Useful Information When Choosing a Telescope
What is the best telescope and which telescope should I buy? The answer to this question is not always a straightforward answer. There are a lot of things that need to be taken into consideration, such as: what you want to do with your telescope and how much money you’re willing to set aside. If you would like to involve yourself with astrophotography, for example, then you should be prepared to spent solid amount of money and to turn towards the leaders in the market. The reason for this is not that they have better optics than our telescopes, but because for astrophotography a special kind of telescope is needed – that automatically tracks the object you are viewing. In order to accomplish this, an expensive and high quality equatorialmount is needed, often with a computerized system as well as additional gadgets and accessories.
But for those that want to feel the excitement of a real outdoor viewing, the joy of finding an object all through your own efforts, then the Dubsonian telescope is right for you. The good news is that you will be able to see images come alive right before your eyes for only a fraction of the price people usually pay. In addition you’ll be able to lean a lot about, the position of [objects] in the night sky, as well as gain the immense satisfaction of being able to find them yourself instead of just using a computer to do it for you. However, the telescopes we offer could have additional computerized GO TO components added to them for automatic tracking.
So generally speaking, what are the characteristics to look for when choosing a telescope: Usually most people think the magnification is most important and the bigger it is the better the telescope is. This, however, can’t be further from the truth. Magnification is not the most important characteristic, nor is it even on the list. As you will soon learn, it can be easily achieved with almost any telescope. There are two important things you should consider and they are aperture and focal length. Everything else is dependent on these two.
1. Aperture - this is the diameter of the lens (with refractor telescopes), or the diameter of the main mirror (with reflector telescopes) and it is the most important characteristic of the telescopes. Why is it so important? Because it provides the following key capabilities.:
- Penetrating – The ability for an astronomical objectwith a specific size, to become visible to a given telescope. The bigger the aperture, the more light that a telescope can gather, which in turn makes the hard to see with the naked eye objects, now visible. The astronomical objects especially the ones from deep space, don’t emit a lot of light or have a lot of light shining on them. Since the human retina has a small diameter, the light that reaches our eyes is insufficient for us to tell these objects apart or in some cases even see them. That’s precisely what the purpose of a telescope is, to gather and send as much light to our eyes as possible. We talk about a bigger penetrating of a telescope when the aperture increases.
- Resolution – the ability for two astronomical objects which are close to each to be distinguished as separate objects rather than as one. The bigger the aperture the better the resolution which is due to the way light behaves in physics and the characteristics of the optics. The value of this parameter, measured in angular seconds, can be calculated when we divide the number 140 by the diameter of the main mirror (measured in millimeters).
This is why the simplest rule when choosing a telescope is: the larger the aperture the more powerful the telescope is and thus you can observe fainter objects in greater detail and higher quality. For example a telescope with 200mm can collect 800 times more light than the human eye. In addition to the cost, which increases with a larger aperture, there is another challenge concerning larger telescopes – they require solid mount and are hard to transport. Luckily however there are some very clever designs, one of which is the one we use for our telescopes. It allows for easy disassembly – assembly and transport, even in the trunk of small car.
2. Focal length – this is the distance from the main mirror or the lens to the point where the light coming from the night objects is gathered. In other words, where the eyepiece is. The focal length depends on the radius of the curve of the main mirror (lens).
3. Along with the focal length and the aperture, there is another important characteristic of a telescope – its brightnessfocal ratio. The focal ratio is the ratio of the aperture of the telescope to its focal length. If the telescope has an aperture of 200 mm and a focal length of 1200 mm, then its brightness will be 1/6 or more commonly written as f/6. Telescopes with a small focal ratio (a bigger number in the denominator) are more suitable for planetary observations, while telescopes with greater focal ratio are more suitable for astrophotography and observations of deep sky objects - galaxies, nebulae, star clusters. Usually focal ratio in the range of f/5 - f/6 equally satisfies both types of observations.
4. Finally we get to the magnification of the telescope. It depends solely on the focal length and on the chosen eyepiece. Since there are different types of eyepieces, which are interchangeable, you can pick and choose whichever ones you need, in order to achieve the desired magnification. The problem, however, is that telescopes with a small aperture don’t allow for a large magnification, since due to the small brightness, the image becomes unclear and dim. That’s why the larger the aperture is, the larger the magnification can become. However this also has its limitations, because of factors such as turbulence in the Earth’s atmosphere, it’s often impractical to use magnification larger than 300-350 times.
Some other characteristics which are derived from these main ones:
- Maximum possible magnification: the way to come up with this value is by doubling the diameter of the main mirror (measured in mm). For example for a telescope with a 200mm mirror, the largest magnification that should be used is 400x
- The system’s field of vision and others