Buying Your First Telescope

Buying your first telescope is something that, like many things, requires a certain amount of research in order to ensure that you buy the right one for you.

When I bought my first telescope the starting point was budget. Browsing around the web it became pretty apparent very quickly that if I wanted to I could spend a lot of money on a telescope, so I set myself a budget of £200 (UK Sterling) - or rather my wife set me a budget of £200!

I knew absolutely nothing about astronomy and telescopes, so I was starting very much from scratch. Having decided on my budget I began to look at what was available for my money.

There are two main types of telescopes: refractors, which uses lenses, and reflectors, which uses mirrors. Being as a lens is more expensive to produce, refractors tend to be more expensive that reflectors. For my budget of £200, it became apparent that if I bought a refractor, I might be a little disappointed. For this reason I decided to go for a reflector. If my budget had been larger, it may well have been a different story.

Aperture

The most important single factor when assessing how good a telescope is, is its light gathering capability, which is determined by its aperture. This is the diameter of the telescope. Looking at it very simply, a telescope with an 8" aperture will gather more light than one with a 5.1" aperture (such as the Sky-Watcher Explorer 130P), and is therefore a better telescope.

Magnification

It is the eyepiece that, in conjunction with the focal length of the telescope, determines the magnification. The formula to use to work out the magnification is to divide the focal length of the telescope by the focal length of the eyepiece. So, for example, a telescope with a 650mm focal length will produce a magnification of 65 when it is used with 10mm eyepiece (650 / 10 = 65).

The rule of thumb regarding the maximum magnification that can be sensibly achieved with a telescope is to double the aperture (in mm). Therefore, a telescope with a 130mm aperture will have a maximum magnification of 260 (2 x 130).

What I have come to realise in the short time I have had my telescope is that magnification really isn't that important - getting a nice sharp, bright image is more important, and as you increase the magnification, the brightness and sharpness of the image reduces. The important thing is to experiment until you find the magnification range that works best for your telescope. Mine has a 130mm aperture and I find that magnifications around the x130 to x160 seem to give me the best results. I have tried cranking the magnification up to x260, but it is very difficult to get a good sharp image at that magnification.

However, it is useful to a have a range of eyepieces so you can use different magnifications when you're viewing an object.

Portability

It's also important to remember that you'll need to be able to easily move your telescope around, assuming that you don't have your own observatory! I keep my telescope in the house but I use it outside (as we all should). If it was too awkward to move around I think there would be a possibility that I might stop using it.

I've heard it said many times, but it is true, and that is that the best telescope for you is the one that you'll use the most - there's no point in buying a great big scope if it's too much trouble to use it.

More About Telescope Mounts

Although when anyone ever thinks about astronomy they automatically think about the telescope, it is also important to have a good mount and tripod. It is essential that the tripod is sturdy and does not wobble, otherwise the object you are viewing will move about when you don't want it to, and also that the mount that goes on top of the tripod is of a good quality and is easy to use.

As with telescopes, there is a wide range of mounts available from cheap alt-azimuth mounts to expensive computerized GOTO mounts. The type of mount you go for depends on what you intend to do and, perhaps more importantly, your budget. It is important to not skimp too much here though.

Alt-azimuth Mount: Alt-azimuth mounts are the simplest design, having two axes: altitude, which lets you move the telescope up and down, and azimuth, which lets you move the telescope from left to right (horizontally). The trouble with alt-azimuth mounts is that in order to track objects in the sky, you need to move the telescope in both the alt and azimuth directions. If you're doing this by hand it can be a bit tricky, especially at high magnifications.

Equatorial Mount: The other type of mount available is the equatorial mount. This type of mount also has two axes, but one of them - the polar axis - is tilted so that it lines up with the tilt of the Earth's axis. This means that it can follow objects as they move across the sky much more easily than an alt-azimuth mount.

Motorized Mounts: Motorized mounts enable your telescope to track objects automatically, and if you go for a GOTO mount, you will be able to go straight to an object and then track it. All you need to do is to enter the name of the object into a handset, and the mount will point the telescope at the object. The advantage of a GOTO mount is that you can find pretty much anything in the sky; the disadvantage is that you miss out on the enjoyment of trying to find a particular planet, star, nebula, galaxy, and so on, which is part of the fun.

More About Eyepieces

The purpose of an eyepiece is to magnify the focused image, and different eyepieces can be used to give different magnifications and fields of view. You will need to experiment with different ones to find the best one for a particular application. For example, if you want to observe a large area of sky, you will need to achieve a low magnification and a wide field of view.

Magnification: To work out the magnification that is achieved by using a particular eyepiece, use the following formula:

telescope focal length / eyepiece focal length

For example, if your telescope has a focal length of 650 mm, such as the Sky-Watcher Explorer 130P, and you use an eyepiece with a 10 mm focal length, the resultant magnification will be:

650 / 10 = 65x

If you use a Barlow lens, see below, this magnification can be doubled to 130x, or trippled to 195x.

Barrel Size: Different barrel sizes are also available, namely either 1.25 inches or 2 inches diameter. Adapters are available for both, enabling you to use a 1.25 inch eyepiece in a telescope designed for 2 inch eyepieces, and visa versa.

Focal Length: This is measured in mm, and the longer it is, the wider the field of view, but the lower the magnification. Short focal lengths are used for high magnifications, while long ones are used to obtain a wide field of view.

Eye Relief: If you wear glasses you can get eyepieces that offer long eye relief. This means that you do not have to place your eye right up against the eyepiece in order to see something. The shorter the focal length, the shorter the eye relief.

Barlow Lens: A Barlow lens is a special type of eyepiece that is used to either double or tripple the magnification. It is inserted in between the telescope barrel and the eyepiece.

How Many Eyepieces Do I Need? In order to cover a range of magnifications from, say, 20x up to 300x, you would typically need four or five eyepieces plus a Barlow lens. It's also best to go for good quality eyepieces. If you buy cheap ones, you may well find that you have to go out and buy new, more expensive ones, later on.