The Half Flux Diameter (HFD) is a measure of the size of a star in a FITS image. Specifically, the HFD is the diameter of a circular aperture that contains half of the total flux or intensity of the star. In other words, it provides a measure of the size of the central part of the star's light distribution. A smaller HFD value indicates a sharper and more compact star, while a larger HFD suggests a broader and less well-defined star.


Analyzing the HFD in FITS images is valuable in assessing the quality of the image and the precision of the telescope's optics.


Please be aware that the HFD value for a star is exclusively computed by the ASTAP solver. If an alternative solver is utilized, no information pertaining to the HFD of a star will be presented.