Astronomy FAQ

Frequently-asked questions

What is aperture, and why is it important?

The aperture of a telescope is simply the size (diameter) of its main lens or mirror.  Aperture is important because the bigger the diameter the more light that can be collected.  More light means that fainter objects can be seen, and that less faint ones can be seen more clearly.  Bigger is also better because it means you can resolve more fine details (see below).  You get more per inch of aperture from a refractor than a reflector (mainly because part of the light in a reflector is blocked by the secondary mirror);  common wisdom suggests a 3″/80mm refractor, or a 4″/100mm reflector as the smallest worthwhile size to begin with.  Nowadays many  people would suggest 6″ as a realistic minimum for a reflector.

What is focal length, and what difference does it make?

A telescope’s focal length is the distance from the main objective lens (or mirror) where it forms a focused image.  A simple way to think of it is holding a magnifying glass up to a window (not the sun!) and moving a white sheet of paper behind it, until it forms an image. Common focal lengths for astronomical telescopes range from 400mm to 2000mm.  Focal length is important because it determines the magnification of the scope (see below). Generally speaking, long focal lengths are considered good for the planets and the moon, and shorter ones for deep-sky objects.

What is the magnification of my telescope?

The scope itself doesn’t have a magnification, as such.  To get your magnification, just divide the focal length by the focal length of the eyepiece.  For instance, if the focal length is 900mm, and you’re using a 10mm eyepiece, then your magnification is x90.  Using the same telescope with a 20mm eyepiece would give x45 magnification;  a 5mm eyepiece would give x180.

So I can just keep using smaller eyepieces and get higher magnification?

Up to a point, yes.  Small, cheap telescopes are often advertised as having ‘power’ (magnification) in the range of many hundreds of times.  In fact, there is a limit to how much magnification you can usefully obtain from a given size of lens or mirror.  As a rule of thumb, you can usefully expect to magnify up to twice your aperture in milimeters (or 50 times aperture in inches).  Much more than this and you’re not going to see any more detail;  the image will just be bigger and dimmer, because it’s more spread out.

On top of this, the atmosphere itself imposes another limit to useful magnification.  Above around 10″ of aperture you run into this limit.

What about the ‘f’ number, or focal ratio?

The focal ratio is similar to the f-number used in cameras.  It defines the field of view and brightness of the image.  To calculate your focal ratio, divide the focal length by the aperture.  Eg:  a 100mm (aperture) telescope with a focal length of 900mm has a focal ration of 9 (written f/9).  An 80mm telescope with a focal length of 750mm would have a ratio of  (approximately) f/9.4.

A small f-number (f/2.8, f/4, f/5 say) gives a ‘fast’ scope;  larger f-numbers (f/15, f/22) a ‘slow’ scope.  Basically, the faster the instrument, the wider the field of view (ie – you can see more of the sky in the eyepiece).  It also lets in more light per second, so exposure times for astrophotography are lower.  Fast scopes are best suited for deep sky objects.

Slower scopes (larger focal ratios) give a smaller field of view, and tend to show contrast and detail better.  They are thus best suited for the moon and planets.

What’s a Barlow lens?

Put very simply, a Barlow is an additional magnifying lens.  You screw the eyepiece into the Barlow, and the Barlow goes into the focuser of the scope.  If you’re using a telescope at a magnification of x60, adding a 2x Barlow will produce an effective magnification of x120.  However, the image will be dimmer for two reasons – first, because it’s spread out over twice the area, and secondly because some light is lost in every piece of glass in the system.

I just want to buy a telescope – why is it so complicated!

It’s a bit like buying a car.  You might prefer power over practicality;  comfort over style;  2-door coupe over hatchback.   You can buy a cheap second-hand Fiat, or a Rolls Royce.  Some cars have GPS SatNav;  some need you to rely on the road atlas. All will get you from A to B, but each has its pros and cons.

If you have a clear idea of what you want to observe then the choice becomes easier. If you’re really into planets and the moon, go for a long focal length and high f-number.  If you’re planning on doing photography, a solid mount with motor drive is pretty much essential.  You may be able to afford a real monster aperture, but not have space to locate it.

There is no perfect scope for all purposes.  However, for around £300 you can get a decent 3″ – 6″ telescope which will be a good all-round performer.  If you’re unsure make sure you run it past one of the many helpful forums on the web – the Sky at Night forum is particularly welcoming to beginners.

One final point – and I learned this myself the hard way – there’s an old saying that “the best telescope is the one that gets used.”  Make sure you are prepared (and able) to carry the scope in and out to use it.  Also make sure you see it in the flesh before ordering – many scopes are much bigger and heavier than they look on the web site!

Still not sure?

If you need more detailed guidance on choosing a telescope, have a look atStephen Tonkin’s guide.  If you think you’ve made your mind up, and you’re in the shop with sweaty palms and a credit card, read his article on evaluating a scope in the store.