Reflecting vs Refracting
A telescope is an instrument that collects and focuses electromagnetic radiation — usually visible light — from distant objects, making them appear larger, brighter, and more detailed. There are two fundamental designs: refracting telescopes, which use lenses to bend light to a focus, and reflecting telescopes, which use curved mirrors. Both exploit the same optical principle — gathering light over a large area and concentrating it to a point — but they do so in different ways, each with distinct advantages and limitations. The key figure of merit for any telescope is its light-gathering power, which scales with the area of the primary aperture. A telescope with a 200 mm diameter mirror or lens collects about 800 times more light than the human eye (pupil ~7 mm), allowing us to see objects far too faint for naked-eye observation. A second figure of merit is angular resolution — the ability to distinguish two closely spaced objects — which is limited by diffraction and scales as λ/D, where λ is the wavelength of light and D is the aperture diameter. Larger apertures therefore give both more light and sharper images. Real optical systems suffer from aberrations: systematic distortions that prevent all rays from converging to a perfect point. The two most important for astronomers are chromatic aberration (affecting lenses) and spherical aberration (affecting both lenses and mirrors of incorrect shape). Understanding these defects explains why all large modern research telescopes are reflectors, and why Isaac Newton's 1668 invention of the reflecting telescope was a pivotal moment in the history of astronomy.
- The Refracting Telescope
- The Reflecting Telescope
- Comparing the Two Designs
The Refracting Telescope
A refractor uses a large objective lens at the front of a long tube to collect parallel rays from a distant star and bend them — by refraction — to a focal point. A smaller eyepiece lens then re-collimates or magnifies those converging rays for the observer's eye. The angular magnification is M = f_objective / f_eyepiece, where f denotes focal length. The problem is dispersion: glass bends violet light more than red light (higher refractive index at shorter wavelengths), so each colour focuses at a slightly different distance. The result is chromatic aberration — bright stars are surrounded by coloured halos, and fine detail is smeared. Lens makers partially correct this by cementing two glass elements (an achromatic doublet) — a converging crown-glass lens paired with a diverging flint-glass lens — which brings two wavelengths to a common focus. This reduced but never eliminates chromatic aberration; the residual error is called secondary spectrum. Another practical problem: a large lens must be supported only at its rim, so it sags under its own weight, distorting its shape. The largest successful refractor, the Yerkes 40-inch (1.02 m), was completed in 1897 and represents the practical upper limit of the design.
The Reflecting Telescope
A reflector replaces the objective lens with a concave primary mirror. Light reflects from the mirror's surface and converges to a focal point without passing through glass, so there is no dispersion and therefore no chromatic aberration — all wavelengths reflect at the same angle (angle of incidence = angle of reflection). Mirrors can also be supported across their entire back surface, not just at the rim, so they can be made very large without sagging. The most common mirror shape is a paraboloid, which brings all on-axis parallel rays to a perfect focus. A spherical mirror is cheaper to manufacture but suffers from spherical aberration: rays striking the outer zones focus closer to the mirror than rays near the centre, producing a blurred image. The Hubble Space Telescope famously suffered from this defect — its primary mirror was ground to the wrong prescription by 2.2 micrometres — before corrective optics were installed in 1993. Common reflector arrangements include the Newtonian (a flat secondary mirror diverts light to the side of the tube), the Cassegrain (a convex hyperbolic secondary sends light back through a hole in the primary), and the Gregorian design.
Comparing the Two Designs
Refractors produce high-contrast, stable images ideal for planetary observation and are low-maintenance — the sealed tube keeps dust off the optics. However, they are expensive to build beyond ~0.5 m aperture, always suffer some chromatic aberration, and are physically very long. Reflectors are free of chromatic aberration, can be built to any size (the ELT primary mirror is 39 m across), and are more compact for a given focal length. Their disadvantages include mirror coatings that tarnish over time and require recoating, and the secondary mirror or its support ('spider') obstructs a small fraction of the incoming light, causing faint diffraction spikes around bright stars. For research astronomy above ~0.3 m, reflectors are universally preferred.
The Key Equation
- M — Angular magnification (dimensionless): The factor by which the apparent angular size of the object is increased compared to viewing with the naked eye.
- f_{\text{obj}} — Focal length of objective (m (or mm)): Distance from the objective lens or primary mirror to the point where parallel incoming rays converge to a focus.
- f_{\text{eye}} — Focal length of eyepiece (m (or mm)): Focal length of the magnifying eyepiece lens through which the observer views the image.
Worked Example
Problem. A Newtonian reflecting telescope has a primary mirror with a focal length of 1200 mm. An astronomer wants to observe Jupiter at a magnification of 150×. (a) What focal length eyepiece should she use? (b) She then swaps to a 6 mm eyepiece — what is the new magnification?
Answer. 8 mm eyepiece for 150× magnification; the 6 mm eyepiece gives 200× magnification.
Origins · 1 minIsaac Newton and how this was discovered
Newton built the first practical reflecting telescope in 1668, using a concave spherical mirror of polished speculum metal (an alloy of copper and tin) to avoid the chromatic aberration plaguing contemporary refractors. He understood that because white light is composed of all colours and glass disperses them by different amounts, no single lens could bring all wavelengths to the same focus — a problem he considered insurmountable for refractors. His design, presented to the Royal Society in 1671, demonstrated that a mirror-based telescope could be far shorter than an equivalent refractor while delivering sharper colour-free images. The Newtonian reflector remains one of the most popular telescope designs for amateur astronomers today. Newton's first reflecting telescope was only about 15 cm long, yet it magnified roughly 40 times — comparable to a refractor over a metre long. He hand-ground the mirror himself.
Related Concepts
- Diffraction and Angular Resolution — The Rayleigh criterion (θ ≈ 1.22 λ/D) sets the ultimate resolving power of any telescope aperture, directly linking to why larger mirrors are built.
- Snell's Law and Refraction — Refraction at lens surfaces — governed by Snell's law — is the physical basis of how an objective lens focuses light and why dispersion causes chromatic aberration.
- Curved Mirrors and Focal Length — The mirror equation (1/f = 1/v + 1/u) and the geometry of paraboloidal mirrors underpin the design of all reflecting telescopes.
- The Electromagnetic Spectrum in Astronomy — Telescopes are built for every region of the spectrum; understanding why different wavelengths require different mirror/detector combinations extends the concepts here.