What is meant by angular resolution?

Angular resolution refers to the ability of an optical system to distinguish or separate two closely spaced objects.

In more detail, angular resolution, also known as the resolving power, is a measure of the smallest angle between two objects that a system, such as a telescope, microscope, or camera, can distinguish as separate entities. It is typically measured in degrees, arcminutes, or arcseconds. The smaller the value, the better the resolution, meaning the system can distinguish between objects that are closer together.

The concept of angular resolution is crucial in fields such as astronomy, where it determines the level of detail that can be observed in distant celestial bodies. For instance, a telescope with a high angular resolution can distinguish the individual stars in a galaxy, while one with a lower resolution may only see the galaxy as a single point of light.

Angular resolution is determined by the wavelength of the light being observed and the size of the aperture through which the light is collected. According to the Rayleigh criterion, the minimum resolvable angle is approximately equal to the ratio of the wavelength to the aperture diameter. This means that to increase the angular resolution, one can either decrease the wavelength (use light with a higher frequency) or increase the aperture size.

However, practical limitations often exist. For example, increasing the aperture size of a telescope makes it larger, heavier, and more difficult to manage. Similarly, observing light at higher frequencies may require more advanced technology or may be limited by atmospheric absorption.

In addition, the atmosphere itself can limit the angular resolution achievable from the ground, due to the blurring effects of atmospheric turbulence. This is known as the 'seeing limit'. To overcome this, astronomers use techniques such as adaptive optics, or place telescopes in space, above the Earth's atmosphere.

In summary, understanding and optimising angular resolution is key to maximising the amount of detail that can be observed in a given optical system.