easily.
Equatorial Mount
With an equatorial mount, the telescope is aligned so that one axis is parallel to the Earth's axis. This allows you to compensate for the apparent motion of celestial objects caused by the Earth's rotation using only one axis. After proper alignment (polar alignment), stars and planets can be tracked much more easily.
This type of mount is particularly suitable for longer observation sessions and astrophotography. However, it is somewhat more complex to operate and requires some practice at the beginning.
