What is a Telescope? What are the Types and Features of Telescopes?

When we think of a telescope, we generally envision a long and slender tube mounted on three legs, with an eyepiece at the back for direct observation, or complex instruments surrounding it in observatories. However, there are many types of telescopes. The primary factor determining which type of telescope to choose is the purpose for which you intend to use it.

Telescopes have two main functions: to magnify objects and to collect more light, allowing us to observe celestial bodies that are normally invisible. The telescopes sold in the market, often referred to as toys, usually focus on the magnification feature. However, the most important aspect of a telescope is its light-gathering capacity. If the light-gathering capacity is low, magnification will result in the loss of existing light, leading to poor image quality. Moreover, most of the time, you do not need or even want to magnify.

If you are considering purchasing a telescope or want to gain extensive knowledge about telescopes, we recommend that you read our article carefully. Otherwise, there is a high likelihood that you might make an incorrect choice.

What is a Telescope?

Telescopes are devices used to examine objects that are too distant to be seen with the naked eye. The most common type, optical telescopes, collect and focus visible light from distant objects to create magnified images of them. Additionally, there are telescopes that can gather different types of radiation that the human eye cannot detect, such as radio waves, X-rays, ultraviolet, and infrared rays.

What are the Types of Telescopes?

Telescopes are used to observe celestial bodies such as the moon, sun, and planets in space. From Earth, only four galaxies can be seen with the naked eye, and telescopes are used to see more. Telescopes vary according to their intended use and each has its own unique feature. They are generally divided into three main categories: reflective, refractive, and catadioptric. Let’s take a closer look at these types of telescopes together.

1.     Reflective Telescopes

Reflective telescopes are more economical compared to refractive telescopes. As the diameter of the mirror increases, the cost does not increase disproportionately as it does with refractive telescopes. Reflective models are generally preferred for diameters of 15 cm and above. Among amateur astronomers, it is possible to find those using reflective telescopes with diameters of 20, 25, 30, and even 35 cm.

The difference in light-gathering capacity significantly increases in direct proportion to the diameter of the mirror. Of course, other factors such as the reflectivity of the mirror are also important. However, you do not need to worry about this. The difference in reflectivity due to the coating remains quite small compared to the advantage you will gain from the size of the diameter.

2.     Refractive Telescopes

Refractive telescopes are often the first type that comes to mind for many of us. These telescopes have a lens at the front that collects light. The cheapest telescopes available in the market are usually refractive telescopes, although one might expect them to be the most expensive.

Refractive telescopes are generally more costly, and the larger the diameter of the lens, the higher the price. This high cost is due to the additional processes required to correct the natural imperfections of the lenses. One of the biggest drawbacks of refractive telescopes is chromatic aberration, which occurs when light refracts differently at various wavelengths, leading to unwanted color distortions in the image and reducing observation quality. If you see blue and red halos around celestial bodies like the Moon, this is a sign of chromatic aberrations caused by the low quality of the lens. High-quality lenses contain special structures that prevent this problem, and unfortunately, they are more expensive.

3.     Catadioptric Telescopes

Catadioptric telescopes combine the features of both lens-based (refracting) and mirror-based (reflecting) telescopes. There are various models under this category that differ according to their designs. To select the model that best suits your needs, we recommend that you carefully examine the telescope features we have outlined below.

Telescope Features

We can categorize telescope features into three groups: light-gathering capacity, focal ratio, and focal length, as well as eyepiece magnification calculation. Let’s examine these features in detail together.

1.     Light-Gathering Capacity

The most critical factor in choosing a telescope or binoculars is the light-gathering capacity. This feature should be able to show you celestial objects that are not visible to the naked eye. Information related to this feature is usually not written on the telescope. The main indicator of this capacity is the diameter of the lens or mirror.

Considering the fact that the area of a circle increases proportionally with the square of the radius, we can conclude that as the diameter increases, the light-gathering capacity will increase at the same rate. Given that the area is proportional to the square of the radius, we deduce that a telescope mirror with a diameter of 20 cm can gather four times more light than a telescope mirror with a diameter of 10 cm.

2.     Focal Ratio and Focal Length

The focal ratio is an important parameter in telescopes and similar optical devices. It is calculated as the ratio of the focal length to the diameter of the lens or mirror. The focal length represents the distance from the point where light begins to focus to the focal point. You can see why this definition is a general statement in catadioptric telescopes. For such telescopes, the distance from the lens to the mirror does not accurately reflect the focal length. The light path folds and progresses, and the length of the tube is not equal to the focal length.

To calculate the focal ratio of a telescope, for example, consider a telescope with a focal length of 800 mm and a mirror diameter of 200 mm. In this case, the focal ratio is found to be 800/200=4, and it is generally expressed as F/4. Observers who want to make wide-angle observations or take photographs of deep space objects often prefer focal ratios between F/3.5 and F/8. These types of telescopes are referred to as fast telescopes. The most commonly preferred focal ratio for deep space observations is approximately F/5.

3.     Eyepiece and Magnification Calculation

The eyepiece, also known as the ocular, is the final component between the observer’s eye and the telescope. It comes in various sizes, which determine the degree of magnification achieved with the telescope. The standard sizes are generally accepted as 4 mm, 10 mm, and 25 mm. For example, if you are using a telescope with a focal length of 800 mm, a 4 mm eyepiece would mean a magnification of 800/4=200 times.

If you want to observe your field of view with a wide angle and less magnification, 40 mm eyepieces can be a good choice. Using an eyepiece of this size, you can achieve less magnification in your observation area. There are also adjustable eyepieces available that allow you to reach different levels of magnification.