Buying your first telescope

Buying a telescope for the first time can be daunting. We reveal what you should look for as you take your first steps into the world of amateur astronomy.

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If you need further help deciding which scope to buy, have a look at our regularly updated equipment reviews section.


What type of telescope should I buy?

A good all-round beginner’s telescope is a Newtonian reflector (below). It’s a simple design and is relatively cheap for the size of mirror you’ll get for your money – ideal if you’re just starting out.

A Newtonian reflector with a 6-inch (150mm) mirror will give you good views of the brighter galaxies and nebulae, and should also perform well when you train it on the Moon and planets.

But what about the other types of telescope? Well, refractors are perhaps the most recognisable of all designs. They use a series of lenses to bring what you’re looking at to focus at the eyepiece end of the tube, and are great for observing the Moon or rich star clusters.

There’s also the Dobsonian, which is a type of reflector that has a simple mount and because of that, often has a much larger mirror for its price. As this collects more light it will let you see dimmer objets, so it’s a great choice if you want to see faint galaxies and nebulae.

Finally, there are the catadioptric telescopes that use a combination of corrector lenses and mirrors. Their compact size makes them relatively portable and their high focal ratios mean that they’re ideal for lunar and planetary observing.

 What do all the numbers on a telescope mean?

When you’re buying a car you want to know how fast it can go, how big the boot is and how many miles per gallon you’ll get. A telescope is no different, so let’s look at some of the specifications you’ll need to know.

The first important spec is the size of the telescope’s front lens or main mirror – its ‘aperture’, which is measured in millimetres today, but is often given in the more traditional inches. The bigger a telescope’s mirror or lens, the more light can be captured and the brighter distant celestial objects will appear.

The quality of the lenses is also very important. Before you buy, ask to have a look through it in daylight at a distant object. When it’s in focus, a telescope with quality lenses should show clear crisp edges to objects in its field of view.

You’ll also encounter the ‘f number’, which is the focal ratio of the scope – the focal length divided by the aperture. The higher the focal ratio, the narrower the field of view that you see in the eyepiece. So if you want to study small features on the Moon, then a scope with a high focal ratio is for you.

You’ll sometimes hear people referring to a telescope’s focal ratio as its speed. Scopes with low focal ratios, like f/5, are said to be fast, while high focal ratios like f/10 are slow.

Finally, don’t be fooled by claims of massive magnification – that’s not the measure of a good telescope. Even poor telescopes can magnify things many times.

What is the mount for?

The mount is the most important part of the telescope. A telescope with superb optics will always be let down by a poor-quality mount: if you can’t keep the optics stable, your view of the night sky will be completely ruined, so make sure that the mount is sturdy. The heavier the mount is the better it will be, because it will be a solid platform for the telescope to rest on.

However, its weight has obviously got to be offset by how portable you want your telescope to be. It shouldn’t have any flimsy plastic parts and it should never flex or wobble noticeably.

There are two main types of mount that you’ll come across: the altaz and the equatorial. Altaz stands for altitude and azimuth, and is the simplest of all mounts. The telescope moves on a base parallel to the ground, up and down (in altitude) and left and right (in azimuth).

The equatorial mount is different and more complex to use. One of its axes is tilted to your latitude, your position on the Earth north or south of the equator, and the other is parallel to the celestial equator, like Earth’s equator but in the sky.

It moves in units of right ascension and declination, which are similar to longitude and latitude but mapped onto an imaginary sphere on the sky.

Most Newtonians come on an equatorial mount, and they’re really good for astronomy because you only need to adjust one axis, the right ascension one, to keep track of an object in the sky as it moves through the night. With an altaz mount (right), you’d need to adjust both axes to keep one object in view for any longer than a few seconds.

What’s the little telescope on top of the big one?

That’s the finderscope. It has a wider field of view than the main telescope, with crosshairs over it, so you can find things and aim the main scope much more easily.

Before observing you’ll need to align the main telescope and the finderscope. In the daytime, point the main telescope at a distant object like a telegraph pole. Be careful not to look at the Sun or you could damage your eyes.

Centre the top of the telegraph pole in the main eyepiece and then adjust the little screws around the finderscope, one by one, to centre the top of the pole in the crosshairs. Once done, you’ll be able to point at something with the finderscope and it will appear in the main eyepiece.

How much money should I spend?

A good Newtonian on a sturdy equatorial mount costs around £200 in the UK, while a good refractor costs around £300. A 6-inch Dobsonian can cost as little as £155. What’s most important is that you buy from a well-respected astronomical dealer, like the advertisers in this magazine.

Avoid the cheap, poor-quality models you sometimes see in mail order catalogues or in high street stores.

I’ve unpacked the scope – how do I set it up?

For detailed instructions on your own make of telescope, it’s best to have a good read of the manual, but here are a few tips.

Set up the tripod and mount arrangement first, and make sure any leg screws are screwed in and secure enough to take the weight of the telescope.

If you have an equatorial mount make sure that the ‘north’ leg is indeed pointing north. Then put the telescope tube on the mount.

Check that the tube is secured firmly with the tube ring screws and that it can’t slip out of them. If you’re setting up a Newtonian, position the telescope tube in the rings so that you can easily look into the eyepiece –it’s best to have it parallel to the ground or slightly above parallel.

And remember, it’ll help if you balance the telescope and counterweights before observing.

What is the eyepiece for?

Most telescopes will have one or two eyepieces included in the price. They come in two standard barrel sizes to fit into the telescope focuser – 2-inch or 1.25-inch – and have several optical designs such as Plössl, Nagler or orthoscopic.

The number on the eyepiece is its focal length, measured in millimetres. The smaller the number, the greater the magnification it will give when used.

Two good quality eyepieces with a small (10mm) and a larger (20-40mm) focal length will provide you with a decent range of magnifications to get you started.

However, if you had to choose just one eyepiece, get a 20mm Plössl. Only use or buy eyepieces that have glass lenses, because plastic lenses will almost always disappoint.

Should I get a computerised Go-to scope?

Go-To telescopes come with an in-built computer and handset that, once set up correctly, automatically aim the telescope and track an object. Although this is helpful, we’d recommend that you get a scope without Go-To as your first-time buy.

If you aren’t familiar with using a telescope, setting up a scope without Go-To is much easier. A non-Go-To is also much cheaper and will be a gentler introduction to how telescopes work.

How do I align my equatorial mount?

Find out below, or read our How to guide here.

First, check that the north leg of the tripod is pointing roughly north, or simply make sure that the polar axis itself is pointing north. You then tilt the polar axis (the one that moves in right ascension) to the angle of your latitude – your position in degrees north or south of the equator. There will be a graduated scale on the side of the mount head to help you do this, as shown in the image on the left.

The polar axis should now be pointing roughly towards Polaris, which is in Ursa Minor. Check this by looking along the axis, or through the finderscope. For simply observing the night sky this setup is fine and you can now rotate the right ascension (polar) axis with the motion of the stars.

More advanced equatorial mounts include a polar scope, fitted inside the polar axis, that allows for a far more accurate alignment.

To use it, set up your telescope as above and when you look into the polar scope you’ll see a diagram on a reticule inside the eyepiece and the star Polaris. You should now align the diagram with the polar axis itself.

Point the crosshairs in the reticule so they are on Polaris. Now unlock the polar axis and rotate it. If Polaris doesn’t move, your reticule and polar axis are already aligned and you don’t need to adjust it. If it does shift, adjust the alignment of your polar scope until it doesn’t move when you rotate the axis.

The final step is to offset Polaris using the reticule so that the polar scope is pointing at the north celestial pole, which is a very short way from Polaris. To do this, tweak the altitude and azimuth knobs on the mount until Polaris is in the correct position in the polar scope’s reticule. See your manual for exact details.

 


What is collimation and how do I do it?

Find out below or read our How to guide here.

Collimation is when you accurately line up all the optics in your scope. This alignment can be upset if the scope gets knocked, or after it’s been moved around a lot. You’ll get the best views out of your telescope if it’s well collimated, so it’s good to occasionally do it.

If you followed our advice and bought a Newtonian reflector, then to collimate it you need to carry out the steps below.

But first things first, check whether you need to do it in the first place by looking at a star out of focus. If you can see a circle with some practically perfect concentric and symmetrical rings around it, your telescope’s collimation is good and you don’t need to do anything.

If they’re not concentric or symmetrical, then you can collimate your scope indoors during the day using a homemade collimation tool following the steps below. Just point the telescope at the ceiling to get a bright, uniform view.

Collimate with a homemade tool

1. Get an old plastic film canister and pierce the base in the centre with a 1-2mm drill bit. Put a paper-reinforcing ring on the exact centre of the telescope’s main mirror.

2. Take out the eyepiece and replace it with the collimation tool. Adjust the set of screws on the secondary mirror that aren’t on the back of it so that the paper ring is in the centre of the view, seen through the tool.

3. Make sure that the secondary mirror appears circular, not slightly elliptical, when seen through the collimating tool by adjusting the same screws on the secondary mirror.

4. Centre the main mirror by adjusting the screws on the back of the mirror holder. Look directly through the collimation tool. The view should look like the third image above, with the hole in the collimation tool in the middle of the paper-reinforcing ring. Your scope is now collimated.

How do I focus the telescope?

With a reflector, you can use the tiny spikes that you’ll see coming out of the stars to help with focusing: when the view is defocused the spikes will be doubled up. Adjust the focus knob until you only see one set of spikes to bring the scope to focus. These spikes come from the ‘spider’ brackets that hold a little secondary mirror in the tube.

Refractors don’t have a secondary mirror so when you look at stars they don’t have spikes coming out of them – they appear as round points. You can tape a cross of string in front of your refractor’s main lens to create these spikes.

Another focusing aid, for refractors and reflectors, is a Scheiner mask. This is a mask that you can make from card that fits over the front of the scope. The mask has three circular holes cut into it, so that when you look through the eyepiece you see three of every star if the scope isn’t focused. Focusing the scope will bring the views together and when it’s focused you’ll see each star as one. Don’t forget to take the mask off afterwards!


If you need help deciding which scope to buy, have a look at our regularly updated equipment reviews section.


 

 
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