Starguide - A Practical Guide to Astro-Photography

HFR is a useful metric for determining your telescope's focus. It's important to note that HFR is only relative to other images of the same object through the same imaging train.

The half flux diameter or HFD is a definition used by astronomers to define the star size in an astronomical image. Mainly due to the seeing, stars are not imaged as a dot but spread out like a Gaussian shape.[1] The half flux diameter defines the diameter of a circle around the bright center in which half of the star flux or energy is contained. The other half of the flux is outside this circle. The half flux diameter unit is pixels.

Source: https://en.m.wikipedia.org/wiki/Half_flux_diameter)

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