telescope mirror

Why Do Telescopes Use Mirrors Instead Of Lenses?

What picture comes to your mind when someone mentions the word “telescope”? A long tube-shaped device featuring a bunch of lenses? You’ll probably be surprised to find that most modern telescopes employ mirrors instead of lenses — And for very good reasons too.

Why do Modern Telescopes use Mirrors Instead of Lenses?

From the world-famous Grand Canarias Observatory to tiny ultra-compacts designed for personal use, all telescopes work the same way. They collect light from distant objects — light that cannot be perceived naturally by the human eye — and focus it onto a central point (AKA focal point) to help us see them better. This can be done using either lenses or mirrors; in order to understand why one’s more effective than the other, it’s worth examining how the telescope has evolved through the years.

Background

The idea of magnifying light to make faraway objects visible can be traced back to the early 17th century. It’s here that a Dutch spectacle maker submitted a patent for a new optical instrument that used lenses housed inside a tube, and which had the capability to magnify objects up to 4 times. This instrument was aptly named ‘telescope’ — a Greek portmanteau of the words ‘far’ and ‘seeing.’

This early version of the telescope is based on the principle of refraction, or the tendency of light to “bend” as it travels from one medium to another. Refraction is what makes objects that are lying beneath a water surface seem bigger than they really are. It also allows us change how light travels to our advantage.

In the case of a refractor telescope, you have two lenses that gather dispersed rays of light and focus them to make faraway objects look bigger and clearer. The first lens, known technically as the objective lens, has a convex shape that makes incoming rays converge around a focal plane. The second lens (eyepiece) then spreads out the beam to magnify the image. And it’s this revolutionary mechanism that paved way for some of the most significant astronomical discoveries. Refractor telescopes basically expanded the boundaries of the known universe.

Still, there was a huge flaw in the design of these instruments. Lenses, while being effective in focusing light, also introduce an effect known as chromatic aberration, or color distortion of the viewed image. This happens because different wavelengths of light (colors) are bent to varying degrees as they travel across the lens. As a result, an object viewed through a refractor telescope will usually appear to be surrounded by a rainbow-like halo.

Now, chromatic aberration can be counteracted by introducing a compensating lens behind the objective and/or lengthening the tube to minimize distortion. But refractor telescopes still have other weaknesses:

  • Lenses are notoriously difficult to get right
  • Glass absorbs some light as it passes through
  • Because lenses can only be supported at their edges, they’re bound to sag under their own weight in the middle

Out With Lenses, In With Mirrors

As clever as the lens-based scope was, its drawbacks would soon inspire other thinkers to explore alternatives. A few individuals began to investigate the potential benefits of using mirrors in place of lenses. This gave birth to what’s now known as the reflector telescope.

Here’s how it works: Light coming from a distant object goes into a tube that has a concave (inward-curved) mirror at the extreme end. Known as the primary mirror, this is designed to make incoming beams converge towards the focal point. A secondary mirror is then placed here to reflect the image onto the tube of the side so it can be viewed.

So why do away with lenses and use mirrors instead? It all comes down to the two most crucial properties that determine a telescope’s effectiveness:

  • Light-gathering capability: With more light comes a better view.
  • Magnification: This describes the extent to which the instrument can make objects appear larger.

Both of these attributes are determined by a scope’s aperture size; aperture is the width/diameter of the light-collecting region. A larger aperture is not only able to capture more light, but also offers more magnifying potential. The key benefit of reflector telescopes here is that they don’t impose as cumbersome limitations as refractors do.

On one hand, a mirror doesn’t need to be milled to spec on both sides. It can also be supported fully on its base to eliminate the risk of sagging. But most significantly, it’s far easier to counteract distortion with a large mirror. There’s no need to think about how thickness will affect light penetration — a huge problem with lenses. The reflective surface can be as big as you want it while still being wafer-thin.

So it’s no surprise that reflectors are the scope of choice for today’s crop of researchers. Refractor telescopes seem to have hit a size cap of 40 inches across the objective lens. By contrast, reflectors have spanned dozens of feet in diameter, and there are plans in the works for 90-to-100-foot designs.

And reflector scopes don’t just rule the professional domain either — they’re also very popular among recreational astronomers. That’s partly down to their relative affordability — mirrors are cheaper to produce than lenses. They also allow for easier cleaning and polishing as well; being one-sided and all.

It’s not to say that reflector telescopes don’t have any drawbacks. Their mirrors are highly-susceptible to misalignment, for one. And thanks to the open-tube design, the optics need much more frequent cleaning.

Still, it doesn’t take a genius to see that these are relatively-minor issues to deal with — especially in light of the shortcomings suffered by lens-based scopes. You’d rather have an instrument that, while being somewhat demanding in maintenance, functions pretty much the same way it’s intended to. All without the need to introduce secondary features in order to overcome weaknesses in the original design.

Wrapping Up

The idea of using a combination of lenses to bring faraway objects within view was quite the groundbreaker. But no sooner had the concept been developed than there emerged a mechanism that could get the job done much better. Today, you’ll be hard-pressed to find a serious scope that doesn’t use mirrors. Reflective telescopes are preferred because they not only offer more potential, but are also cheaper to design and produce at any given price range.

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