Reflecting Vs Refracting: Telescopes That Reveal the Wonders of the Cosmos

Capturing moments once unimaginable, these astronomical tools transform how we perceive the universe and deepen our understanding of space and time. In this blog, we explore the differences between two dominant telescopes that help us achieve these goals, reflecting and refracting.

‍Let’s begin by tracing the origins. Throughout the ages, telescopic technology has evolved. Galileo made a game-changing discovery as the first to point a telescope toward the sky. Johannes Kepler then improved upon this by integrating two Convex Lenses, expanding a user’s field of view and allowing for higher magnification. Later, Isaac Newton revolutionised the field by switching from lenses to mirrors. Together, these ‘Fathers of Modern Astronomy’ laid the groundwork for something far greater than they could have ever foreseen.

Interstellar travel, black holes, neutron stars, dark matter, and the Big Bang theory are just a few concepts that would astonish these three pioneers of modern astronomy and physics. And it all started with a refracting telescope.

What Are Refracting Telescopes, and How Do They Work?

True to their title, refracting telescopes – like Galileo’s and Kepler’s – use lenses to bend light and form an image. The optical instruments consist of two lenses. The objective lens is responsible for gathering light from subjects and converging it at a focal point. Meanwhile, the eyepiece lens magnifies the image for viewing. Together, they enable users to view faraway objects with superior clarity and detail. In the 18th and 19th centuries, refracting telescopes were scaled up for use in giant observatories. Nowadays, they’re normally used by amateur astronomers and for educational purposes, and many observatories have replaced their refractors in favour of reflectors.

Optical Components for Refracting Telescopes

What lenses are used in refracting telescopes?‍
Typically, you’ll spot Convex or Achromatic Lenses used as the objective lens, with the latter commonly incorporated in more progressive models to correct chromatic aberration. Kepler modified Galileo’s original setup, which used a convex objective and a concave eyepiece. After his change to a dual-convex lens configuration, most modern refracting telescopes now use Convex Lenses for both the objective and the eyepiece.

What Are Reflecting Telescopes, and How Do They Work?

Chromatic aberration. It’s an issue that even plagued the legendary Galileo and Kepler. Yet, Isaac Newton wasn’t willing to settle. By substituting lenses for mirrors, he successfully eliminated colour fringing and blurred visuals. From this point forward, the Concave Mirror, recognised for reflecting all colours equally, gave birth to the reflecting telescope. With clearer scenes and the potential for larger-scale designs, the breakthrough paved the way for the state-of-the-art telescopes we know today – some of which are orbiting right above our heads at this very moment.

Optical Components for Reflecting Telescopes

What mirrors are used in reflecting telescopes?‍
Consisting of a primary and secondary mirror, you’ll find concave varieties (often Parabolic Mirrors) and smaller Convex or Plane Mirrors used, respectively. Lenses aren’t strictly prohibited from a reflecting telescope’s blueprints, however. Like a refracting version, you’ll encounter Convex Lenses used for the eyepiece here, too.

What Are Reflecting Telescopes Used For?

Due to the ability to avoid chromatic aberration and the ease of manufacturing larger mirrors compared to lenses, reflecting telescopes are popular – especially among professional astronomers and vast observatory complexes. You might even recognise their contributions in the visuals below.

The Cosmic Reef. Captured by Hubble. Image courtesy of NASA, ESA and Space Telescope Science Institute

The Cosmic Reef and Cosmic Cliffs are mind-blowing snapshots from contemporary reflecting telescopes that would have Newton doing a double take. Two big names lead the charge: The James Webb Space Telescope (JWST), regularly making headlines, and Hubble, the original stargazer. Although both remarkable scopes capture striking representations, the critical difference lies in the spectrums they discern.

Cosmic Cliffs. Captured by NIRCam on the James Webb Space Telescope. Image courtesy of NASA, ESA, CSA and Space Telescope Science Institute (STScI)

Infrared (IR)‍
Let’s start with the most recently launched. James Webb is best known for delivering regular updates and findings from space – such as supermassive black hole-powered quasars and stunning nebulae. Since its deployment almost three years ago, it has quickly become one of science’s most significant contributors. Thanks to its IR-focused system, Webb can peer through galactic dust and witness cooler stellar bodies, such as forming stars and distant galaxies. Unlike Hubble, James Webb’s optics are optimised for IR detection. Its mirrors, coated in Gold to improve the reflection of IR light, are just one example of the specialised materials and coatings that set Webb apart.

The Hubble Space Telescope. Photo by NASA on Unsplash.

Visible, UV, and NIR‍

Hubble, on the other hand, primarily captures shots using a different range of wavelengths. From its first-ever photo, the binary star HD96755, to its most recent close-ups of Uranus, it’s been wowing us with peeks into the cosmos for 34 years.

This is rendered possible by an Optical Telescope Assembly (OTA) that enables Hubble to observe the celestial sphere mainly in visible and UV – but also in near-infrared (NIR) wavelengths, albeit on a smaller scale than Webb. Central to its arrangement is a primary Concave Mirror coated with a thin layer of Aluminium for reflectivity and a coating of Magnesium Fluoride (MgF2) to prevent oxidation and enhance UV light reflection. A smaller convex secondary mirror then redirects light from the primary mirror to Hubble’s equipment for detailed analysis.

The Final View

Whether reflecting or refracting, these scopes share a collective aim: To sharpen our knowledge of the cosmos, with optics and coatings playing a key role.