Aperture synthesis

Lore & Background

The concept of aperture synthesis was first formulated in 1946 by Australian radio astronomers Ruby Payne-Scott and Joseph Pawsey. Working from Dover Heights in Sydney, Payne-Scott carried out the earliest interferometer observations in radio astronomy on 26 January 1946 using an Australian Army radar as a radio telescope. The technique was later developed at radio wavelengths by Martin Ryle and coworkers from the Radio Astronomy Group at Cambridge University. Martin Ryle and Tony Hewish jointly received a Nobel Prize for this and other contributions to the development of radio interferometry. The Cambridge group went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s, and during the late 1960s and early 1970s, as computers like the Titan became capable of handling the computationally intensive Fourier transform inversions, they used aperture synthesis to create a 'One-Mile' and later a '5 km' effective aperture using the One-Mile and Ryle telescopes, respectively. Aperture synthesis is possible only if both the amplitude and phase of the incoming signal are measured by each telescope. For radio frequencies, this is achievable by electronics, while for optical frequencies, the electromagnetic field cannot be measured directly and correlated in software but must be propagated by sensitive optics and interfered optically, requiring accurate optical delay and atmospheric wavefront aberration correction—a demanding technology that became possible only in the 1990s. This explains why imaging with aperture synthesis has been used successfully in radio astronomy since the 1950s and in optical/infrared astronomy only since the turn of the millennium. Most radio frequency aperture synthesis interferometers use the rotation of the Earth to increase the number of different baselines included in an observation, as discussed in detail in the 1950 paper 'A preliminary survey of the radio stars in the Northern Hemisphere.' Some instruments use artificial rotation of the interferometer array instead, such as in aperture masking interferometry.

Reader's Guide

Aperture synthesis has been fundamental to high-resolution astronomy, enabling observations across radio, optical, infrared, submillimetre, and radio wavelengths. The technique was originally thought to require measurements at essentially every baseline length and orientation out to some maximum to fully sample the Fourier transform, which would formally contain information equivalent to an image from a conventional telescope with an aperture diameter equal to the maximum baseline. However, it was rapidly discovered that useful images could be made with a relatively sparse and irregular set of baselines, especially with non-linear deconvolution algorithms such as the maximum entropy method. This shift in emphasis led to the alternative name 'synthesis imaging,' acknowledging the move from trying to synthesize the complete aperture to synthesizing the image from whatever data is available using powerful but computationally expensive algorithms. To avoid this computational bottleneck, telescopes such as the Deep Synoptic Array use a large number (2000) of randomly distributed antennas to nearly fully sample the uv plane, providing a point spread function that allows much simpler reconstruction, referred to as a radio camera. The technique was subsequently further developed in very-long-baseline interferometry to obtain baselines of thousands of kilometers and even in optical telescopes. Aperture synthesis is technically and historically independent of synthetic aperture radar, which is a Doppler technique developed independently in the early 1950s by Carl A. Wiley.

Did You Know?

• The Very Large Array has 27 telescopes, giving 351 independent baselines at once.
• The Event Horizon Telescope project used aperture synthesis to derive the first image of a black hole.
• Aperture synthesis imaging in optical/infrared astronomy became possible only around the turn of the millennium due to the need for accurate optical delay and atmospheric wavefront aberration correction.

Origins in the Workshop, Not the Laboratory

The telescope emerged not from a single scientist's eureka moment but from the accumulated craft of lens-makers across centuries. Ancient Greek philosophers theorized about refraction and reflection, and those ideas were preserved and expanded in the medieval Islamic world before reaching a sophisticated state in early modern Europe. The practical breakthrough that made the telescope possible was the development of spectacle lens manufacturing, first in Venice and Florence during the thirteenth century, then in the Netherlands and Germany. In 1608, the Netherlands became the birthplace of the first documented refracting telescope when spectacle maker Hans Lippershey filed a patent, followed shortly by competing claims from Jacob Metius and an anonymous third applicant. Within a year, Galileo Galilei had built his own improved version using a convex objective and concave eyepiece—the design now known as the Galilean telescope—and became the first to publish astronomical findings through one. Johannes Kepler then proposed swapping in a convex eyepiece, creating what we call the Keplerian design.

Three Families of Optical Design

Optical telescopes fall into three principal categories based on how they gather and focus light. Refracting telescopes rely on lenses, and occasionally prisms, to bend incoming visible light toward a focal point. Reflecting telescopes substitute a curved mirror for the objective lens, using catoptric principles to form an image. Catadioptric designs blend both approaches, employing a combination of lenses and mirrors as their primary optical elements. In every case, the objective—whether a convex lens or a concave mirror—collects light from a distant object and concentrates it onto a focal plane, producing a real image. An eyepiece then acts as a magnifying glass, allowing the observer's eye to perceive an enlarged virtual image. A critical design relationship governs performance: the resolving power scales directly with the diameter of the objective, while light-gathering capacity scales with its area. Larger objectives therefore collect more photons and reveal finer structural detail, making aperture size the single most consequential parameter in telescope design.

The Long Road of Reflectors and Hybrid Optics

The theoretical groundwork for reflecting telescopes was laid well before any working instrument existed. Alhazen's work on curved mirrors, widely disseminated through Latin translations, established that mirrors could behave analogously to lenses. After the refracting telescope appeared, Galileo, Giovanni Francesco Sagredo, and others discussed mirror-based designs, and James Gregory published a notable parabolic-mirror proposal in 1663 that became known as the Gregorian telescope, yet no functional model was ever built. Isaac Newton is generally credited with constructing the first practical reflector, the Newtonian telescope, in 1668. However, the difficulty of fabricating speculum metal mirrors and their mediocre performance meant reflectors did not gain widespread popularity for over a century. Subsequent centuries brought parabolic mirror fabrication perfected in the 18th century, silver-coated glass mirrors in the 19th, durable aluminum coatings in the 20th, segmented mirrors for larger apertures, and active optics to counter gravitational sag. Mid-20th-century catadioptric innovations like the Schmidt camera added wide-field imaging capability, while late-20th-century adaptive optics and space-based telescopes addressed atmospheric distortion.

From Observatory to Backyard

Optical telescopes and their smaller cousins—monoculars and binoculars—serve a remarkably broad range of human activities. Outdoors, they support observational astronomy, birdwatching, maritime navigation, hunting, and military reconnaissance. Indoors or in semi-outdoor settings, they are standard tools for following performance arts and spectator sports. The most recent transformation has been the electronics revolution of the early 21st century, which produced computer-connected telescopes in the 2010s. These instruments link to a smartphone, tablet, or laptop and leverage digital astrophotographic techniques originally developed by professional astronomers. By stacking multiple exposures and subtracting noise, amateur observers can now capture images of Messier objects and faint stars as dim as apparent magnitude 15 using consumer-grade equipment. This democratization means that non-professional skywatchers can observe stars and satellites with relatively low-cost hardware, a capability that would have been unimaginable to the spectacle makers of 1608.