Things to know and do before buying a telescope
Binoculars are inexpensive, simple, and easy to use, and they’ve brought you thousands of objects in our Milky Way and beyond. Every astronomer should own a pair of binoculars.
But there may come a time when you want more, when you want to see brighter, bigger, and more distant objects. That’s when you should consider a telescope.
Many beginning astronomers looking to buy a telescope before they’ve learned the basics of what to see in the sky (and how to see it) often fail and drop out of astronomy before they can actually get started. It’s just too complicated and leads to failure. By learning a bit of background, new astronomers can enjoy their first telescope in a rewarding and enjoyable way.
So how do you know you’re ready to buy and use a telescope? Here is a list of 10 things you need to know and do before you start telescope viewing:
- Discover some of the bright stars and about 10 major constellations.
- Know the main points of the celestial sphere: the horizon, the zenith, the meridian, the position of the north or south celestial poles, the celestial equator and the ecliptic.
- Learn how to find and observe with binoculars, especially the Moon, Jupiter and bright “deep objects” such as the Orion Nebula, Andromeda Galaxy and the Pleiades. You have to practice looking through the eyepiece, and binoculars are more comfortable than a telescope.
- Look through someone else’s telescope (or the astronomy club’s at events) to get an idea of what you can see. Many beginners have been 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. You can try it out with a friend’s telescope or join a star party hosted by your local astronomy club. If possible, have someone guide you and help you look at dark objects with a telescope to get an idea of what to expect.
- Learn the main types of telescopes, and the pros and cons of each (you’ll learn more quickly in this guide…)
- Learn the main functions and specifications of the telescope: what they are, what they mean (you will also read in this guide). Knowledge is power.
- Determine where you will observe and how to get your telescope there. Don’t force yourself to carry a horribly big and heavy hose if you struggle with the stairs in your accommodation every night.
- Determine where you will store the telescope. You will need a dry, convenient place to move the glasses in and out of the viewing area.
- What do you want to see with your telescope? Just the Moon and the planets? Fuzzy layers like 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 place with a radius you really trust and get advice from astronomical groups like the Hanoi Astronomical Society – HAS, Astrophysics.
Image of the Moon seen through an ordinary telescope
Guide to choosing a good telescope
Choosing a telescope is an important decision for those who want to study astronomy in depth. Before diving into the pros and cons of telescoping walls, first understand a few unwritten rules so you can choose the best tool for your needs.
First, don’t buy a cheap “junk” telescope that’s advertised as focusing on large magnifications (>300x, 400x or more), and the type of stand used with the telescope can make images worse. you feel dizzy. These “discarded” glasses are sold in many department stores and in 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 should be careful when buying a telescope under 3-5 million.
Don’t worry about magnification. As noted above, one of the most important characteristics of a telescope is the aperture, which is the diameter of the lens or mirror that collects the light. A telescope with a large aperture will let in more light, give you a brighter image, and allow you to see better detail. Your first glasses should have a minimum aperture of 80-90mm. If not, images of anything other than the Moon and possibly Jupiter will be very blurry and less sharp. For example, you can see dozens of galaxies beyond our Milky Way through an 80mm aperture telescope from a dark area. They will be dark, but you will still see them. A 6-inch or 8-inch (150-200mm) telescope will make out-of-focus objects much more visible, 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 impressive than the view of the same subject through a smaller telescope. The disadvantage of apertures is that the larger the aperture, the higher the cost and the bulkier the telescope. We leave the finances to you. But always note the size and mass of the telescope, as well as the distance it must travel to the viewing area. Some large beginner goggles can be 1.2 to 1.5 meters long, 200 to 230 mm wide and come in two main sections, each weighing between 9 and 14 kg. Can you store them all or move them? If that’s not possible, you’ll need to consider a more compact lens. You may need to reduce the aperture a bit or buy a more expensive but more compact lens. Remember: a tall glass is no good if you can’t use it.
Some telescope terms
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, multimillion-dollar professional telescopes have apertures up to 10 m (400 inches), or about the size of a small fish pond.
The lens or mirror collects light from a distant object and brings it to a point of focus (focus). The distance between the lens and the focal point is called the focal length.
A second lens, called the eyepiece, is placed near the point 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 1200mm (about 48 inches) and it has an eyepiece with a focal length of 25mm (about 1 inch), the telescope will have equal magnification To
. Almost all telescopes allow the eyepiece to be changed for different magnifications. If you want 100x magnification in this example, you are using an eyepiece with a 12mm focal length.
The eyepiece aperture of this simple telescope is D. The focal length of the objective is F. The focal length of the eyepiece is f. So the magnification is F/f. The focal length is F/D. Credit: OneMinuteAstronomer.com
Another unwritten rule: the useful magnification of a telescope is about 50 times the aperture in inches. The higher the magnification, the blurrier and darker the image becomes. So a 4 inch lens can achieve 200x magnification before the image becomes too blurry and dark, a 6 inch lens will provide a useful 300x magnification, and so on. It is not a strict rule. Sometimes when the atmosphere is unstable you can only achieve 20x or 30x per inch of aperture. With atmospheric stability and high optical quality, you can get up to 70x or even 100x per inch of aperture, and using the example above, you can go up to 400x with a 4-inch lens. But it is rare.
The third important specification of a telescope is the focal length ratio, which is the ratio of the focal length divided by the diameter of the objective.
A long focal length ratio, i.e. higher magnification and narrower field of view with a given eyepiece, is good for observing the Moon, planets, and binary stars. For each of these subjects, a focal ratio of around f/10 is ideal. But if you want to look more broadly at star clusters, galaxies, and the Milky Way, a low focal ratio is best. You’ll get less magnification, but you’ll see more of the sky. Wide field telescopes have focal ratios of f/7 or less.
That’s enough for the math and the rules! Let’s get into the details of the three main types of telescopes available to amateur astronomers: refractors, reflectors, and composite telescopes, and look at the pros and cons of each.
Theo Hiền Phan – VLTV, Nguồn: A Beginner’s Guide to Choosing Binoculars and Telescopes for Stargazing
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