The mysterious language of telescopes

When you get a new telescope there’s lots of numbers and things thrown around like focal lengths, focal ratios etc. This articles looks at what those things mean.

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When I first got a telescope I all of sudden felt like I’d been thrown into a world with a completely different language that I couldn’t understand. People were going on about focal lengths, focal ratios and exit pupils and piles of other stuff that I didn’t know anything about. It didn’t take long to pick it up and learn the lingo but for someone just starting out, it can be a little daunting when you first get your telescope and you wonder what all of the numbers mean. So in this article I’ll try and demystify some of the jargon which some people might find helpful.


We’ll start with aperture.

This is how wide your telescope is, the diameter of the optics. So for reflectors, this is how big the mirror is and is a good indicator of how much light your telescope can collect, same deal for other types of telescopes – the size of the optics. Basically the bigger the aperture the more photons it’ll collect. But that’s not the complete picture because there’s a whole lot more to it than the size of the mirror and other optics. The picture below is the mirror in my 10″ (250mm) reflector, a long way down the tube.

So the next important number is how long is the focal length. This is the distance that it takes for the reflected (or bent) light to reach a focal point. In the case of a reflector this is determined by the shape of the mirror. A deeper figuring will make the light come to a focus closer to the mirror and shallow figuring will have the light come to a focus further away. Don’t forget for a reflector the light still has to be bounced off a secondary mirror. Telescope manufacturers are a helpful bunch so will often print the details of the telescope on the instrument like this:

So you can see from the above picture that the diameter of the mirror is 406mm (16″) and the focal length is 1829mm (that’s nearly 2 metres, which is why this particular telescope is so tall). The other number is the focal ratio which is found by dividing the focal length by the aperture. This is good to know for further calculation, stay tuned! So for his telescope that ratio comes out at 4.5, called f4.5. This is quite a low number and the number means that this telescope is known as a fast telescope – which is strange because nothing happens fast in a telescope this size, especially if you try to move it. This telescope is a compromise between power and usability so that the focal length could have been longer but then the telescope would have been ridiculously tall and quite unusable without a ladder. The trade off comes with a more deeply figured mirror, but that can introduce optical problems, especially around the edges. The aberration is also known as coma and can make the image distorted around the edges.

Oh, and how many times magnification is it?

Don’t you love this question?

Now that you know these magic numbers, you can actually answer the next most common question that astronomers get, which is, “what’s the magnification?” You can spend ages trying to explain, it’s not as simple as saying what the magnification is, as there are a lot of factors involved in seeing stuff in the night sky. Or you can just tell them the magnification and send them on their way.

The formula is:

Magnification = focal length / focal length of the eyepiece

Eyepiece manufacturers are helpful people, just like telescope manufacturers and they print the focal length of the eyepiece somewhere on the eyepiece. The picture below is an example of how handy they can be:

At Milky-Way.kiwi, we really like the 2″ eyepiece in the 203mm (8″) reflector. So if you want to know the magnification that this eyepiece achieves you just put the numbers in the formula:

Magnification = 1200mm / 26mm

and presto! The answer is 46 X magnification for that particular eyepiece. But as I mentioned earlier, it’s not that simple. You can’t just go putting in more and more smaller eyepieces and expect an awesome view. For example, we know that the 8″ reflector gives really good views with the 20mm eyepiece (60 X magnification) and the 9mm eyepiece (133 X magnification). With the 6mm eyepiece (200 x magnification) it’s good for trying to split double stars like Alpha Centauri but the 4mm (300 X magnification) is really rubbish. That’s because it’s pushing the telescope beyond what the optics can do – and actually beyond what the atmosphere will let it do anyway.

Field of view

The next useful thing to know about with your telescope is the field of view for different eyepieces. With a bit of research on the internet you should be able to find the apparent field of view (APOV) measure that most manufacturers give. For the Meade 26mm eyepiece pictured above it has an APOV of 50 degrees. To figure out what the field of view will be visible in that eyepiece for a given telescope, we use this formula:

Field of view = APOV / magnification of that eyepiece

With the Meade 26mm at 50 degrees and the magnification this has in our 8″, which we worked out before, of 46, then:

Field of view = 50 / 46

So our field of view in the 8″ wth that eyepiece is 1.09 degrees, which is quite wide.

Exit pupil

This is the size of the cylinder of light that is beaming into your eye from the eyepiece. As we age so does the stretchyness of our pupils so that we can’t make use of all the light coming out of an optical system that delivers a big exit pupil. The formula for working this out is:

Exit pupil = Focal length of eyepiece / focal ratio of telescope

So for the big huge 16″ telescope the exit pupil with the 26mm eyepiece will be:

Exit pupil = 26mm / 4.5

This gives an exit pupil of 5.8mm, which is pretty good for my eyes. I wouldn’t want it too much bigger than that. This really only becomes a problem with the fast telescopes and bigger eyepieces. For example a 32mm eyepiece in the f4.5 16″ is not going to be too good for my eyes as a bit of light simply won’t get to the back of my eye, where I can appreciate it.

That’s enough formulae for now – get out there and start calculating!