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The Full Width at Half Maximum, or "FWHM", is an important
measure for quantifying
image sharpness. For example, the FWHM is used to determine the size
of apertures for use in 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.
The pictures below compare the FWHM measured using a Radial Profile
plot for stars of different
brightness. Since all stars were taken from a small region of the
image, the point spread functions should be similar except for
scaling in brightness. Therefore, the FWHM values should all be the
same, within random variation due to noise. In this experiment, we used objects
covering a range of 13,200 ADU above background down to 11 ADU above
background (3 sigma's). Does your current software produce results
this accurate and consistent? |
