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How Do Telescopes Work?

Understanding how telescopes work is key for both getting the best performance from the one you own and deciding what you want from a new one.

Telescopes work by collecting as much light as possible through a large aperture, much more than our eyes. Since they collect so much light, we can see objects with them that are invisible to the naked eye.

There is much more detail to workings of a telescope than this simple summary though. So, in this article, we share all the information you need to understand why telescopes work so much better than your eyes.

Do Telescopes Magnify Planets?

The simple answer is, yes. Telescopes do magnify planets, and galaxies, stars and nebulae. Anything you point a telescope at is magnified, but they need an eyepiece to do it and – as you’ll soon see – it is not the most important thing they do.

If you’ve ever bought or shopped for a telescope, particularly a cheap or low-end one, you’ll have seen that the advertising often shouts about how much magnification the scope can deliver.

This, sadly, leads unsuspecting shoppers down the wrong path:

When buying a telescope, the amount of magnification it can offer is irrelevant!

Telescopes are effective because they have a huge pupil (compared to the ones in your eyes). It’s the size of this aperture which we need to pay most attention to.

What Telescopes Do

Compared to a telescope, our eyes are puny for collecting light!

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.

Objects in space, from planets to galaxies, are so far away that only a tiny amount of the light the produce reaches us 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.

Telescopes, however, are designed to collect light.

The number one thing any telescope does to improve night sky observing is gather (significantly) more light than our eyes. The bigger your telescope’s aperture, the more light is collected and the fainter the objects we can see with it.

How Much More Light Does a Bigger Telescope Collect?

The amount of light an objective collects grows based on a known formula which says that doubling the size of the telescope’s objective (aperture) increases its light-gathering power four-fold.

This means a small increase in objective size gives a big increase in light gathering power. For example, an 8″ telescope pulls in about 77% more light than one with a 6″ opening

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, which only becomes relevant after the light is gathered.

It’s actually the job of a telescope’s eyepiece to magnify the image collected by the scope. To use higher magnification requires more light, so the same rule still applies. If you want more magnification, get a bigger aperture.

What Are The Different Types of Telescope?

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

A third design, the Catadioptric (cat), or hybrid, uses a combination of mirror and lens to deliver some advantages in size.

Hubble space telescope
Hubble Space Telescope works in the same way as a backyard scope (source)

All telescopes, from the smallest starter scope to the Hubble Space telescope, work on the same three principles:

  1. Collect a lot of light with a big aperture
  2. Focus that light to a small, sharp image
  3. Magnify the image using an eyepiece

Reflectors use mirrors as their light-gathering objective and refractors use a glass lens to collect and focus the light.​ Cat’s use a combination of lens and mirror.

Over the rest of this article, we’ll look at how each of these three telescope types work.

How Refracting Telescopes Work

A refractor is the classic telescope design of movies. It simply consists of a long tube and relatively narrow tube (also known as an optical tube assembly, or OTA) with an objective lens at one end and an eyepiece at the other.

Refractors are so called because they use a lens to refract (bend) incoming light rays to a focus.

The detailed diagram below shows 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
How refracting telescopes work. Click to enlarge (source)

An eyepiece is used to magnify the image on the ‘focal plane’ and the resulting 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 an OTA which is 60 feet (18.3m) long!

Yerkes observatory refractor
40″ refractor at Yerkes Observatory (source)

The limiting factor with refractors is the size of the objective lens and bigger refractors quickly become too expensive for backyard astronomers.

This is because the glass used to make the lens has to be of very high quality. There can be no imperfections on its surface or interior, and it must be perfectly shaped to focus the light it collects.

We’ve already said that ‘bigger is better’ when it comes to aperture, so you may be wondering why anyone would buy a refractor.​

The reason astronomer’s buy refrators is their resolving power.

The physics behind a refractor telescope makes them brilliant at resolving fine detail, such as binary stars, lunar craters and wisps of nebulae. This quality means that even a smaller, cheaper scope can give good views.

Refractor scopes are also perfect for astrophotography. Because the camera takes a big part of the light-gathering role, it’s more important that an astrophotography telescope delivers sharp, crisp images than has a big aperture. However, lens quality is much higher in these scopes, which means much more expensive too.

How Reflecting Telescopes Work

Reflectors are fundamentally different beasts to refractors.

As their name suggests, reflectors use mirrors to gather and focus light, instead of a lens. These mirrors reflect 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 (source)

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.

With the objective mirror placed at the back end of the OTA, it is not possible to have the eyepiece mounted on the back end of the telescope like it is on a refractor.

Instead, the primary mirror is curved to reflect light to a smaller, secondary mirror, which is the blue rectangle shown in the diagram at a 45° angle. The secondary mirror diverts and further focuses the image to an eyepiece positioned at the side of the telescope.

Newton's second reflector telescope
A replica of the second ever Newtonian scope (source)

Sir Isaac Newton invented the reflector the 1680s to combat the poor image quality he was getting from the lenses in refractor telescopes. This scope design is know as a Newtonian, after its inventor.

Refractors were so poor in Newton’s day because they did not have the technology to beat chromatic aberration, which is where the lens bends different colors of light at different angles, separating them out and making image quality terrible.

​Newton knew that mirrors reflect all the colours of light at the same angle, so images are not blurred by chromatic aberration. Sir Isaac Newton’s reflector telescope significantly improved images of the night sky.

The Downside of Newtonian Telescopes

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, which is why we’ve written a detailed guide to collimating your telescope.

The second challenge is coma.

This optical aberration is particular 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 some way towards eradicating it.

Types of Reflector Telescope

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
A table top wide 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 can make them a good choice for wide area nebulae and capturing large globular clusters or displays of beauty such as the pleiades.​

However, they tend to be cheap, of lower quality and don’t offer great astronomy.

2) Dobsonian Reflector Telescopes

It is actually the way this reflector is mounted that makes it a ‘Dob’. Dobsonians have simple (i.e. low cost) but very effective mounts.

The huge benefit the low cost mount is that you can have an incredibly large aperture for a relatively low price. Here are plenty of examples on Amazon (opens new page).

It is a very simply designed scope: a long tube, altazimuth mount and a big mirror. It’s easy to see why they are known as ‘light buckets’.​

10inch Dobsonian telescope
Example of a 10″ Dobsonian telescope (source)

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, and they can 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) telescope is quite different from the Newtonian, in that the secondary mirror reflects the image from the primary back through a hole in the centre of the primary to a waiting eyepiece.

How a Schmidt Cassegrain telescope works
How a Schmidt Cassegrain telescope works (source)

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
The NextStar range of scopes from Celestron is the classic SC 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
  • Top end refractors give superior image quality, but the price of a refractor lens the same as a SC – as we saw above – is going to be prohibitive.

Schmidt-cassegrain and a tracking mount is a great all-rounder choice. See our review of the Celestron NexStar 8SE to learn more about it (opens in new tab).

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 money. 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.

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

If you’ve learned enough to help you make a choice, why not check out our best telescope reviews!