Sparrow's resolution limit
When a star is observed with a telescope, the light is spread into an Airy disk. The resolution limit is defined as the closest separation of two stars that can still be perceived as separate by an observer, and is limited because the Airy disk causes the image of one star to merge with the other.
Rayleigh's resolution limit is reached when the two stars are separated by the radius of the Airy disk, but many astronomers say they can still distinguish the two stars even when they are closer than Rayleigh's resolution limit. Sparrow's Resolution Limit improves on this by saying that the ultimate resolution limit is reached when the combined image from the two stars no longer has a dip in brightness between them, but instead has a roughly constant brightness from the peak of one star's image to the other. But because of the extended image, it is still distinguishable from a single star.
Sparrow's resolution limit is about half Rayleigh's resolution limit. For example, for an eight-inch telescope, Rayleigh's resolution limit is 0.70 seconds of arc, but Sparrow's resolution limit is 0.35 seconds of arc.
Sparrow's resolution limit is also used for optical microscopes.
- C. M. Sparrow, 1916, ApJ, 44, 76
- Eugene Hecht, 2002, "Optics"
- Rainer Heintzmann & Gabriella Ficz, 2006, "Breaking the resolution limit in light microscopy", Briefings in Functional genomics, Vol. 5, pp 289–301.