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So you want buy a telescope!
Got the bug to take a close up look at the heavens, but you don't know where to start? Well here is your guide to deciding on just the scope for you!
There are a few things that you need to think about and you will want to buy a telescope that you will use. There is no point in spending good money on a great telescope that stays in its box. If this is the first time that you are thinking about a telescope for star gazing you probably know how much you want to spend, but not what to spend it on.
For rote beginners I would always recommend a good set of binoculars and a good book first. My favourite one (that is the one I keep going back to) is called "Sky watching" by David H. Levy. Good binoculars will cost between one and two hundred dollars and are useful for more than just stargazing, this is important if you are buying a gift for a friend or child and it isn't clear whether they either have, or will develop an interest in Astronomy.
Binoculars are just as useful for bird watching and sightseeing, but the most important thing for a budding sky watcher is that the field of view is right side up. Yes, a trap for the unwary is that astronomical scopes often display the image upside down. Of course when looking up at the sky this is not a problem, but for terrestrial use a correcting prism needs to be used to flip the image right side up and these do not come as standard with astronomical scopes. They ARE built into binoculars and so for beginners it makes finding your way around the sky a little easier.
Learning the sky
Much of the fun and challenge of amateur astronomy is finding your way around the sky. Knowing where to look is a skill that develops each time you go out. Having a field of view that is the same way up as your naked eye view makes it so much easier to go from a star chart to looking up to aiming your binoculars. After a few nights and a little guidance you learn more and more interesting objects and how to quickly find them.
When you have learned how to read a star chart and relate it to what you see in the sky, you can then think about trying a telescope. The big difference with a telescope is the magnification over a set of binoculars. Typically binoculars provide a view around 10 times the normal size to your naked eye (10x). It is possible to get up to 20x binoculars, but these become very hard to hold steady and difficult to enjoy for any length of time. As a result it quickly becomes apparent that when working at higher magnifications the 'mount' that holds the telescope is very important as we often use magnifications of 50x to 200x.
It is a good idea to go out with an experienced Amateur Astronomer and see how they put a telescope to use and the different methods of finding the objects that you want to look at. If you don't have a friend that is a regular user of a telescope you should contact your local Astronomical Society and they would be only too glad to help you. They will be able to give you some practical demonstrations.
When trying to decide what sort of telescope is right for you there some important terms and concepts that you need to be aware of.
What is telescope aperture
This is the diameter (in mm or inches) of the primary optical element. This could be either the main mirror in a reflecting telescope or the primary lens of a refracting telescope. All other things considered the bigger the better. The aperture is a measure of the light gathering power or 'grasp' of the telescope and in Astronomy this is key, as the larger the aperture the 'brighter' the image will appear. Normally the larger the aperture, the more costly the telescope. For astronomy this needs to be at least 90mm to provide any advantage over a good set of binoculars, but typical values for amateur telescopes are 100 to 200mm.
cal LengthThis is the distance in mm of the 'image plane' from the primary optical element. The 'image plane' the position that a focused image is formed (the 'prime focus') and the bigger the focal length the larger the image formed. In combination with a selection of eye pieces, different magnifications can be achieved and this is the main difference with binoculars which have a fixed magnification. Typical focal lengths are 800mm to 3000mm, but for reasons that will be explained later bigger is not always better.
The telescope mount
This is important for 2 reasons. Firstly it is impossible to hold a telescope still enough to be useful and secondly because of the rotation of the earth objects of interest move across the sky and need to be followed. So a good mount will hold the telescope steady enough to look through, but will also allow you to steer it to your intended target. There are a wide variety of mounts at the low end of the market they are purely hand guided mounts much like camera tripods all the way up to computer guided precision mounts that can automatically point the scope for you. Typically the more features that you include the more expensive the mount, but the most important thing to remember is that no matter how magnificent your gleaming new telescope looks in the showroom you need to be confident in using it in the dark.
Yes once you have chosen your first telescope it is a good idea to practice using it during daylight hours when you can clearly see all the controls and attachments. Learn how to site your scope, how to focus it, how to change eyepieces, where to store the covers because there is nothing worse than dropping a small attachment in the dark and having to find a torch when what you really want to do is to star gaze.
So which scope should you buy? This is a good question and as with all such good questions the answer is "It depends".
OK that was a trite answer, but not invalid. It depends on what you want to look at and it is now important to consider a concept called the 'Focal Ratio'.
The Focal Ratio is Focal Length divided by the Aperture of the telescope. It is precisely the same as the 'F-stop' setting on older style cameras. In general a lower number allows a scope to gather light 'faster', but to a viewer this makes for brighter images. So a scope that has a focal length of 1000mm and an aperture of 100mm has a Focal Ratio of 10, normally referred to as F10. If we double the aperture to 200mm with the same focal length the scope is now F5. Since the Focal Length is the same in both cases the image in the eyepiece will be the same size, but the faster scope will produce a brighter image. Though we won't go into the reason why here, the bigger aperture will also produce a clearer image.
So returning to the question of "what you want to look at?" If you want to look at planets which are relatively bright, but small in size you will not need a very fast scope, but one that can produce a large image so the longer focal length is important. If you want to look for "deep sky" objects such as Nebula, Star Clusters and Galaxies then you will want as fast a scope as you can get. These objects are typically dim, but also quite large in some cases so a large focal length is not crucial.
So now to the nub of the matter, what is best for me the beginner? Well start with a budget and look around to see what you can get for it. It also makes sense to go to stores that specialize in Astronomical equipment and ask.
If you have used binoculars and have decided to take the next step and are keen then you should budget for about $1000 Australian dollars for a good scope that will last you for years. This may seem excessive, but once you take into account that quality eyepieces can be hundreds of dollars it quickly adds up. For this price I would expect to get a good Astronomical mount, a scope of between 100 to 200mm aperture and a selection of eyepieces. There are some compromises that can be made, for instance a simpler mount would allow you to afford a larger aperture. Or a smaller aperture would allow you to afford a computerized mount. it is very much up to you. If looked after you can expect a quality instrument to last decades and this is important to consider.
Another important point is something that one of our new members remarked recently, there is no point in having the greatest telescope if you don't use it. Make sure you are comfortable with how to use it and maintain it and it will open the door to a lifetime of enjoyment.
For more information please feel free to contact the Bendigo District Astronomical Society at Contact Us.
Mike Lyons
Bendigo District Astronomical Society
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