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

First Telescope Specifications

When you're buying a car you want to know how fast it can go, how big it is and whether it's going to be any good for what you want. Your first telescope is no different, so let's look at some of the 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 itself, 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 focal ratio of the scope. This tells you what the scope is good for. A lower focal ratio of 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 is never going to be achievable in real-life use.

Magnification is a product of the telescope's focal length and the focal length of the eyepiece in use. For example, a telescope with a 500mm focal length and a 40mm eyepiece will have a magnification of 500/40 = 12.5x. By swapping in a more powerful eyepiece, say 25mm, this magnification then becomes 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 is, it depends on what you want to look at in the night sky. Each of the different designs of the telescope has its advantages and disadvantages. You have to weigh up each one and decide on the overall best telescope for you. Many astronomers have more than one telescope, but to start with, I believe 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 use 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 in turn focuses the light into the eyepiece. 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. 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 type of 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 that they're ideal for lunar and planetary observing. 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 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. 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 do have to weigh up the benefits of a heavier mount with portability and ease of setting it up. Its weight has got 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 of all 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 units of Right Ascension and Declination, which are 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, and 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 is much easier, especially the modern computer-controlled mounts since they do not require perfect polar alignment. A non-Go-To is much cheaper and will be a gentler introduction to how telescopes work as well as helping you to get to know the night's sky and aligning the scope to the sky.

If you are looking for accurate tracking of celestial objects then 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 just a lot 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 being among the best. Personally, 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

There are a few useful accessories which can help you when starting in astronomy. Although not essential, they did make a big difference to my observations and 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. When the object is centred in the finder, it should be in the main scope field of view.

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 simply 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 a 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 and brighter, useful for clearing up when night vision is not required anymore.

Motorised Auto-Focuser

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

Motor focusers use a small stepper motor to control focus wheels. These, when activated, can turn the focus dial smoothly without causing any vibrations and allow for the target to remain in view. They also allow for very small adjustments to be made, 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 that it automatically focuses but instead is motorised. The package consists of the motor, control unit, brackets and screws. The installation was very easy, taking only 3 minutes and only 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.