Tips for Buying Your First Telescope - What Type? How Big?

Buying your first telescope can be a daunting task. We show you what you should look for and how to quickly get up and running.

Many factors are involved in selecting a telescope for home use, the most important of which is "What do you want to look at?". This question affects the type of telescope design; for example, planetary observation is suited more to a slow (large f/ number) refractor, as this will give a large depth of field. This makes focusing easier and increases the ability to resolve surface detail. The trade-off is limited light-gathering ability since refractors generally have a much smaller aperture due to the cost of producing large lens elements. This limits observation of deep space objects (nebulas, galaxies, etc.), which require large apertures to capture the faint light.

For observation of deep space objects, the aperture is of greater importance for light gathering and for cost/performance, a Newtonian reflector is more suitable as mirrors are relatively inexpensive. The downside is that they are often bulky instruments and suffer from low f/ numbers, making planetary observation difficult. A compromise would be a compound design, such as a Maksutov Cassegrain, which maintains large apertures with a short tube and uses secondary mirrors to double the focal length and increase the focal ratio. However, these telescope designs tend to be more expensive. Newtonians are comparatively inexpensive and make good beginner telescopes.

First Telescope Specifications

When buying a car, you want to know how fast it can go, how big it is and whether it will be any good for what you want. Your first telescope is no different, so let's look at some specifications you'll need to know.

Focal Length

The focal length of a telescope or lens is the length over which light directed by the curvature of the optic lens or mirror comes into focus. At this point, called the focal plane, you can see a real image of a distant object. In traditional telescopes, the focal length is usually around the same length as the telescope. However, modern optics can "fold" the light path with mirrors to reduce this distance, resulting in a more compact telescope.

Focal length is measured in mm and is the same as camera lenses. The larger the focal length, the greater the overall magnification will be.

Mirror or Lens Size (Aperture)

The bigger the mirror or lens, the more light can be captured and the brighter a distant celestial object will appear. You'll also encounter the "f" number, which is the scope's focal ratio. This tells you what the scope is good for. A lower focal ratio around f/5 is great for observing large, faint objects like galaxies, while higher focal ratios around f/11 are good for looking at brighter things like the planets and the Moon.

Magnification

Don't be fooled by claims of massive magnification. That's not the measure of a good telescope. Even poor telescopes can magnify many times. What's important is the quality of the lenses and the size of the telescope's lens or main mirror. In many cases, the magnification quoted is an absolute maximum under optimal conditions, which will never be achievable in real-life use.

Magnification is a product of the telescope's focal length and the eyepiece's focal length in use. For example, a telescope with a 500mm focal length and a 40mm eyepiece will magnify 500/40 = 12.5x. Swapping in a more powerful eyepiece, say 25mm, results in a magnification that is 20x.

Resolving Power

The resolution of a telescope is a measure of its ability to differentiate the details of an object or to distinguish two very closely spaced objects from each other, such as binary stars or fine detail on the Moon or a planet. The resolving power is calculated as 116 divided by the telescope's objective diameter. A 200mm scope can resolve details as close as 0.58 arcseconds, twice as well as a 100 mm scope, all other things being equal.

What type of telescope should I buy as a First Telescope?

The simple answer depends on what you want to see in the night sky. Each of the different designs of the telescope has its advantages and disadvantages. You have to weigh each one and decide on the best telescope. Many astronomers have more than one telescope, but to start with, it's best to invest in one good-quality telescope rather than two lesser-quality telescopes.

Newtonian Telescopes

A good all round first telescope is the Newtonian reflector. These reflecting telescopes were invented by Sir Isaac Newton and used a specially curved main mirror to collect celestial light. The light gathered by the large primary mirror is focussed and aimed at a smaller secondary mirror, which focuses the light into the eyepiece. It's a simple design and relatively cheap for the mirror size you'll get for your money - ideal if you're starting. 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 looking at the Moon and planets.

Refractor Telescopes

A refractor is the oldest and perhaps the most recognisable of all designs. The refractor telescope was the type used by Galileo to record the phases of Venus, among other things. Refractors have a curved lens at the front, which focuses the light down the tube directly into the focuser. A star diagonal is often added between the focuser and eyepiece to make viewing more comfortable. Refractors are great for observing the Moon, planets or rich star clusters.

Dobsonian Telescope

The Dobsonian telescope is another reflecting telescope, but mounted on a simple alt-azimuth mount (we'll see mounts later) popularised by amateur astronomer John Dobson in the 1960s. If you want to chase faint galaxies and nebulae, this type of telescope has a much larger mirror for its price compared to any other design because it has a simple mount. However, you won't be able to track objects across the sky without a much more expensive mount.

Catadioptric Telescopes

Finally, there are the catadioptric telescopes, or compound telescopes, which use a combination of corrector lenses and mirrors. Their compact size makes them relatively portable, and their large focal ratios mean they're ideal for lunar and planetary observation. Popular designs include the Schmidt-Cassegrain and Maksutov-Cassegrain.

What is the telescope mount?

The mount is the most important part of the telescope. A poor-quality mount will always let down a telescope with superb optics. If you can't keep the optics stable, your view of the night sky will be completely ruined, so make sure the mount is sturdy. For astrophotography, the heavier the mount, the better. This is because it will be a solid platform for the telescope, and the heavier, the better. You have to weigh the benefits of a heavier mount with portability and ease of setting it up. Its weight has to be offset by how portable you want your telescope to be. It shouldn't have any flimsy plastic parts, and under no circumstances should it flex or wobble noticeably when setting up.

There are two main types of mount that you'll come across: the alt-azimuth (alt-az) and the equatorial (eq). Alt-azimuth and Alt-az stand for altitude and azimuth and are the simplest mounts. The telescope moves on a base parallel to the ground up and down (in altitude) and left and right (in azimuth). This motion is the same as most camera tripods.

The equatorial mount (typically the German equatorial mount, or GEM) is different; one of its axes is tilted to your latitude, and the other is parallel to the celestial equator. It moves in Right Ascension and Declination units, similar to longitude and latitude and is mapped onto an imaginary sphere in the sky. Most Newtonians come on a German equatorial mount.

Some more expensive equatorial mounts feature motors and gears which, when correctly aligned, will track an object across the sky. This means you do not have to keep adjusting it to keep an object in the field of view, which is essential for astrophotography.

Go-To telescopes come with an inbuilt computer and handset that, provided they are set up correctly, automatically aims the telescope and tracks a specific object. If you aren't familiar with using a telescope, setting up a scope without Go-To, especially the modern computer-controlled mounts, is much easier since they do not require perfect polar alignment. A non-Go-To is much cheaper and will be a gentler introduction to telescope work. It will also help you get to know the night sky and align the scope to the sky.

Suppose you are looking to track celestial objects accurately. In that case, an equatorial mount makes life a lot easier since when it is properly aligned, the telescope moves on the axis of the earth. That's not to say that alt-az mounts can't track objects; it is much easier and more accurate.

From personal experience, my Go-To mount has proven invaluable in light-polluted areas where deep sky objects are nearly impossible to see. Long exposures with a camera can pick them up. The Go-To mount makes finding these objects much easier, often within 30 seconds, as opposed to an hour of star hopping and trial and error with the camera.

How much should I spend on my first telescope?

A good Newtonian on a sturdy 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 reputable astronomical dealer or a major retailer, and above all, avoid the cheap, poor-quality models you sometimes see in mail-order catalogues or high street stores.

The best brands to buy include SkyWatcher and Celestron at the lower end of the market; Orion and Meade are towards the higher end, with Takahashi generally regarded as among the best. I have two SkyWatcher telescopes and have been very pleased with them. I've used Celestron, and they are very similar to SkyWatcher.

Useful Accessories

A few useful accessories can help you when starting in astronomy. Although not essential, they did make a big difference to my observations, and I wish I bought them earlier.

Finderscope

Telescopes typically come with a smaller telescope called a finder, which is a low-power lens or even just plain glass, which, when aligned, lets you aim the telescope easily by looking through the finder. The object should be in the main scope field of view when centred in the finder.

Night Vision Torch To Help Preserve Night Vision

In low light and darkness, our eye's pupil expands to a larger aperture to allow as much light as possible to enter the eye. The dark eye adaption process can take up to 20 minutes for your eye to become accustomed to low light levels. While this is good for viewing the sky as many more stars will be visible, it does not allow you to read any text or star maps. A conventional torch will ruin your "night vision" in a fraction of a second as the intensity of the light will cause the pupil to contract back to normal. It will take another 20 minutes to become accustomed to the low light again.

This is a simple tool that makes a big difference at night. This is nothing more than an LED torch, but it has dual functionality as it emits red or white light. Red light at night will preserve night vision because it offers enough light to see by, but not at an intensity to cause your pupil to contract. This is also why red light is used in dark rooms, as the intensity is not great enough to spoil the film. Many star maps are specially designed to be viewed under red light. The white light function will illuminate a further distance brighter, which is useful for clearing up when night vision is not required anymore.

Motorised Auto-Focuser

At high magnifications, the vibrations caused by touching the focuser make the image un-viewable, and the target rapidly moves out of the field of vision. Focusing is nearly impossible.

Motor focusers use a small stepper motor to control focus wheels. When activated, these can turn the focus dial smoothly without causing vibrations and allow the target to remain in view. They also allow for small adjustments, which cannot be done with an unmodified focuser.

I have purchased and installed the SkyWatcher Auto focuser. Despite being called "Auto", this does not mean it automatically focuses but is motorised. The package consists of the motor, control unit, brackets and screws. The installation was very easy, taking only 3 minutes and requiring a screwdriver. The control box, although large, functions perfectly with two buttons for in and out focus and variable speed control for making fine adjustments. Even at high magnification (100X), there is no visible vibration caused by focusing.