How to Choose a Telescope
Buying a telescope for beginners can be a daunting task. There are hundreds of models, divided into several types on the market today. Then, when reading the characteristics one encounters a number of unfamiliar words and it adds extra tension. That’s why you’ll first meet the main characteristics of telescopes and once you know the basics, it will be quite easy to make selections and choose the best telescope for beginners. So let’s start!
Aperture of 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 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
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 “zoom” 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 one 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). So you have to watch out for 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.
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:
- The equatorial
- The altazimuth
The Equatorial is designed so you can easily follow the movement of the sky as the Earth rotates on its axis and its movement indicates celestial north-south and east-west in the eyepiece. This is a big help if you’re looking at an object in space using a star map.
The Altazimuth is simple and just moves up and down, left and right. It is lighter and cheaper than the equatorial and offers the same stability.
What are focal length and the f-number?
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.
Catadioptric telescopes are different because thanks to their optical system can offer long focal length with a big f-number at the same time.
Designs of Telescopes
You can choose between 3 types of telescopes, depending on their design:
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.
The 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.
- 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
- 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
The Reflecting Telescope or Reflector
Reflector telescope use a huge concave parabolic mirror instead of a lens and focus the light on secondary flat mirror which reflects it in a hole in the wall of the main tube. Eyepiece is on the side of the tube near the top.
- 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
- 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
C A T A D I O P T R I C
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 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
- More expensive for equal aperture than the reflectors
- Slight light loss due to secondary mirror
- The appearance may not be liked by everyone
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.
You and your 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. I believe the best telescopes for beginners are those with GoTo function. So let’s look at some of them, and go forward to make the best buy telescope.