Measuring FWHM

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The Full Width at Half Maximum, or "FWHM", is an important measure for quantifying image sharpness. This feature is available in Mira Pro x64 and all Mira Pro platforms. One application of FWHM is for determining apertures size for aperture photometry. FWHM is also important for characterizing the size of the point spread function as a measure of  focus, atmospheric turbulence, or optical performance. Although FWHM is a mathematically well-defined quantity, it is difficult to measure accurately in real data. FWHM is measured from the image of a point source such as a star or a specular highlight reflection. A common shortfall of imaging software is to produce accurate estimates of the FWHM, especially noticeable in the variations reported for bright and faint sources. The technique Mira uses involves fitting a Gaussian + Constant model to data around the source. This method leads to scientifically accurate results, independent of brightness as they should be. Mira provides two ways to estimate the FWHM: You can directly measure and tabulate FWHM and other parameters by clicking on a point source, or you can measure it indirectly using a Radial Profile plot. The Radial Profile technique shows data from all azimuths around the point source. This method gives visual verification that the FWHM value is correct for the data.