Telescopes are one of the most fascinating and useful optical devices that humans have ever created. From the very earliest models to the basic amateur telescopes that are in use today, most have used a system of lenses to provide magnification.
But magnification is a little more complicated than it sounds, so just how many lenses does a telescope actually need?
Here’s how many lenses a telescope has
Most telescopes that use lenses have either one or two of them. Refractor telescopes have one for focusing the image and another for magnifying it. Catadioptric telescopes have just one for correcting the image. However, reflector telescopes have no lenses at all, since they rely strictly on mirrors instead.
Whatever the combination of lenses and/or mirrors, a telescope does two basic thing. First, it gathers much more light than the human eye can capture, and focuses it onto a smaller area. Second, it magnifies that focused image.
This is most traditionally accomplished with two lenses, but designs vary significantly. Below, let’s take a closer look at how and why that is.
How many lenses are in the simplest telescopes?
Lens-based telescopes focus incoming light in order to make objects that are far away appear more magnified, clearer, and brighter. Today, we call these telescopes “refractor telescopes”. A large majority of refractors have two primary lenses — there’s a small lens that serves as a viewing piece, dubbed the “eyepiece lens”, and a bigger one which is named the “objective lens”.
With this in mind, you’re probably wondering how telescopes use these lenses to form the celestial imagery that we all know and love. Well, the refractor telescopes, as their name suggests — create images via refraction. Their lenses are generally made out of two possible materials — plastic and glass. Furthermore, the surfaces of the lenses are ground in a way that makes them segments of either planes or spheres.
As a consequence of the principles of refraction: if we’ve got a convex lens, it will converge the rays of light that come to us from distant objects. The converging happens once all of these light rays are bent into a single point. We call it “the focal point”. And the distance from your telescope’s lens to the abovementioned focal point is the measurement we call “the lens’ focal point”.
Conversely, a lens may also have a concave shape — which results in the opposite effect of diverging. That means that there will be parallel rays which are bended in a way that makes them spread out. In this situation, it will seem like the rays all come from a single point found in front of your telescope’s lens; this is also a focal point, but it is measured and expressed in negative units.
Why do telescopes have two lenses?
As you might have gathered by now, if you were to want to create the simplest possible telescope that works, all by yourself — you will have realized that this contraption amounts to nothing more than two lenses fixated inside a tube.
This pair of lenses is necessary due to basic laws of light refraction in physics. You need a front lens, that we’ve already established as the objective lens — this is the lens that works to focus your image when you’re observing something.
The other lens that’s found in the back is there to magnify the image — that’s the eyepiece lens. While we’re only describing the crudest possible kind of telescope, it’s still perfectly capable of showcasing the basic principles that govern the engineering behind more powerful and complex instruments used for astronomy.
Remember — light generally passes through its environment in straight lines. However, we can create conditions where that is not the case — which is the basic principle on which refracting telescopes are based. Remember how everything you see when you look through the ocean’s surface seems distorted? This happens due to light bending upon passing between the air and the water.
And while astronomers have not always understood the precise reasons why light acts in this way, this has not prevented them from utilizing the effect. The fact that light bends between surfaces has allowed us to create artificial lenses — as optical devices which are capable of both spreading out light and converging it into a single point.
So, for a working telescope, one lens is not enough — it would merely focus the image, without magnifying it. In terms of close observations of the surface of other planets and celestial bodies; that would be pretty much useless. That’s why we have another lens in our telescopes — the eyepiece one. When light passes from the objective lens to the eyepiece one, it magnifies the image as perceived by the human eye.
How do telescopes mirrors work with lenses?
We have already talked about the fact that refracting telescopes use a system of lenses to magnify distant images. However, there’s another kind of telescope — the reflector ones. They use mirrors instead of lenses.
Now, we’ve already established that refractors utilize two lenses in order to focus and magnify a distant object — these lenses are convex, and bend the incoming rays of light accordingly. Conversely to this, there are reflecting telescopes. These devices don’t contain any lenses in the first place.
In their place, there are mirrors — which can also be used to focus beams of light. These are glass surfaces that are concave; this shape allows the telescope to bend light rays together, just as refractors would. The only difference is that the light is reflected upon passing through, rather than experiencing refraction.
There are plenty of situations where reflector telescopes are useful, but they have one slight issue…
If you were to build a truly powerful reflector, you would need a huge amount of space. We’re talking the sort of space that allows people to sit in right in the middle of the scope! One of the first reflecting telescopes was constructed a couple of years after the French revolution by a renowned British astronomer named William Hershel — predictably, it was quite big.
Crucially, we should point out that there are also quite complex telescopes; these use more complicated systems that combine the principles of both refraction and reflection — thus containing mirrors and lenses.
These telescopes are named catadioptric telescopes, and they (generally) have a pair of mirrors plus a single lens to correct aberrations caused by the mirrors.
There are two most commonly utilized designs — the Maksutov-Cassegrain and the Schmidt-Cassegrain one — MCT and SCT, respectively.
While complex to design, this kind of telescope is extremely popular among amateur astronomers. We strongly recommend it for most home astronomers, assuming budget isn’t too tight of a constraint. Visit our buyer’s guide to catadioptric telescopes for some models that we’re particularly excited about.
Brief recap and conclusion
As we’ve covered, the most basic telescopes contain two lenses, and the reason for that is simple — due to the basic laws of optics, one lens serves to focus the object that is being viewed, and the other one magnifies it and straightens the picture.
These are “refractor” telescopes — there are also “reflector” ones, that use mirrors instead of lenses. Generally, their design requires them to be huge, but they have their benefits as well.
Finally, there are also catadioptric telescopes; these are designed with a combined array of mirrors and lenses, and they’re quite popular among astronomy hobbyists that require something practical, but not necessarily too powerful.