Astronomical data usually consists of electromagnetic radiation because it is a form of information that can travel distances as big as the ones between planets, galaxies, etc. This means that the study of the treatment of light (electromagnetic radiation) is of crucial importance to astrophysics.
The devices we use to gather astronomical data are called telescopes. Depending on how they process electromagnetic radiation, they have different advantages, disadvantages, and uses. The main division used in the study of telescopes is to classify them based on the main optical effect that takes place inside them: reflection (for reflecting telescopes) and refraction (for refracting telescopes).
What are reflection and refraction?
Reflection is the change of direction of a wave at an interface separating two different mediums so that the wave is redirected towards the medium in which it had been propagating.
Refraction is the change of direction of a wave when it leaves a medium and enters another one.
These are the two changes of direction an electromagnetic wave can undergo when encountering different objects or media. Along with diffraction, they offer a precise and complete description of the phenomena associated with light coming from the universe and measured by telescopes.
When an electromagnetic wave encounters a new medium, there is usually a fraction that is reflected and a fraction that is refracted. It is helpful, in this context, to note Maxwell’s laws, the basic laws of electromagnetism that describe every classical (not quantum) electromagnetic phenomenon, which concern issues such as:
- The angle at which light is refracted (refraction indices).
- The conditions specifying how much light is refracted and reflected.
- Why the angle of reflection is the same as the incident angle of the incoming wave in certain circumstances.
What are the main characteristics of reflecting telescopes?
In simple reflecting telescopes under the assumptions of ideal mirrors, we can study the disposition of mirrors and the magnification power (the amplification of images provided by the telescope) to classify them. Studying the disposition is a way to understand more complex designs while studying the magnification allows us to understand the mathematical structure behind telescopes, what we can do to improve them, and which problems we can encounter.
Reflecting telescope diagram and structure
The ideal optical object that refracts all light and does not reflect any fraction of it is called a lens. These are used in seeing glasses or in telescopes. The ideal optical object that reflects all light and does not refract any fraction of it is called a mirror. See the following diagram of a concave parabolic mirror.
Figure 1. Concave parabolic mirror. Source: Gustavo Girardelli, Wikimedia Commons (CC BY-SA 4.0).
Concave mirrors are very simple devices that can gather electromagnetic data. In the image, the electromagnetic radiation coming from astronomical objects is represented by the horizontal lines. When they encounter the mirror, they are reflected towards a point F called the focus.
The reason why we consider a parabolic mirror is because it can reflect all incoming parallel rays towards the same point. Intuitively, we can see that this device takes information from a region of space and concentrates it into a small region, which allows us to magnify images with high resolution.
Below you find a diagram of one of the most common models of reflecting telescopes: the Cassegrain telescope.
Figure 2. A Cassegrain telescope. Source: HHahn, Wikimedia Commons (CC BY-SA 3.0).
The functioning of these telescopes is based on the gathering of electromagnetic radiation by a concave parabolic mirror (primary mirror) and the placement of a convex mirror (secondary mirror) before the focus of the primary mirror. This last mirror once again reflects the light and focuses it towards the observer’s eye or a device that can process the incoming signal.
Magnification of reflecting telescopes
Magnification is the amount of growth of the image of an object relative to the size of the object after being processed by an optical system like a telescope.
The ability of a telescope to gather light is determined by the mirror’s area. Modern reflecting telescopes use primary mirrors with diameters of up to 10m. The diameter of the primary mirror determines how much electromagnetic radiation the telescope can collect, but it does not determine the power of magnification of the telescope.
The derivation of the magnification power of a reflecting telescope is not covered here, but it is based on the same laws that are described in the explanation for Astronomical Telescopes. We should, however, note that the magnification of a reflecting telescope is determined by the following equation:
[M = \frac{f_p}{f_s}\]
Here, fp and fs are the focal distances of the primary and secondary mirrors, respectively. The focal distance is a characteristic quantity of lenses and mirrors that is determined by the point towards which the rays of light are deflected. For instance, for parabolic concave mirrors, it is the distance between the centre of the mirror and the point where all the rays converge.
This suggests that the best-reflecting telescopes are achieved by using a primary mirror with a big focal distance and a secondary mirror with a small focal distance. See the example below for simple proof.
Consider a primary mirror with a diameter of 5 metres that we have already mounted to use as part of a Cassegrain reflecting telescope. The focal distance of this mirror is 10 metres. We are given three mirrors to act as secondary mirrors whose characteristics are the following:
- A mirror with a 5 cm diameter and 5 cm focal distance.
- A mirror with a 5 cm diameter and 1 cm focal distance.
- A mirror with a 5 m diameter and 1 mm focal distance.
Which mirror is best to use as a secondary mirror and to build a reflecting telescope?
Looking at the magnification, we obtain the following results:
- \(M = \frac{f_p}{f_s} = \frac{10 m}{0.05 m} = 200\)
- \(M = \frac{f_p}{f_s} = \frac{10 m}{0.01 m} = 1000\)
- \(M = \frac{f_p}{f_s} = \frac{10 m}{0.001 m} = 10000\)
This means that objects appear 200, 1000, and 10,000 times bigger with each of these mirrors. Although the logical choice seems to be the third mirror, we have to take into account that it has a very large diameter that prevents us from receiving light properly (picture it with the help of figure 2). Thus, the best choice is the second mirror. Usually, however, we consider small secondary mirrors whose blockage of light is almost irrelevant.
Comparison between reflecting and refracting telescopes
The telescopes used in astronomical observatories are mostly reflecting telescopes because they are better suited to the task. Let us consider why that is the case and what the best uses are for both reflecting and refracting telescopes.
Refracting telescopes
The main disadvantages of refracting telescopes have to do with how the lenses are made and handled. While a mirror can be easily handled with a device behind it, lenses can only be held by their edges (so there is no interference with the refracted light). This is the reason why lenses cannot be made arbitrarily big. Lenses are also very dense objects (especially if their focal distance is to be large), which makes them very heavy. All of these factors make refracting telescopes not suited for scientific purposes.
Let us now consider optical aberrations.
An optical aberration is a property of an optical system that causes the light that goes through it to spread rather than to focus on a point.
In other words, aberrations are imperfections of optical systems. They are not caused by defects in the making of devices but rather because of their physical conditions. For refracting telescopes, we find two kinds of aberrations:
Chromatic aberration: this is the anomalous spread of colour after being refracted as colours have slightly different refraction indices. It distorts images since it is ‘spreading their colour’.
Spherical aberration: this is the anomalous spread of light rays caused by the fact that lenses are not ideal and thus do not take parallel rays of light to the exact same point.
Reflecting telescopes
Reflecting telescopes have several advantages because of the properties of mirrors. First, mirrors are much easier to make and handle. They can be held from behind without any trouble, they can be made very thin, and modern techniques allow us to use several mirrors and compose images and information with them (segmented mirrors). Mirrors also do not suffer from chromatic aberration since (ideally) there is no refraction, only reflection. And the use of parabolic mirrors allows us to deal with spherical aberration as well.
Although mirrors are never ideal, and there are still small aberration effects because of some refraction and aberrations in the eyepiece and the secondary mirror, reflecting telescopes have more advantages than refracting telescopes for scientific purposes. However, it is worth mentioning that many of the optical telescopes used in amateur astronomy are refracting telescopes because they are cheaper and can perform very well.
Reflecting Telescopes - Key takeaways
- Telescopes gather electromagnetic radiation coming from different parts of the universe. They usually process this radiation by means of refraction and reflection.
- Reflecting telescopes use mirrors while refracting telescopes use lenses.
- A common model for a reflecting telescope is a Cassegrain telescope, which uses two mirrors. One collects electromagnetic radiation, and the other redirects it. The ratio of their focal distances determines the magnification power of the reflecting telescope.
- In general, reflecting telescopes have more advantages than refracting telescopes, which makes them better suited for scientific purposes.
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