point spread function

The night sky looks calm, but in reality, invisible particles, cosmic rays, are constantly flying through us. Among them are electrons and their antimatter counterparts, positrons. When they collide with detectors on satellites or balloons, the instruments record their energy.

A camera looks at two distant sparks. They're so close together that they merge into a single fuzzy blob. It seems impossible to tell them apart — the classic limit of optical microscopy has been reached. But quantum mechanics hints that the light field still holds a secret about where each spark sits.

A flashlight beam cuts through fog. The denser the fog, the more the light scatters and the later it reaches your eye. A similar principle works with fast radio bursts — mysterious, super-powerful pulses from distant galaxies. As their radio waves travel through space, they interact with free electrons — particles stripped from atoms.

Photographing an exoplanet orbiting another star is like trying to spot a firefly in front of a spotlight. Earth's atmosphere is constantly churning, which makes stars appear as dancing speckles through a telescope.

The optical sky background is a cosmic palimpsest, where light from billions of unresolved galaxies is layered, each layer corresponding to a different epoch. Until now we could only distinguish the brightest "letters"—individual stars and nearby galaxies.

When an electric charge moves with acceleration, it emits electromagnetic radiation, i.e., light — a familiar phenomenon in physics. However, researchers have found that when moving along a specially chosen trajectory, the radiation acquires unusual properties.

Photographing the vicinity of a black hole is like shooting a fountain: a short exposure freezes the droplets, a long one blends everything into blurry streaks. Light from the superheated gas swirling around the hole reaches us along different paths. Some rays curve around it in an arc, others make multiple loops due to spacetime curvature.

A solar telescope is like an eye trying to spot an ant through a pot of boiling soup. The daytime sky dazzles, the air shimmers, and the image swims. To freeze it, you need adaptive optics — a system that predicts distortions and corrects them hundreds of times per second.

Any light measurement runs into a fundamental limit: photons hit the camera sensor unevenly. This natural ripple (shot noise) blurs fine details, like trying to pick out a quiet instrument in an orchestra through the noise of the hall.

Light passing through a turbid medium like milk or living tissue scatters in all directions. Reconstructing where light was absorbed more strongly is like guessing the shape of a fallen object from a puddle on the asphalt. Optical tomography solves this problem, but usually requires large computational resources.

Astronomers, like chefs, have long been preparing a description of the Universe using the same cumbersome recipes. These recipes — complex mathematical formulas — estimated how many galaxies of different luminosity and mass fill the cosmos, as well as how the invisible frameworks of dark matter are distributed. But the dishes lacked accuracy.

Hidden in cold interstellar gas clouds are long, thin filaments, like threads in a tangle. It is within these filaments, as they collapse, that new stars are born. Astronomers selected seven such clouds, located between 140 and 920 light-years away, and counted how many filaments of various thicknesses each contains.

A conventional light microscope hits a fundamental limit: it cannot distinguish two points closer than 200 nanometers — roughly the thickness of a soap bubble divided by a hundred. To see deeper, scientists have made molecules in the sample blink or fade, but these tricks are slow and harmful to living cells.

Light from a distant star, after passing through air and telescope lenses, loses its sharpness, like looking at a streetlamp through wet glass. Astronomers call this blur the telescope's "fingerprint": each instrument has its own, and you need to know it down to the smallest detail.

When a bright spot flares up on a star, astronomers want to know how much energy was released into space. They used to picture a flare as a growing campfire: the area of the fire increases, but the heat remains the same. That made calculations simpler, but reality turned out to be different.

When an X-ray burst erupts on a neutron star, its atmosphere heats up to millions of degrees. The light we catch with telescopes passes through this dense layer of gas, and gets dramatically altered along the way. The spectrum no longer looks like that of an ideal radiator: some colors get amplified, others dim.

In the dark clouds of interstellar gas where stars are born, matter gathers into dense knots — future cradles of stars. Astronomers measure distances between these knots from sky photographs, but a photograph is flat while the cloud is volumetric. Converting from plane to volume is not as simple as it seems.

A light beam sent from Earth to a satellite scatters in the atmosphere, like a flashlight in fog. To keep the signal intact, you need short-wavelength light that spreads less. Such light is born in special crystals, but shrinking them to chip size is tough.

Astronomers read the history of galaxies by their portraits. Previously, they used either lengthy analysis of light by colors or quick filtered snapshots, losing the details of shape.

Fast radio bursts are brilliant cosmic flashes that, like a starting signal, set off a race of radio waves across the universe. Astronomers used them as a natural laboratory to test whether the photon, the quantum of light and the carrier particle of the electromagnetic interaction, has mass.