Before discussing how telescopes are classified, it's important to first define what they are. A telescope is a device that facilitates the observation of something at a great distance, providing a more detailed view than if observed directly with the naked eye. It thus offers a magnified image of the object in question.
Its history is linked to various optical and physical discoveries. The first of these mechanisms was invented in 1608 by the German scientist Hans Lippershey (1570–1619). It was a telescope with a concave eyepiece and a convex objective lens: the modification of the light intensity in the lens of these modules causes the rays, which travel parallel to each other, to converge at a single point that forms part of the focal plane.
Over time, reflecting telescopes came out that, instead of lenses, used mirrors to summarize the orientation of light and achieve the creation of images. In the same way there are others that are cited as catadioptrics and that adjust the positions of the mirrors and lenses.
The first astronomical telescope was registered in 1609 by Galileo Galilei, who was able to observe the planet Jupiter, the Moon, and many stars. From then on, telescopes became fundamental to the advancement of astronomy (the science dedicated to the study of celestial bodies).
The diameter of the objective (whether of the mirror or the main lens of the telescope), the focal length (the effective path between the mirror or lens and the focus where the eyepiece is located), the filter (an accessory used to darken the image of the observed body) and the magnification (the number of times the assumed diameter of the object can be reproduced) are some of the measurements that help define a telescope.
How are the telescopes
Below I will mention how telescopes are classified, as these are:
refractor
It is the oldest and therefore most popular telescope design. It is sometimes also called a Galilean telescope. It consists of a set of lenses that attract light and focus it at the focal point, where the eyepiece is placed . There are different optical arrangements for this lens assembly—doublets, triplets, Petzval design, among others—and each offers a different configuration and correction of optical errors.
The main defect in the telescope is its color, which changes to a reddish and a bluish halo appearing on either side of the brightest objects. This optical distortion is caused by the alteration of light in the telescope's lenses , although it can be mitigated to a greater or lesser degree.
According to this approach, refractors are categorized as achromatic, meaning their color distortion is too pronounced, and apochromatic, when this distortion is remarkably small. Despite this minor flaw, the lenses possess excellent optical properties and produce very sharp and accurate star trails.
Reflector
It consists of a concave mirror located at the lower end of the tube, which gathers the light and directs it towards a smaller, flat secondary mirror located at the mouth of the tube. This secondary mirror reflects the light back at a 45° angle and directs it towards the eyepiece. It's important to note that a portion of the objective lens's diameter is hidden by the secondary mirror, so its optimal viewing angle is typically between 10 and 20% smaller.
There are several types of telescopes, but the most common design is the Newtonian system. They are bulkier than refractors and less mechanically controlled, since the eyepiece is located near the telescope's aperture. These telescopes tolerate coma and other less significant, but still present, errors. Their optical efficiency is generally lower than that of eyeglasses; however, in recent years, extraordinary capabilities have been achieved. Despite this, the star images are less precise.
Another way to classify telescopes is by catadioptric telescopes . These are made up of lenses and mirrors, designed to overcome the shortcomings of earlier designs. The objective lens is a curved mirror, but at the aperture, there is a censor lens that directs the light to a hole in the center of the primary mirror at the end of the tube.
This design results in a very long focal length for this type of telescope, while the tube itself is very compact. They are short and heavy tubes, but easily portable due to their compact size. Their optical efficiency is good, but it doesn't surpass that of a good refractor , falling somewhere between the two designs and becoming a versatile telescope without a specific field of view to excel in. There are various designs and optical ratios available.
Recommendations, tips, advantages and disadvantages with telescopes
For learning astronomy, a minimum aperture of 70mm for refractors and 130mm for reflectors is recommended. However, for catadioptric telescopes, it's best to go below 150mm or even 200mm, which is why they aren't as commonly used as learning telescopes. These figures are minimum guidelines, and if your budget allows, it's advisable to exceed these limits.
It's important to know that with the same budget, you can achieve a larger diameter reflector than a refractor, since mirrors are much easier to refine and their manufacturing costs are minimal compared to lenses . On the other hand, lenses generally offer higher quality. If you'd like to learn more about the different types of telescopes , there's very useful information that will complement your learning.
This means that certain types of analysis or objects are better suited to each type of telescope. Thus, refractors are typically used for planetary and double star observation due to their greater optical efficiency, while reflectors are generally used for deep -sky observation (nebulae, galaxies, clusters, etc.) because of their larger aperture.
However, this isn't a determining factor, especially for learning astronomy, since all beginner telescopes are somewhat versatile and will allow us to cover a bit of everything in the exam. Catadioptric telescopes don't have a very specific purpose and fall somewhere between the advantages of eyeglasses and reflectors.
In this respect, telescopes don't require excessive support, but it's true that they need to be properly collimated for optimal performance. Reflectors and catadioptrics need to be collimated very frequently, while refractors don't. This is a significant difference, but it shouldn't alarm us or influence our decision when purchasing a telescope.
Finally, it should be noted that each type and design of telescope has certain optical errors. These may be better or worse mitigated, but they are common to every type of telescope . Thus, a reflector will tolerate coma and spherical error, while a refractor will tolerate a greater or lesser degree of chromatic aberration.
Finally, if you want to buy a telescope, you need to consider other factors such as the mount, weight, dimensions, viewing area , and, of course, the cost. That's why, if you want to learn more about this world, it's important to understand how telescopes are classified.
