Things to know and do before buying a telescope
Binoculars are inexpensive, simple and easy to use, and they have exposed you to thousands of objects in our Milky Way and beyond. Every stargazer should own a pair of binoculars.
But there may come a time when you want more, when you want to see brighter, larger, and more distant objects. This is when you should consider a telescope.
Many beginning astronomers who buy a telescope before learning the basics of what to see in the sky (and how to see it) often end up frustrated and give up on astronomy before they can observe it. It’s just too complicated and leads to failure. By learning a little background beforehand, new astronomers can make their first telescope experience rewarding and enjoyable.
So how do you know if you’re ready to buy and use a telescope? Below are 10 things you need to know and do before you start telescope observing:
- See some of the bright stars and about 10 major constellations.
- Understand the main points of the celestial sphere: horizon, zenith, celestial meridian, position of the north or south celestial pole, celestial and ecliptic equator.
- Learn how to find and observe with binoculars, especially the Moon, Jupiter and bright “deep objects” such as the Orion Nebula, the Andromeda Galaxy and the Pleiades cluster. You have to practice looking through the eyepiece and binoculars are more comfortable than a telescope.
- Look through someone else’s telescope (or at an astronomy club meeting) to get an idea of what you can or cannot see. Many beginners are surprised to see only 0.5 to 1 degree in the sky at a time. It’s almost like looking at the sky through a reed tube. You can try it with a friend’s telescope or attend a star party hosted by your local astronomy club. If possible, you should have someone guide you and help you find and observe faint objects with a telescope to get an idea of what to expect.
- Discover the main types of telescopes, as well as the advantages and disadvantages of each (you will quickly discover them in this guide…)
- Learn the main functions and specifications of telescopes: what they are, what they mean (which you will also read about in this guide). Knowledge is power.
- Determine where you will observe and how to get your telescope there. Don’t force yourself to carry an incredibly big and heavy tube if you have to climb the stairs in your residence every night.
- Determine where you will store the telescope. You will need a dry, convenient place to move the goggles in and out as well as to the viewing area.
- What do you want to observe with your telescope? Just the Moon and the planets? Layers of nebulae and galaxies? Birds and mountains? A little for everyone?
- Count the money…and decide how much you’ll spend on a telescope. Find a really trustworthy place and get advice from astronomy groups like Hanoi Astronomical Society – HAS, Astrophysics.
Image of the Moon seen through a normal telescope
Instructions for choosing a good telescope
Choosing a telescope is an important decision for those who want to study astronomy in depth. Before knowing the advantages and disadvantages of telescopic walls, let’s start by understanding some unwritten rules so that you can choose the tool that best suits your needs.
First, don’t buy a cheap “junk” telescope that is advertised to focus at high magnification (>300x, 400x or more), and the type of mount used with the telescope may cause distortion of the telescope. The image of shaking and vibrating objects makes one dizzy. These “junk” glasses are sold in many department stores and some sports, technology, or camera stores. Never buy such telescopes. You will definitely regret it.
You can spend a few hundred to a few million for a good telescope, or a few tens of millions for a great telescope. But as this guide recommends, you need to think carefully when buying a telescope under $3-5 million.
Don’t worry about the magnification. As mentioned above, the most important feature of a telescope is the aperture, which is the diameter of the lens or mirror that collects the light. A large aperture telescope will collect more light, give you brighter images, and allow you to see finer details. Your first pair of glasses should have a minimum aperture of 80-90mm. If this were not the case, the images of anything other than the Moon and possibly Jupiter would be very blurry and less sharp. For example, you can see dozens of galaxies outside our Milky Way through a telescope with an 80mm aperture from a dark area. They will be blurry, but you will still see them. A 6 or 8 inch (150 to 200 mm) telescope will make it much clearer to distinguish faint objects, especially if you have to deal with light pollution.
(Note: when astronomers aim at a 150mm telescope, for example, they point to the diameter of the objective.)
The view through a large aperture telescope is almost always more spectacular than the view of the same object through a smaller telescope. The disadvantage of the aperture is that the larger the aperture, the higher the cost and the telescope is also bulkier. We leave the financial questions to you. But always keep in mind the size and mass of the telescope, as well as the distance it must travel to reach the viewing area. Some ideal beginner scopes can measure between 1.2 and 1.5 meters in length, 200 to 230 mm in width and have two main parts each weighing between 9 and 14 kg. Can you store or move them all? If this is not possible, consider a more compact pair of glasses. You may need to reduce the aperture a bit or buy a more expensive but more compact pair of glasses. Remember: a tall glass is useless if you can’t use it.
A few terms about telescopes
We have already mentioned that the most important optical property of a telescope is the aperture, i.e. the diameter of its main mirror or lens. The larger the aperture, the brighter the image. A good home telescope has an aperture of 20 to 300 mm (3.15 inches to 12 inches) or larger. Some large multi-million dollar professional telescopes have apertures up to 10 meters (400 inches), or about the size of a small fish pond.
The objective lens or mirror picks up light from a distant object and brings it to a convergence point (focal point). The distance between the lens and the focal point is called the focal length.
A second lens, called the eyepiece, is placed near where the light from the objective is focused at the focal point. The eyepiece magnifies the image coming from the objective and also has a focal length. The magnification of a telescope and eyepiece is easy to calculate. If the focal length of the lens is noted as “F” and the focal length of the eyepiece is “F“, then the magnification of the telescope/eyepiece combination is
. For example, if a telescope has an objective lens with a focal length of 1,200 mm (about 48 inches) and an eyepiece with a focal length of 25 mm (about 1 inch), then the telescope will have a magnification of
. Almost all telescopes allow the eyepiece to be changed for different magnifications. If you want 100x magnification in this example, you’re using an eyepiece with a 12mm focal length.
The aperture of the eyepiece in this simple telescope diagram is D. The focal length of the objective lens is F. The focal length of the eyepiece is f. The magnification is therefore F/f. The focal ratio is F/D. Credit: OneMinuteAstronomer.com
Another unwritten rule: the useful magnification of a telescope is about 50 times the aperture in inches. Higher magnification makes the image too blurry and dark. So a 4 inch scope can achieve 200x magnification before the image becomes too blurry and dark, a 6 inch scope will give a useful 300x magnification, and so on. This is not a strict rule. Sometimes when the atmosphere is unstable you can only get 20 or 30x per inch of aperture. With a stable atmosphere and high optical quality, you can achieve 70x or even 100x per inch of aperture, and following the example above, 400x can be achieved with a 4-inch lens. But it is rare.
The third important technical characteristic of a telescope is the focal ratio, which is the focal length divided by the diameter of the objective lens.
A long focal ratio, meaning higher magnification and a narrower field of view for a given eyepiece, is ideal for observing the Moon, planets and binary stars. For each of these subjects, a focal ratio of f/10 or higher is ideal. But if you want a wider view of star clusters, galaxies, and the Milky Way, a low focal ratio is best. You will have less magnification, but you will see more of the sky. Wide field telescopes have a focal ratio of f/7 or less.
Enough math and rules! Let’s get more specific with the three main types of telescopes available to the home stargazer: refracting, reflecting, and compound telescopes, and consider the advantages and disadvantages of each.
Theo Hiền Phan – VLTV, Nguồn: A Beginner’s Guide to Choosing Binoculars and Telescopes for Stargazing
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