Convert fwhm_eff to fwhm_geom. This conversion was calculated by Bo Xin and Zeljko Ivezic (and will be in an update on the LSE-40 and overview papers).
- fwhm_eff: `float`
the single-gaussian equivalent FWHM value, appropriate for calc_neff, in arcseconds
FWHM geom, the geometric FWHM value as measured from a typical PSF profile in arcseconds.