To begin with let’s make a simple effort to describe the term Full width at half maximum in simple terms. This term is used to measure the width of an object that does not have sharp edges. We can have specific dimensions for a box which will have height width and volume also. Other one dimensional stuff such as a square or rectangle on a paper will have a specific length and breath. But an object such as a star in the universe may not be as concrete as a box. And when we are measuring, we just measure its image. This measurement is closer to a Gaussian curve and can be described as the distance between points on the curve at which the function reaches half its maximum value. Usually quoted in nanometres, it can also be measured as a velocity by applying the Doppler Effect.
Now lets begin to understand in a brief way what solar astronomy is all about and what all do we measure in solar astronomy. Astronomy is the study about all celestial objects and the phenomenon that takes place in the universe outside the boundaries of earth. Solar astronomy revolves around the study of the Sun and its solar system.
When there is no atmosphere, a small star would have a round about size in a telescope image caused by the diffraction which then would be inversely proportional to the diameter of the telescope. However when light enters the atmosphere the different temperature layers and different wind speeds distort the light waves. This leads to distortions in the image of the star. The effects of the atmosphere can be modeled as rotating cells of air moving turbulently. At most observatories the turbulence is only significant on scales larger than r0 (r0 is 1020 cm at visible wavelengths under the best conditions) and this limits the resolution of telescopes to be about the same as given by a space-based 1020 cm telescope.
The distortion changes at a high rate, typically more frequently than 100 times a second. In a typical astronomical image of a star with a limited exposure time of seconds or even minutes, the different distortions average out as a filled disc called the point spread function. The diameter of the seeing disk, most often defined as the (Full width at half maximum), is a measure of the astronomical seeing conditions.
It follows from this definition that what different people see always varies greatly due to difference in place to place, difference in times etc. Astronomers often mention “good” nights with a low average seeing disc diameter, and “bad” nights where the seeing diameter was so high that all observations were of no use.
The Full Width at Half Maximum of the seeing disc is usually measured in arc seconds, abbreviated with the symbol (“). A 1.0″ seeing is a good one for average astronomical sites. The seeing of an urban environment is usually much worse and with obvious reasons, too much powerful light and pollution add to the cause. Good seeing nights tend to be clear, cold nights without wind gusts. Warm air rises degrading the seeing as does wind and clouds. At the best high-altitude observatories the wind brings in steady air which has not previously been in contact with the ground, sometimes providing seeing as good as 0.4”.