Reflecting Telescopes
Almost every major observatory built in the last century, and every large telescope in orbit, uses mirrors rather than lenses to collect its light. Reflecting telescopes solve several stubborn physical problems that limit how large — and how good — a refracting telescope can ever practically be.
The Cassegrain arrangement
A reflecting telescope uses curved mirrors, rather than lenses, to collect and focus light. One widely used design is the Cassegrain telescope, which arranges two mirrors along the same tube to fold the light path back on itself, keeping the whole instrument compact.
- A large primary mirror, shaped as a concave paraboloid, sits at the back of the tube facing the incoming light. Parallel rays from a distant object strike this mirror and are reflected back up the tube, converging toward a focus.
- Before reaching that focus, the converging rays are intercepted by a small secondary mirror, mounted near the front of the tube facing back down toward the primary. This secondary mirror is convex, and reflects the light back down the tube a second time.
- The primary mirror has a small hole bored through its centre. The light reflected from the secondary mirror passes back through this hole and comes to a focus just behind the primary mirror, where it can be viewed with an eyepiece or recorded by a detector.
Tracing the full ray path: parallel rays enter the open end of the tube, reflect off the concave primary mirror and converge toward a focus, are intercepted by the convex secondary mirror before reaching that focus and reflected back the other way, then pass out through the hole in the primary mirror to form the final image at the eyepiece. Folding the light path this way lets the telescope have a long effective focal length — and so high magnification — while the physical tube itself stays relatively short.
Why the primary mirror is parabolic
The primary mirror is shaped as a paraboloid rather than a simple sphere because of spherical aberration. If the primary mirror were spherical, parallel rays striking it further from the central axis would be reflected to a focus slightly closer to the mirror than rays striking nearer the axis — the mirror would not bring all the incoming parallel rays to exactly the same focal point. This spreads the image of even a single point source out into a small blur rather than a sharp point, degrading the image.
A parabolic mirror is shaped precisely so that every ray travelling parallel to the optical axis, regardless of how far from the axis it strikes the mirror, is reflected to converge at exactly the same focal point. Spherical aberration is avoided by construction, rather than needing to be corrected afterwards. It's worth noting that spherical aberration can affect simple lenses in refracting systems too, for the same underlying reason — a spherical refracting surface doesn't focus all parallel rays to one point either — though the effect is treated only qualitatively at this level.
Reflectors versus refractors
Reflecting telescopes have several practical advantages over refracting telescopes, which is why almost all large modern optical telescopes are reflectors.
A lens refracts light, and the amount of refraction depends slightly on wavelength — this is the same effect that splits white light into a spectrum in a prism. As a result, a simple lens does not bring all colours of light to exactly the same focus: red light and blue light from the same object focus at slightly different points, blurring and colour-fringing the final image. This is chromatic aberration, and it is an unavoidable consequence of refraction through a lens. A mirror works by reflection, not refraction, so it reflects every wavelength to the same focus — reflecting telescopes are free of chromatic aberration entirely.
- Structural support. A lens can only be held in place around its edge, since light must pass straight through the rest of its surface. A large, heavy lens supported only at its rim tends to sag and distort slightly under its own weight, which becomes a serious problem at large sizes. A mirror, by contrast, only needs its reflecting front face left clear — it can be supported across its entire rear surface, making large, rigid mirrors far more practical to build and maintain than large lenses.
- Number of precision surfaces. A lens has two surfaces (front and back) that both need to be ground and polished to a precise curved shape, and both surfaces contribute to any optical imperfection. A mirror has only one reflecting surface that needs to be figured to high precision, roughly halving the manufacturing difficulty for a given aperture size.
- Freedom from chromatic aberration. As above — since mirrors reflect rather than refract, every wavelength converges to the same focus, which particularly benefits observations that need to compare or combine light across a range of wavelengths.
Every one of these advantages becomes more important as telescopes get larger, which is exactly the direction observational astronomy has pushed for centuries — bigger apertures collect more light and resolve finer detail. A large lens becomes structurally impractical and increasingly blurred by chromatic aberration long before a mirror of the same diameter runs into comparable problems, which is the underlying reason reflecting designs, including Cassegrain-style arrangements, dominate large-telescope construction.
Key takeaways
- A Cassegrain reflecting telescope uses a large concave parabolic primary mirror and a smaller convex secondary mirror to fold the light path back through a hole in the primary.
- The ray path is: parallel rays in, reflected by the primary toward a focus, intercepted and reflected back by the secondary, then out through the hole in the primary to the eyepiece.
- A parabolic (not spherical) primary mirror is used so all parallel rays converge to exactly the same focal point, avoiding spherical aberration.
- Spherical aberration can also affect simple lenses and spherical mirrors generally, and is treated qualitatively.
- Refracting lenses suffer chromatic aberration because refraction depends on wavelength; mirrors reflect all wavelengths to the same focus and avoid this entirely.
- Large lenses can only be supported at their edge and sag under their own weight; large mirrors can be supported across their whole back surface.
- A lens has two surfaces that must be precisely ground; a mirror has only one reflecting surface, simplifying manufacture at large apertures.