The Beginner’s Guide to Choosing a Telescope


Thinking of getting your very first telescope? It can be overwhelming when you first start searching because there are so many to choose from. It’s so easy to get caught up in the excitement of a new telescope that you might not stop to think about how practical it will be for your lifestyle. But if you arm yourself with a basic understanding of different telescope sizes, types, and mounts, it will be much easier to pick a telescope in your price range that you will get the most use out of.

To make that choice easier, we advise you pay attention to only a few important factors that suit your needs and budget. They are:

  • Aperture size, not Magnification
  • Mounts
  • Telescope Types

Aperture of a Telescope

How to Choose a Telescope

Telescopes rely on the principles of aperture to help people see far-off objects in space. Image quality and magnification all start with the aperture, which is very different from the kind found in cameras. Telescopes have two basic jobs – to collect light, and to magnify images. That second job depends entirely on the first, so in order to build a quality telescope, a manufacturer ensures that the device can gather as much light as possible. A larger aperture means the scope has more light-capturing potential.

With telescopes, the term aperture doesn’t refer to an opening as it does in cameras. Aperture indicates the diameter of the main optical element, which is either a lens or a mirror. As the aperture size increases, more light enters the scope, making your target objects to appear brighter. This results in better visual information for your eyes, and means that you can see fine details and faint objects in space that a telescope with a smaller aperture would miss. Thus, provided it’s made with high-quality construction methods and materials, a large-aperture telescope creates superior magnification and extremely sharp details.

Unfortunately, many future astronomers think that getting a telescope with a bigger aperture is the best possible choice. In fact, things are not that simple. If your decision is based only on the size of the aperture you will probably get a huge telescope that collects dust in a forgotten corner of your closet.

Ask yourself, where you are going to use your telescope. If the answer is in the backyard, then a large and powerful telescope will be the best choice for you. If you would like to hang out away from the city lights, and enjoy the dark night sky, then you definitely need something more compact. However, remember that the assembly of the telescope in the dark can be a daunting task, and the one that is too large for outside urban areas or difficult to assemble, most likely will not be used very often.


Magnifying Power of a Telescope

How to Choose a Telescope

Bearing in mind the aperture, it is good to learn about some other important aspects such as power and design before you decide to make a purchase. The power is actually the magnification that the telescope can offer. So the size of the planets and other objects in space you are going to observe, depend on the power of the eyepiece. It sounds like a critical factor but it is not. You can always change the power by simply using different eyepieces.

Eyepieces are small interchangeable lenses, or in other words the piece you look through your telescope. Most of the good telescopes for beginners go with a few eyepieces, but don’t worry, any time you want you will be able to buy more additionally. Remember that for clear images, the eyepiece must match the aperture of the telescope. If you use a very powerful eyepiece with a small aperture, the telescope will not show very clear images, while a large aperture telescope can use eyepiece with even 200 times magnification.

A good rule to go by is the maximum magnification should not be more than 2 times the aperture in millimeters (or 50 times per inch aperture). As an example, a 4″ (100mm) telescope shouldn’t be used be used to magnify objects more than 200x (i.e. 4″ x 50, or 100mm x 2). So you have to be careful with a small telescopes that offer magnification much more than 100x. Don’t be grasping for power, even advanced astronomers with large and powerful telescopes rarely use eyepieces with magnification more than 250x.


What are focal length and the f-number?

How to Choose a Telescope

The focal length is the distance between the lens and eyepiece, which for most telescopes will give you an idea of its length. In other words, if a telescope has a focal length of 900 mm, it means that its length will be slightly less than 1 meter. F-number is the focal length divided by aperture and tells you whether the telescope is short and wide or long and thin. So if a telescope has a small f-number, for example F6, it means the telescope is short and wide and provides clear images with low power, while large f-number (F12) is long and thin and will show faded images at high power  in the same aperture for both instances.


The Importance of A Stable Mount

How to Choose a Telescope

Once you know about the aperture and the power in different types of telescopes, let’s look at something that is often overlooked, but it is actually quite important- mounting. Remember that an unstable image is all it takes to kill your enthusiasm! There are two main types of mounting: altazimuth and equatorial.

Altazimuth mounts are a tripod design that only swing up and down (altitude) and left and right (azimuth). You will have to move it in both of those planes to search for and follow the planets and stars as the Earth turns. These mounts are often supplied with cheaper scopes and suffer from vibrations, but you can make alterations to dampen these at home.

An equatorial mount, on the other hand, makes it easy to follow celestial body. Once set up you only need to turn it in the left/right plane to keep any object in view, the mount automatically handles the up/down plane. They also tend to be of more solid and robust build, making viewing less prone to vibration and more pleasurable.

While equatorial mounts are more expensive than their altazimuth counterpart, we highly recommend them if they fit inside your budget.


Types of Telescopes – Refractors vs. Reflectors vs. Catadioptrics

There are three different types of telescopes: refractors, reflectors and catadioptrics. The key difference between a refractor and a reflector is that a reflector uses a mirror as the primary component of its design, while a refractor uses only a lens at the end of a long tube. That means reflectors can be much shorter in length while also allowing for wider apertures, although they can require more setup and maintenance to keep everything in alignment.

Compound (Catadioptric) telescopes, as you might have guessed, are simply a combination of the two, and employ both mirrors and lenses that allow for even smaller and more portable telescopes (albeit at a higher cost). All three types have the same light collection properties despite the differences in size and weight. They all have the same goal: to gather light and carry it into a point of focus where it can be magnified with an eyepiece, but they do it differently. Therefore, any type of telescope has its pros and cons that may match your needs.

Reflecting Telescope or Reflector

reflector uses its primary mirror to gather and focus light. The two typical examples are the Newtonian reflector (so called because it was developed by Isaac Newton) and the Dobsonian telescope. They each feature a concave mirror at the base and a smaller diagonal mirror near the top. They provide great image quality and you get plenty more light collected for your cash. The Dobsonian comes mounted on an easy to use base that is a ‘point and shoot’ style.

Advantages

  • Easy to assemble and use
  • Excellent for observation of galaxies and nebulas because of the large aperture which collects more light
  • Very small failures, delivers clear and bright images
  • Relatively compact and portable
  • Reflecting telescope has the lowest cost per inch for aperture as mirrors cost less than the lens

Disadvantages

  • They are not very suitable for terrestrial observations
  • There is some light loss due to secondary mirror located inside the tube
  • The tube is open and allows entry of dust
  • Reflecting telescope may need a little extra maintenance and care

Refracting Telescope or Refractor

This is the most common type of telescope. The light travels in straight thin tube directly from the front lens to the eyepiece at the opposite end of the tube.

Advantages

  • Very easy to use due to the simplicity of its design
  • Very good for terrestrial observations
  • Excellent for lunar and planetary observations, even binary stargazing if there a large aperture
  • Sealed tube prevents optics and significantly reduces interference image caused by air currents
  • Durable, needs very little or no maintenance

Disadvantages

  • Relatively small aperture – 3 to 5 inches
  • Less suited for observation of small objects or those in deep space as galaxies and nebulae
  • Larger and more bulky than the other 2 types with the same aperture
  • The quality refractor telescope has big price for aperture per inch compared to reflector and catadioptric telescopes

 Catadioptric ( Compound) Telescope

How to Choose a Telescope

Catadioptric telescopes use a system of mirrors and lenses to focus the image before it gets on the eyepiece. This is the most popular type of telescope, with the most modern design providing the larger aperture. There are two popular models:  Schmidt-Cassegrain and Maskutov Cassegrain

In the Schmidt-Cassegrain model light first passes through a thin aspherical lens, called Schmidt corrective lens, then falls into a spherical primary mirror which reflects it to the secondary mirror. From there, the light is directed to the end of the tube where it is mounted to the eyepiece.

The difference between the Schmidt-Cassegrain and Maksutov-Cassegrain is not very big. They have the same advantages and disadvantages. In Maskutov-Kasergen the lens is quite thick and strongly curved, also the secondary mirror is smaller than that of Schmidt which gives good results in planetary observations. Because of the thickness of the lens at Maskutov-Kasergen the telescope needs more time for convergence of temperature, but this only applies to large aperture telescopes.

Advantages

  • The most universal type of telescope
  • The best focus at short distances
  • First rate for deep space observations and astrophotography as well
  • Excellent observations of the moon, planets and stars, land sites and terrestrial photography
  • Closed tube, which means less impact on air currents and dust
  • Compact and durable

Disadvantages

  • More expensive for equal aperture than the reflectors
  • Slight light loss due to secondary mirror
  • The appearance may not be liked by everyone

How to Choose a Telescope

So now when you know so much about telescopes it’s right to ask yourself several questions and depending on the answers to make your choice. First ask yourself what will be interesting for you to observe. If the answer is the moon and planets then a reflector or refractor would be a good choice for you. If you are interested in distant galaxies and nebulas then a 6 or 8 inch Schmidt Kassegrain is the best telescope for you. You are very lucky for choosing a telescope in this time of our history- now with a “Go To” function and with the advances of the technology it is not necessary to know all the sky map to find what you want to see. So let’s look at some of them – check our Review of Best Starter Models for 2021.


16 Responses

  • Very helpful blog for novices like me! Thank you for explaining in detail some of those terms.

    I am still wondering about the different mountings. You said that we should not get a telescope with an unstable image–and I do not want that. However, you just told us that one mounting is more expensive but that they both work.

    Is one mounting better than another?

    • Hello Richard. Thank you for the comment. Both mountings are good, but in fact The equatorial is not offered very often any more. Most telescopes today goes with altazimuth moutning. Just be careful when you choose a telescope and be sure it is heavy enough and stable.

  • Hello,
    thank you for sharing an article in which you explain how to choose a telescope. I have to admit, I am a big fan of telescopes, and I am really glad that I ran into your website because you really provide unique articles. Keep up the good work.
    Kind regards

    • Hi Karlo,

      getting a telescope is not easy job, this is why I’m trying to help here with this article. But one thing is for sure- once you get it, its going to blow your mind every time you look trough the eyepiece. Personally, my telescope is my favorite toy, and will remain there forever 🙂

  • Sites such as these are invaluable for those about to purchase a telescope, not knowing exactly what to get to suit their needs.
    Often times, unsuitable scopes are bought for youngsters and novices which can actually put them off this fascinating subject so it is important to make sure the scope is easy to use and will produce a sharp image to wow!
    You have described the main types of telescope which is great. My take on a first instrument is perhaps to invest in a good pair of binoculars as a starter, especially for the young who can use them easily and learn the basics of how optics work. Once the child is hooked, a good quality telescope will be a worthwhile investment for the whole family.
    Great post and good read. Ches

    • Hi Ches, and thanks for the comment.

      I totally agree with you, a bad investment can really kill the incipient love to Astronomy, so every novice astronomer needs to be well prepared. If you make a right choice you will enter a new world of wonders. In this post is everything you need to now for getting your first Telescope.

      I never had a binoculars and I have to admit that I always wanted. My grandfather had once, and I remember watching the moon with him. Was lovely…

      All best for you!

  • This post was very helpful for me. I happen to be in the market for a really great telescope.

    I was checking out that Celestron Nexstar 4SE and it seems impressive and really cool! Do you recommend a telescope like that for a novice or is it too advanced?

    I am a novice, but I do have some experience with telescopes and I catch on pretty quickly. I just don’t want to get in over my head.

    What’s your suggestion? Too complicated for a novice or should I just go for it?

    • Hi Dani!

      Honestly speaking my first Telescope was Celestron NexStar 6SE and to this moment I really didn’t find reason to by another one. It’s Amazing devise! No worries that you are novice – I was just like you and didn’t have any problem with my Telescope. Go for it! You gonna love it!

      Thanks for the comment, see you by the Stars! 

  • It’s all very technical. My best use for long-range viewing apparatus is for bird-watching so I use a simple binocular. However, for starters, in case I go into a serious study of the universe, I think I’d start with the moon and the planets. So maybe I’d go for the refractor telescope because of low light loss, and low maintenance So it would be the Celestron NexStar 130 for me. I wonder though, if I could afford it.

    • Hi Essengy6903 

      It’s a good idea to start from the Moon and the Planets, so yes, the Refracting Telescope is the best one for that. Celestron NexStar 130 is Feflecting Telescope and is quite expensive. The good Fefractor is Celestron Omni XLT 102 and here is the review and the best place to get it

      Good luck with the Universe, you going to be amazed!

  • Hello,The truth is that I find it exciting to observe our galaxy. Also before buying a telescope, it is very appropriate to know certain characteristics very well described in your article. I live in the countryside, with very few houses around me. The nights are usually very starry, they give me a wonderful show. What kind of telescope do you recommend? Thanks for the info.

    • Hi cjciganotto,

      I definitely will recommend you to have the telescope I own – Celestron NexStar 6SE I have it already 5 years and don’t have even a though to replace it with some other. It is good enough  for observing everything – the Moon. Planets, Stars, Nebulas, Galaxies… Use this advantage living out of the city light pollution, and get into the Astronomy and the Night Sky!    

       

  • So many telescope out there and they all look the same. I did not know the differences. The bigger the aperture the better. With a 6-inch telescope, I can see craters on the Moon as small as about a mile across. Some lens come with anti UV which really helps for my eyes.

    • Yes Kit, few more think to know and you will be able to make a choice for you. All information you need to know is here in the article. 

  • Hi, I really like the way you have explained about the details of a Telescope, for a lay man it really is very helpful. anybody after going through this article, Iam sure more and more people will get into Astronomy, it really is a amazing subject, and nobody will ever get bored of it. In fact I deal in Telescopes and Binoculars from many years, but to be frank I have never tried to explore with a Telescope even though I have so many Models with me. But now after reading this article it has boosted me and has made me more interested in exploring the Galaxies. Thanks Mate.

    • Happy to know I am helpful. One of the best feelings, isn’t ? Don’t waste any more time, grab some Telescope and start exploring the night sky, mate 🙂 Thanks for the comment!

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