subarcsecond

Subarcsecond refers to angular sizes smaller than one arcsecond, where an arcsecond is 1/3600 of a degree. In astronomy, subarcsecond-resolution imaging reveals details finer than 1 arcsecond on the sky, enabling the separation of closely spaced objects or the characterization of small-scale structures in distant systems.

The actual physical size corresponding to a subarcsecond angle depends on distance. For example, at a distance of about 1 kiloparsec, 1 arcsecond subtends roughly 0.005 parsecs (about 1000 astronomical units). At the distances of other galaxies, subarcsecond features correspond to much larger physical scales, while in nearby star-forming regions they can probe protoplanetary disks and jet structures.

Achieving subarcsecond resolution relies on overcoming diffraction limits and atmospheric blurring. The diffraction limit of a telescope is approximately theta ≈ 1.22 lambda / D, in radians, where lambda is the observing wavelength and D is the aperture diameter. In visible light (~500 nm), a 2-meter telescope yields about 0.05 arcseconds, while a 10-meter telescope reaches ~0.01 arcseconds. Ground-based images are often blurred by Earth's atmosphere to around 0.5–1.5 arcseconds, but adaptive optics can restore near-diffraction-limited performance on large telescopes. Space-based observatories, such as the Hubble Space Telescope, avoid atmospheric seeing and routinely achieve subarcsecond resolution.

Interferometry further extends reach by combining light from widely separated telescopes. Optical/infrared interferometers and radio Very Long Baseline Interferometry can achieve milliarcsecond or even microarcsecond scales, enabling extremely fine measurements for stars, disks, and active galactic nuclei. Subarcsecond imaging remains a cornerstone of contemporary observational astronomy.