The StellarMate Plus Manual
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In order to focus an image, Ekos needs to establish a numerical method for
gauging how good your focus is. It's easy when you look at an image and can see
it as unfocused, as the human is very good at detecting that, but how can Ekos
possibly know that?
There are multiple methods. One is to calculate the Full Width at Half Maximum
(FHWM) of a star profile within an image, and then adjust the focus until an
optimal (narrower) FWHM is reached. The problem with FWHM is that it assumes
the initial focus position to be close to the critical focus. Additionally, FWHM
does not perform very well under low-intensity fluxes. An Alternative method is
Half-Flux-Radius (HFR), which is a measure of the width in pixels counting from
the center of the stars until the accumulated intensity is half of the total flux of
the star. HFR proved to be much more stable in conditions where you might have
unfavorable sky conditions, when the brightness profile of the stars is low, and
when the starting position of the focus is far from the optimal focus.
After Ekos processes an image, it selects the brightest star and starts measuring
its HFR. It can automatically select the star, or you can select the star manually. It
is usually recommended to select stars that are not too bright as they might get
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