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How To Choose A Telescope For Astronomy?

It’s a great moment to become an amateur astronomer. Never before have newcomers to stargazing been offered such a diverse range of telescopes and accessories to help them pursue their interests. But, naturally, this comes with the burden of choice: the overwhelming variety makes it difficult for an untrained buyer to make the appropriate selection on how to choose a telescope for astronomy.

Whether you’re serious about purchasing your first telescope or just fantasizing about it, this guide will help you narrow down your options. We’ll begin by looking at the essential qualities that all telescopes have, and then we’ll look at some unique designs. We’ll also consider the tradeoffs, as each tool has pros and downsides.

Know The Pre-requisites! 

Before you acquire anything, you must first decide what is most essential to you. What do you want to look at the most? How black is your night sky? what is the experience you have as an observer? How much money are you willing to spend? Where are you going to keep your telescope, and how much weight are you willing to carry? Answer these essential questions, become acquainted with what’s available, and you’ll be well on your way to selecting a telescope that will satisfy you for many years to come.

This guide focuses on visual observation rather than astrophotography. Long-exposure photographs of galaxies and nebulae need a significant amount of time, patience, and specialist equipment. If you’re new to astronomy, it’s a good idea to start with visual astronomy before diving into astrophotography. In addition, it is frequently less expensive to purchase a single telescope specialized for visual viewing and a separate scope for astrophotography than to purchase a single scope that is adequate for both activities.

Before delving into the many telescopes on the market, it’s essential to understand the fundamentals of how they function.

Basic Terminology


The diameter of the telescope’s objective (primary) lens or mirror is commonly measured in millimeters. Essentially, the wider the aperture, the brighter the pictures and the further into space you can view.

OTA -Optical Tube Assembly

The major component of a telescope is the OTA. It collects light and houses the optical accessories and eyepiece.


This word is frequently used but is comparatively new to an amateur astronomer. It is used on the motorized mount, which is wholly or partially controlled by the computer. The name refers to the controller’s capacity to “go to” the definite spot by itself, rather than the user physically adjusting the mount.


It is the number of times an item looks larger when viewed by the bare eye. For example, a scope with a focal length of 1500mm and a 25mm eyepiece will create a magnification of 60x, while the 10mm eyepiece will give a magnification of 150x. As you can see, the more the magnification is, the longer the focal length of the telescope and the shorter the eyepiece focal length.

A word on magnification:

a lot of beginner astronomers befall into the “greater power” trap, although this impulse should be avoided when learning astronomy. As magnifications rise, specific unexpected concerns develop. A few areas the following:

  • Smaller exit pupil, making it difficult to see in the darkness.
  • The increased appearance of picture wobbles due to wind or vibrations
  • Limited eye relief, leading the user to bring their eye into the eyecup, creating vibrations.
  • Lower image brightness.


The OTA is coupled to a Mount, which controls how the user moves, aligns, and monitors celestial objects. There can be a lengthy discussion on the various mounts, but for now, there are mainly three principles: Motorized, German Equatorial (EQ), and Alt-Azimuth (Alt-Az or Az). Motorized mounts can be EQ or Alt-Az. However, they usually are designated as such to distinguish them from manual mounts.


Lenses are of glass. Most decent models will have lenses of optical glasses, which is better than standard glass in reducing chromatic and spherical aberrations, resulting in sharp and crisp pictures. For enhanced aberration correction, better telescopes will use exceptionally fluoride glass or low-dispersion.

Spherical and Chromatic aberrations have been mentioned several times in the previous paragraphs. Here’s everything about it:

Focal Ratio

Suppose you’ve never seen a telescope or binoculars before. In that case, the f-number indicates its portability and size—smaller ratios mean more petite focal lengths, resulting in shorter OTAs. So, for example, just by glancing at the focal ratios, one can presume that f/5 will be considerably lower tube length, will most likely be managed by a person, but the f/15 would be gigantic.

The focal ratio is crucial in astrophotography. The more diminutive the ratio is, the faster the telescope, resulting in shorter exposure periods for taking photographs since light within the OTA travels a smaller distance and remains more intense than in a slower telescope. In addition, more diminutive exposure lengths imply that any tracking problems will be a little less prominent while also giving you more time to shoot more shots, which you can then heap in post-production.

Focal length 

It is the distance measured in millimeters from the objective to the eyepiece. When combined with an eyepiece, its length has a direct impact on the telescope’s magnification capacity. When combined with catadioptrics and reflectors, this theoretical distance produces an even more substantial focal length than an actual optical tube, making the OTA more portable while significantly increasing magnification capability over a similarly-sized refractor. As with a refractor, the distance can be a literal, linear measurement from the primary lens to the eyepiece or a theoretical distance depending on how light is reflected from the primary to secondary mirrors and then into the eyepieces.

Spherical Aberration

It has been there since astronomy began. The curve of the lenses or mirrors makes light to be concentrated at one point. To have a crisp image, light invading the system through the lens must be focused on the f/point, making it easier to see it clearly from your eyes. If the mirrors or lenses inside the optical system are not perfectly polished, round, or placed, the light may not focus appropriately and fall inadequate of the f/length. As a result, there will be distortion and difficulty in obtaining precise focus. 

Chromatic Inconsistency

Light of different hues has distinct wavelengths and travels into the glass at various but expected speeds: Because wavelengths that are shorter travel quicker than the longer ones, the various hues of light from a singular object reach the eye at varying times exit the lens.

This aberration can also be caused by the shape of a mirror or lens. In the case of lenses, the lens’s form lets it be thinner or thicker at different locations; as a result, light traveling through the thicker portions takes longer than light passing through, the thinner sections. 

Types Of Telescope

We may now investigate the many types of telescopes accessible after understanding a few fundamental principles influencing their performance.

From the advertisements in the astronomical press, one could be forgiven for believing there is a limitless variety. However, despite their many different forms and sizes, telescopes may be classified into three types: refractors, reflectors, and catadioptrics.

Refractor telescope 

It is the stereotypical image of a telescope. A long, shiny tube with a massive lens in the front and the eyepiece at the rear. When correctly planned and manufactured, Refractors provide sharper and brighter pictures per inch of aperture than any other design. This is partly because lenses are more efficient than mirrors. And because practically all other designs have a secondary mirror upfront. That blocks some of the incoming light. In general, a high-quality 4-inch refractor performs nearly as well as a 5-inch reflector. Catadioptric for deep-sky objects and may even perform somewhat better for planets.

Refractors are most telescopes with apertures of 80 mm or smaller. This is due to the ease and low cost of producing small lenses, and the refractor’s performance edge is most noticeable in those small apertures. As a result, refractors dominate both the low-end market, where people can only afford extremely tiny apertures, and the market for very portable high-performance telescopes.

The reflector telescope

The one that employs a mirror to gather and concentrate light is the second type of telescope. The Newtonian reflector (by Isaac Newton) is the most popular, with a specifically curved concave (dish-shaped) primary mirror at the telescope’s bottom end. A tiny, flat, diagonal secondary mirror at the light of the top guide from the primary to the side of the tube, where a conveniently located eyepiece meets it.

The reflector is the scope to choose if you want the maximum aperture for your money. When properly constructed and maintained, a reflector may produce clear, contrasty pictures of a wide range of celestial objects at a fraction of the price of an equal-aperture refractor.

The catadioptric or compound telescope

It is the third type of telescope. These were designed in the 1930s to combine the most delicate features of refractors and reflectors: they use both lenses and mirrors to generate a picture. The most exciting aspect of these instruments is their modest size in their most frequent configurations. Their tubes are only two to three times as long as they are wide, a configuration made possible by “optical folding” of the light. The smaller tube allows for a lighter, more controllable installation. As a result, you may produce a large-aperture, long-focus telescope that is also relatively portable.

However, there are some limitations here as well. The focal ratio of most Schmidt-Cassegrains is f/10, while Maksutov-Cassegrains have even longer focal ratios. As a result, they are unable to provide vast, low-power fields of vision. Some versions, but not all, allow the insertion of a focal reducer to drop the effective focal ratio to f/6 or so, which helps significantly.

Everything Comes At A Cost.

While it may be tempting, avoid the impulse to purchase the cheapest telescope on the market. Most will be of low optical, mechanical, or both qualities and will disappoint. On the other hand, a good telescope may be purchased for $150 or less if you shop carefully. Even then, you’ll obtain a scope with a small aperture. A 6- or 8-inch Dob, costing between $300 and $500, is the cheapest scope with no significant drawbacks.

However, even if you have a lot of money to spend, don’t go out and buy the most giant, most expensive telescope you can afford just yet. Instead, begin with something smaller and more doable. It makes sense to start with a lower-cost alternative until you’ve thoroughly researched your options and determined where your long-term interests lie.

Also, save part of your astronomy cash aside for extra eyepieces to increase the scope’s magnification range, a complete sky map, decent guidebooks, and other accessories. Many individuals, for example, believe that an adjustable-height chair is worth its weight in gold.

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