How Telescopes Work (and why your eyes don’t)

The Basics of How Telescopes Work

For me, who is rediscovering backyard astronomy after 20 years away from it, understanding how telescopes work has been really useful for deciding what I want from a new one.

In this article, I'm going to share what I've learned so if you don't have one yet, or are thinking of an upgrade, you'll be able to consider which type of telescope might be the best for you, and why.

What Telescopes do that our Eyes Can't

Compared to a telescope, the eyes in our head are puny!

Light comes in through our pupils, which are around 5mm - 7mm in diameter, so their light gathering power is poor. Our pupils dilate in the dark to gather in more light, but that is in no way enough for astronomy...

Which is the number one reason we need a telescope to get the best out of the night sky: because they have much larger diameters than our pupils, telescopes gather so much more light from the dark sky than our eyes. (Which is also we use DSLR cameras to photograph the night sky).

Objects out there in space, from planets to galaxies, are so far away that they only produce a tiny amount of light down here on Earth.

That faint light is such a small proportion of everything our eyes take in that most objects are invisible to us, and those we can see - even the bright and nearby ones like Mars - have no detail that we can make out.

Magnification is Not the Most Important Job of a Scope

If you've ever bought or shopped for a telescope, particularly a cheap or low-end one (like my first one) you'll have seen that the advertising often shouts about magnification.

This is leading unsuspecting shoppers down the wrong path:

When buying a telescope, magnification is irrelevant!

Telescopes are effective because they have a huge 'pupil' (known as an objective) and bigger ​objectives pull in more light, which in turn means more detail.

The amount of light an objective collects grows based on a known formula which says if you double the size of the objective, you 4x the amount of light the objective collects.

This means a small increase in objective size gives a big increase in light gathering power. For example, an 8" objective pulls in about 77% more light than a 6" lens. 

If you take away nothing else from this article, remember this: first decide which type of scope is for you (see below) and then get the biggest objective diameter you can afford.

Everything else in a telescope is secondary to its light-gathering power, especially magnification.

Magnification only comes into play once the light is gathered.

Eyepieces magnify the image collected by the primary to blow details up to a scale our eyes and brain to see, and since more light = more detail = higher magnification, you'll need a bigger aperture objective if you want to use higher magnification.

Different Scopes for Different Folks

Hubble space telescope

Hubble Space Telescope - works the same way as yours and mine (credit below)

All telescopes, from my tiny one to the Hubble Space telescope, work on the same, 3-step basis:

  1. Collect a lot of light with an objective (lens or mirror - see below for details)
  2. Focus that light to a small, sharp image,  
  3. Magnify the image using an eyepiece for our eyes to see details

The two main types of telescope are reflectors and refractors, they both follow the same three steps shown above to deliver their night sky images, but they use fundamentally different processes to achieve it.

Reflectors use mirrors as their light-gathering objective and refractors use a glass lens to collect and focus the light.​

There is a third type, which is a combination of the first two (it uses lens and mirrors to collect and focus light) and is know as a catadioptric telescope.

Over the rest of this article, we'll look at how the reflector telescope works (next) and then, further down, I'll examine how the refractor telescope works followed finally by how a Schmidt-Cassegrain telescope works, which is the classic example of a catadioptric scope.

How Refracting Telescopes Work

A refractor is the classic telescope design that everyone will recognise. It simply consists of a long tube with an objective lens at one end and an eyepiece at the other.

The diagram below is a bit busy (click on it for a bigger version) but you can see the light rays (orange) coming in through the objective lens on the left and being focussed down to a point on the focal plane at the right.

How a refracting telescope works

The detail behind how a refracting telescope works (credit below)

A telescope eyepiece is then used to magnify the image on the  'focal plane' (find out more on magnification here) and the magnified image is delivered to your eye.​

The distance light travels from the objective the focal plane, i.e. the point where the image is brought to a focus for the eyepiece to magnify, is known as the focal length.

The longer the focal length, the larger the image is created - but bigger images need more light and so longer focal planes generally need larger apertures.

But the reverse is also true: bigger lenses need a longer focal length to bring the light they gather to a focal point.

The biggest refracting lens in the world is at Yerkes Observatory, Wisconsin. The 40" (102cm) diameter beast sits inside a telescope which is 60 feet (18.3m) long!

Yerkes observatory refractor

At 40" the Yerkes Observatory Refractor is the biggest in the world (credit below)

A telescope which uses a lens as its objective is called a refractor that lens bends - or refracts - the incoming light rays to a focus.

The limiting factor with refractors tends to be the size of the objective lens. Bigger lenses quickly become more expensive to make as the glass needs to be high quality with no imperfections on its surface or interior and perfectly shaped to focus the light it gathers.

Unfortunately, large refractors quickly become prohibitively expensive for the backyard astronomer.

Since it's the size of the lens which dictates light gathering power​, this also means that if you want to become serious about deep sky astronomy, e.g. galaxies and nebulae, then you are almost certainly better off getting a reflector which will be significantly cheaper at larger apertures.

You might well now be asking 'why would anyone buy a refractor?'.​

How refractor telescopes work means they are fantastic for resolving detail, think binary stars or the craters on the moon, and so give great views through small (and cheap) lenses - which is why one came out top of our 'best budget scopes' review.

They are also a great choice for astrophotography, where the aperture size matters much less because the camera gathers more light by leaving the lens open for extended periods.​.. but be prepared to pay a lot for a high quality lens.

How Reflecting Telescopes Work

Reflectors are fundamentally different beasts to refractors.

How reflector telescopes work, as their name suggests, is by using a mirror to gather light (instead of a lens) which reflects the collected image to your eyepiece.​

In the diagram below, you can see the way light travels inside a classic Newtonian style reflector:​

How reflector telescopes work

How reflector telescopes work - this is a classic newtonian style (credit below)

Light enters the scope from the left (green and purple lines) and hits the primary mirror at the end of the tube, which is shown in blue.

The primary mirror is curved to reflect the all the light back to a smaller, secondary mirror, which is the blue rectangle at a 45° angle.

Because the objective mirror sits at the back end of the tube, unlike a refractor's lens which sits at the front, it is not possible to have the eyepiece mounted on the back end of the telescope.

So, the secondary mirror's role is to divert the image to an eyepiece at the side of the telescope for viewing instead.

​It's known as a Newtonian after its inventor: Sir Isaac Newton.

Newton's second reflector telescope

A replica of Newton's second reflector telescope (credit below)

He created it in the 1680s to combat the poor image quality he was getting from the lenses in refractor telescopes.

There was no such thing as achromat lenses back then, so chromatic aberration - where the lens separates out different colours of light - was a real issue. Image quality was terrible compared to what even the cheapest refractors provide today.

​What Newton used to his advantage was the knowledge that mirrors reflect all the colours of light at the same angle, so images are not blurred by chromatic aberration.

With the design he created, Sir Isaac Newton's reflector significantly improved images of the night sky.

Is not all Roses in the Newtonian Garden

Up to now, it may feel like the reflector has all the answers that a refractor can't provide.

Whilst there are many benefits of reflectors, they do come with  couple of their own challenges that you should be aware of.

The first is collimation.

Most reflectors are not sealed units because the aperture pointing towards the sky is open to the elements to let in as much light as possible for the primary mirror.

What can happen over time is the primary mirror comes out of alignment with the secondary mirror, meaning images become blurred.

Collimation is the process of re-aligning the primary mirror with the secondary and is not straight forward. It can be made easier using a laser collimator like this one​ or you can use more manual methods, as set out here.

The second challenge is coma.

This is optical aberration parti​cular to reflectors. Away from the centre of the primary mirror, the curvature used to focus the image becomes greater and this causes points of light, like stars, to appear with a comet-like tail.

They look, simply, like a coma... which is where the name comes from.​ There is not a lot to be done about this, but the Schmidt-Cassegrain design (see below) goes someway towards eradicating it.

The classic Newtonian comes in three different varieties:

1) Rich Field Reflector Telescopes

Rich field (or wide field) reflectors have a short focal length and so operate more effectively at lower magnifications.

Rich field reflector telescope

Example of a rich field reflector telescope

What they are really effective at is giving you and image of a bigger area of sky than a most other types of scope.

This means they are great for wide area nebulae and capturing large globular clusters or displays of beauty such as the pleiades.​

Click here if you'd like to see an example of one, they are low price but don't get a great rating from users.​

2) Dobsonian Reflector Telescopes

The huge benefit of a Dobsonian telescope is that you can have an incredibly large aperture, even as big as 15", for a relatively low price.

This 8" example on Amazon was less than $400 at the time of writing, which is a stunning price for that much light gathering power.

It is a very simply designed scope: a long tube, altazimuth (point and shoot) mount and one big mirror... which is why they are known as 'light buckets'.​

10inch Dobsonian telescope

Example of a 10" Dobsonian telescope (credit below)

The downside of this scope (as with any open reflector) is you will need to collimate the mirrors to ensure the best quality images, which was discussed above, and the altazimuth mount means it can't be used for astrophotography.

All this might not matter to you when you consider the huge upside of how a Dobsonian telescope works: they give large, bright images of deep sky objects for a fraction of the price of any other telescope style.

3) Classic Newtonian Telescopes

We looked at the classic model above when discussing how and why Sir Isaac Newton created it in the first place.

This classic design generally has a shorter focal length which means a greater field of view.

They can also be mounted on equatorial mounts and, equipped with a tracking motor, they can definitely be used for astrophotography too.

As with all reflectors, inch for inch, this model will be cheaper than a refractor or the Schmidt-Cassegrain style, which we'll look at next.​

How Catadioptric Telescopes Work

Catadioptric telescopes combine the physics of refractors and reflectors to get the best of both worlds.

The classic and most popular catadioptric is the Schmidt-Cassegrain design.​

How Schmidt Cassegrain Telescopes Work

The Schmidt-Cassegrain (SC) reflector telescope is quite different from the Newtonian, in that the secondary mirror reflects the image from the primary back through a whole in the centre of the primary to a waiting eyepiece.

How a Schmidt Cassegrain telescope works

How a Schmidt Cassegrain telescope works (credit below)

In this way, from outward appearance, it resembles a refractor telescope - light gathering at one end of the tube, viewing eyepiece at the other end.

It also resembles a refractor in that it has a lens at the aperture which faces the sky.

This lens is used to angle light rays before they hit the primary mirror at the back of the scope to reduce aberrations and so improve image quality.​

Example of a Schmidt Cassegrain telescope

Example of a Schmidt Cassegrain telescope

One of the reasons for the huge popularity if this design is its compactness.

Due to the curvature and arrangement of the mirrors, an SC scope manages to pack in a focal length which is around 5x its actual length!

This makes it incredibly portable against any given Newtonian reflector or refractor of similar focal length.​

But, perhaps the biggest reason for it being on the 'desired' list of so many backyard astronomers is that it can do everything you want of a telescope:

Inch for inch, a Dobsonian is cheaper, but the Schmidt Cassegrain can be used for astrophotography of deep sky objects, where the Dob can't. 

The SC is also a completely sealed unit, so no collimation is required either.​

If you're a planet hunter, well, a refractor is going to give superior views, but the price of a refractor lens the same as a SC - as we saw above - is going to be prohibitive.

This, for me, is a great all-rounder and one that I aspire to own one day, particularly as I want to get into astrophotography.

How Telescopes Work - Summary

There are two basic types of telescopes: refractors which rely on lenses to provide very high quality images but they become  eye-wateringly expensive at relatively small objective lens sizes.

The second type is the reflector, which works using mirrors. There are a few 'flavours' of reflector.

They have the advantage of giving much bigger apertures for less $​, but they also come with some limitations, like portability and the need to collimate.

Finally, we looked at the Schmidt Cassegrain, which is an amalgamation of the best features of both refractors and reflectors.

They give you great image quality, compactness and all for a price that fittingly sits between the high end refractors and the light bucket Dobsonians.​

Hopefully, that was a useful overview of how telescopes work, but if you're still not sure, then perhaps you'd like to check out this dated video from the 1980s Nickelodeon show, Mr Wizard...


Image Credits

​Hublle space telescope Credit: NASA

​How a refractor telescope works Credit: Tamasflex

​Yerkes observatory refractor Credit: Steve Hubbard

​How a reflector telescope works Credit: Krishnavedala

Replica of Newton's second reflecting telescope Credit: Solipsist

10" Dobsonian Telescope Credit: Wikipedia

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