The language in which we talk about telescopes can be confusing at the best of times.
Consider the number of ways we can describe a telescope’s design – refractor, reflector, compound, catadioptric, Cassegrain, Dobsonian and astrograph, to name a few.
Some telescopes can be considered to be more than one of them. Two of those terms even mean the same thing.
What they all share, however, is a handful of core statistics: four measurements that will give you an idea of how any telescope will perform, regardless of its inner optical arrangement, all other things being equal.
These are aperture size, focal length, focal ratio and useful magnification.
Knowing what these numbers mean isn’t vital to using a telescope, any more than understanding how your brain and eyes work together is a prerequisite to being able to enjoy the gift of sight, but in this case it’s worth the effort.
Once you know what these figures mean, you’ll be in a good position to work out what you can and can’t do with your scope, and what budget you might need to realise your longer-term goals.
Aperture
This figure is the most straightforward – the aperture is the diameter of a telescope’s main lens or mirror, measured in millimetres and commonly converted into inches.
This number describes how much ‘light grasp’ a telescope has, by which we mean how many photons it can collect.
A bigger aperture delivers brighter views, which leads to better contrast and more detail.
This is why aperture is often described as being the most important feature of a telescope; the more light you can gather, the fainter the celestial bodies you’ll be able to see.
The amount of light a telescope can gather is directly proportional to the area of its aperture.
The gains are rapid: based on area, a 6-inch aperture instrument will gather four times as much light as a 3-inch one, for instance.
Focal length
Before you can see your chosen target, the rays of light passing through the aperture have to be focused together, and the point where they converge is known as the focal point.
The distance that the light has to travel between the aperture and the focal point forms our second core measurement, the focal length. This is recorded in millimetres.
There is no fixed relationship between an instrument’s aperture and its focal length; it all depends how the lenses and mirrors within the tube are arranged.
Focal length is useful for two reasons: it’s the major determinant of useful magnification (which we’ll get onto in a moment) and it gives you a rough idea of what sort of field of view you can expect.
Smaller focal lengths deliver wider fields, so lean towards being better suited to observing larger swathes of the night sky and for star hopping, while longer focal lengths offer narrower fields – perfect for planetary disc close-ups – and tend to allow you to use eyepieces with longer eye relief; that’s the ideal distance your eye should be from the lens of an eyepiece, and it’s a particularly important consideration if you wear glasses.
Focal ratio
Our third core number is the focal ratio, also known as the f/number, which describes the relationship between the focal length and the aperture.
You can work it out by dividing the focal length by the aperture; both of these figures should be in millimetres.
Let’s say you have a 130mm aperture instrument with a focal length of 900mm – its focal ratio will be ‘f/6.92’.
Like focal length, focal ratio can tell you a lot about a telescope: larger f/numbers imply higher magnification with a given eyepiece and a narrower field of view, smaller f/numbers the opposite.
Additionally, an f/number can be described as ‘fast’ or ‘slow’, and this reveals how a telescope will perform when used for astrophotography.
The terms fast and slow are a throwback to the days of chemical camera film processing.
Scopes with fast ratios (typically f/5 or below) can capture images more quickly than their slow ratio (encompassing f/9 and above) counterparts, but the trade-off comes in terms of depth of focus; slow scopes are much more forgiving in this regard.
Useful magnification
Like the telescope itself, every eyepiece has a focal length, and it’s the relationship between these two focal lengths that gives you the magnification (or ‘power’) of your setup.
The calculation is simple: divide the focal length of the scope by that of the eyepiece.
So, if you have a scope with a 1,200mm focal length and a 20mm eyepiece, your magnification would be 60x.
The smaller the focal length of the eyepiece, the greater the resulting magnification on any given telescope. Aperture is entirely irrelevant in this case.
It’s worth being able to work out how much magnification you’re using because, unlike aperture, more isn’t necessarily better.
The theoretical useful limit is two times the aperture in millimetres; so for a 150mm aperture, that’s 300x magnification.
Push it beyond the useful magnification and you’ll get a closer view of your chosen target, but that view will be a fuzzy one, not to mention dimmer.