Reflecting Telescopes

Reflecting Telescopes

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Astrers use electromagnetic radiation to study space because it can travel long distances between planets and galaxies. Telescopes are the tools used to collect this data. There are two types of telescopes: reflecting telescopes and refracting telescopes. Reflecting telescopes use mirrors to process the electromagnetic radiation, while refracting telescopes use lenses. Understanding how telescopes work is key to astrophysics.

What are reflection and refraction?

Electromagnetic waves can change direction when they encounter different objects or media. Reflection happens when a wave is redirected towards the medium it was travelling in, while refraction is the change in direction when a wave enters a new medium. These two phenomena, along with diffraction, describe how light behaves in space and how telescopes measure it. When a wave meets a new medium, some of it reflects and some refracts. Maxwell’s laws explain how much light is refracted and reflected, and why the angle of reflection is equal to the angle of the incoming wave in certain situations.

What are the main characteristics of reflecting telescopes?

Reflecting telescopes use mirrors to gather and reflect light. If we assume the mirrors are perfect, we can classify these telescopes based on the arrangement of the mirrors and their magnification power. By studying their arrangement, we can learn about more complicated designs. By studying their magnification, we can understand the math behind telescopes, how to make them better, and what issues we might face.

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.

Concave parabolic mirror
Concave parabolic mirror

Concave mirrors are simple tools that can be used to collect electromagnetic data from astronomical objects. When electromagnetic radiation encounters a concave mirror, it is reflected towards a point called the focus, represented by the convergence of the horizontal lines in the image.

A parabolic mirror is often used all incoming parallel rays. This it can collect information from a wide area of space and focus it on a single point, allowing for high-resolution images to be magnified. The parabolic shape of the mirror is what enables it to achieve this level of precision.

Below you find a diagram of one of the most common models of reflecting telescopes: the Cassegrain telescope.

A Cassegrain telescope
A Cassegrain telescope

Reflecting telescopes work by using a concave parabolic mirror as the primary mirror to gather electromagnetic radiation. A convex mirror is then placed before the focus of the primary mirror. This convex mirror reflects the light and focuses it towards the observer's eye or a device that can process the incoming signal.

The placement of the secondary mirror is crucial in determining the magnification power of the telescope. By adjusting the distance between the primary and secondary mirrors, the magnification can be increased or decreased. The final image produced by the telescope is a result of the combination of the reflections from both mirrors.

Magnification of reflecting telescopes

Magnification is the ratio of the size of an object's image to the size of the object itself, after being processed by an optical system such as a telescope. The ability of a telescope to gather light is primarily determined by the area of its primary mirror. Modern reflecting telescopes use primary mirrors with diameters that can be as large as 10 meters in order to collect as much electromagnetic radiation as possible. However, the diameter of the primary mirror does not directly determine the power of magnification of the telescope.

The magnification power of a reflecting telescope is determined by a formula that takes into account the focal distances of the primary and secondary mirrors. 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 parabolic concave mirrors, it is the distance between the centre of the mirror and the point where all the rays converge. The best reflecting telescopes are achieved by using a primary mirror with a large focal distance and a secondary mirror with a small focal distance.

To illustrate this, let's consider a primary mirror with a diameter of 5 meters and a focal distance of 10 meters that we want to use as part of a Cassegrain reflecting telescope. We are given three mirrors to act as secondary mirrors with different characteristics. After calculating the magnification power of each mirror, we find that the mirror with a 5 cm diameter and 1 cm focal distance is the best choice for building a reflecting telescope with the given primary mirror.

While the logical choice may seem to be the mirror with the largest magnification power, we have to take into account the diameter of the mirror and how much light it may block. In practice, small secondary mirrors are often used in reflecting telescopes to minimize the amount of light that is blocked.

Comparison between reflecting and refracting telescopes

Astronomical observatories mostly use reflecting telescopes due to their advantages over refracting telescopes. Refracting telescopes have disadvantages related to the making and handling of their lenses. Lenses can only be held by their edges and cannot be made arbitrarily large, making them unsuitable for scientific purposes. Additionally, lenses are very dense objects, which can make them very heavy.

Refracting telescopes are also prone to two types of optical aberrations: chromatic aberration and spherical aberration. Chromatic aberration is caused by the anomalous spread of colors after being refracted, while spherical aberration is caused by the fact that lenses are not ideal and do not take parallel rays of light to the exact same point.

Reflecting telescopes, on the other hand, have several advantages due to the properties of mirrors. Mirrors are much easier to make and handle, and can be made very thin. Modern techniques also allow for the use of segmented mirrors to compose images and information. Additionally, mirrors do not suffer from chromatic aberration, and the use of parabolic mirrors can help to deal with spherical aberration.

Although mirrors are never ideal and may still have some small aberrations, reflecting telescopes have more advantages than refracting telescopes for scientific purposes. However, many amateur astronomers may still choose to use refracting telescopes due to their affordability and good performance.

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.

Reflecting Telescopes

How does light travel through a reflecting telescope?

Light is reflected by several mirrors that focus the rays of light. In a Cassegrain telescope, there is one primary mirror whose function is to gather as much light as possible, while a secondary mirror directs the light rays towards a small region that is to be observed.

What is a reflecting telescope used for?

Reflecting telescopes are the most widely used model of scientific telescopes to gather astrophysical data.

Are radio telescopes reflecting or refracting telescopes?

Radio telescopes are reflecting telescopes since a large gathering surface is needed to capture radio waves (which have very long wavelengths).

Who invented the reflecting telescope?

Isaac Newton invented the first reflecting telescope in 1668. The name ‘Newtonian’ is given to the configuration with two mirrors used by Newton.

Why are reflecting telescopes more commonly used in science?

Because they are easier to mount as the size of the telescope grows, and they have fewer defects (aberrations) than refracting telescopes.

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