How to Choose a Telescope

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Buying a telescope for the first time can be a daunting task, but as long as you learn some basic terminology and familiarize yourself with the night sky, you'll just need some luck for clear skies.
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In his last guest article for Discovery News, BBC astronomy presenter Mark Thompson introduced "Six Tips for Perfect Stargazing." In this second installment, Mark shares some tips on how to choose the right telescope before you stare deep into the cosmos.

First things first, before embarking on the purchase of a telescope — if you haven't already — then consider buying a pair of binoculars or at the very least, learn your way around the sky by eye. It will make using a telescope so much easier.

Assuming you're at that stage and now looking at your first all important purchase, there are a few things to consider.

(If you're a parent thinking about buying a telescope for interested children, then ignore my comment about using binoculars! I think it's more important to inspire them with a telescope, no matter how basic it is than to disappoint them with a pair of binoculars. The downside is that they will want to be up late at night — that's not for me to worry about though, so good luck!)

Choosing Binoculars for Stargazing

Binoculars come in many different sizes but they are all "classified" using one very simple method. This method defines how much magnification they provide and also the diameter of the big (or objective) lenses at the front of the binoculars.

For example: 7x50 binoculars would yield a magnification of 7-times, with an objective lens diameter of 50 millimeters.

One very important concept in astronomy is that

Also, the higher the magnification will also make for an unsteady image; magnification doesn't just magnify what you are looking at but also magnifies any movement you impart to the binoculars.

A compromise between magnification and aperture should be aimed for. I started off (and still have) some 7x50 binoculars and they work very nicely for astronomy. Mine only cost around £20 (approximately $40) and were bought from a second-hand shop.

Image stabilization is a new technology for binoculars and adds a lot to the price but does work very nicely to steady the image, if you can afford it, then this is a very good option, although personally I would rather spend money on a better quality pair of binoculars than cheaper ones with stabilization.

Reflectors and Refractors

Eventually you will want to invest in a telescope.

This can be a minefield and daunting to a newcomer, so I hope to demystify it and point you in the right direction. I will introduce some more new terms in this section but will explain as I go along.

Please also be aware, the aim of this is to serve as a guide for the beginner. There are many, many more aspects to telescopes than covered here so this is a basic guide, the rest you will pick up along the way.

Firstly, there are two main types of telescope: refractors, which are telescopes that use lenses (similar to those seen in use by sailors) and reflectors which use mirrors. The key difference for our purposes is cost.

Since light travels through a lens (which itself is generally made up of a few lenses stuck together) there are more surfaces of the glass to be shaped (in a refracting telescope) than with a mirror that has just has one curved surface that the light bounces off (in a reflector).

Ultimately, reflectors are cheaper for instruments of same size and same quality of image. This enables you to get a bigger reflecting telescope than a refracting telescope for the same price. And remember, the most important thing is the size of main mirror or lens: the aperture of the instrument. In this case, bigger is better!

For a complete beginner, having perhaps owned a pair of binoculars, a good starting point would be a modest-sized reflecting telescope, something around 150mm (6 inch) aperture.

Focal Length

You will also find telescopes are given another specification, along with aperture, the "f/ratio" which is related to its focal length. The focal length is simply the distance from the mirror or lens at which the incoming starlight is bought to a focus, this is where the image is then magnified by the eyepiece (more on magnification in a short while).

Generally, the longer focal length, the higher the magnification you can get but the darker the image and smaller the area of sky you can see. This is defined by its f/ratio, or the focal length divided by the aperture. Again, a good starting point is a telescope that does not have too long a focal length ("slow" f/ratio such as f/10) nor too short ("fast" f/ratio such as f/4). So, a good starting telescope would be something like a 150mm f/6 reflecting telescope or something similar.

Magnification and Eyepieces

The next thing to briefly mention is magnification and eyepieces.

Any astronomical telescope is designed to accept a range of eyepieces. Changing eyepieces allows the astronomy to change, amongst other things, the magnification and can thus make the image bigger or smaller. Making the image bigger — or increasing the magnification — has the downside that it makes the image darker. So for some objects, a smaller but brighter image can be better, in which case you would decrease the magnification and thus make it brighter.

Eyepieces are sold according to their focal length. A shorter focal length will give you a higher magnification and a longer focal length will give a lower magnification. To work out what you will get, take the focal length of the telescope and divide it by the focal length of the eyepiece.

For example: a 1000mm focal length telescope and a 10mm eyepiece will give a magnification of 100x. When buying eyepieces, it's as well to have a good range to allow for a range of magnifications. You can spend lots of money on them and some are of very high quality.

In general, spend money on good quality, high magnification eyepieces and spend a little less on the lower power ones. That's simply because quality of optics is of more importance in higher magnifications.

Telescope Mounts

So you have a telescope and some eyepieces, you now need something to stand it on.

A good rule is to spend as much money as you can afford on a good solid mount. Consider that the telescope is magnifying the image often in excess of 200x but it also magnifies any wobbles in the image as a result of vibrations in the ground. I cannot overstate the importance of a good, solid mount to stand the telescope on.

There are two basic sorts of telescope mounts; the first is the most basic. It's called an alt-azimuth mount and has two axes about which the telescope can turn, up and down (allowing you to move from horizon to the point overhead) and left and right (allowing you to move around the sky).

This basic mount is fine and is very similar to the sort photographers or bird watchers use. The only negative is that it is very difficult to attach motors to track things as they move across the sky. Yes, things do move, as the Earth rotates, objects move from East to West and if you point a telescope at an object and watch you will notice it drift slowly out of view.

This problem is resolved in a design called an equatorial telescope mount. This type of mount has one of its axes pointing to the same place in space as the axis of the Earth's rotation. This means that a motor needs to be attached just on this axis and, if it is correctly set up, it will be able to counteract the rotation of the Earth and thus freeze objects in the eyepiece of the telescope. The equatorial telescope mount is perfect for astronomical photography.

While equatorial mounts do make visual observing a little easier, getting used to the way they move and getting them to go where you want them to go can be quite tricky, so for the beginner, a telescope with an alt-azimuth mount is a good starting point. It's always possible to upgrade just the mount at a later date and keep the telescope.

A great example that is the cheap and very easy to use Dobsonian telescope mount. They are often made of wood and are ideally suited to the reflecting telescope.

Try Before You Buy!

So that's it. Having read this, you should now have a basic knowledge and understanding of telescopes and mounts to enable you to make an informed choice. The best thing to do now though is to get out to your local astronomical society and try out some different types of telescope. It will make your choice a whole lot easier.

However, probably the most important piece of advice I have is: the best telescope for you is the one you will use most often. There is no point in buying a big complicated piece of equipment if you can't be bothered spending an hour setting it up! In this case, a smaller more modest setup will mean you can be up and running in minutes.

Also, consider where you are going to be observing. At home, moving a heavy telescope around isn't too much of an issue, but if you have to drive to a dark site then you don't necessarily want too much equipment to lug around.