FWHM

Abbreviation: Full Width Half Maximum. In this case, it is a measurement of the diameter of a star’s image understanding that the star’s image does not have a hard defined edge. In an astronomical image a star has a profile close to a Gaussian curve. Instead of trying to determine the edge of an ever decreasing value, measuring the diameter of the image at half of its maximum value provides a well defined number to compare image quality. The value can be used to compare observing conditions (seeing) as well as the quality of the telescope’s focus and collimation.

Illustration by Arne Nordmann (norro)
[CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]