def calcNeff(FWHMeff, platescale):
    """
    Calculate the effective number of pixels in a single gaussian PSF.
    This equation comes from LSE-40, equation 27.
    https://docushare.lsstcorp.org/docushare/dsweb/ImageStoreViewer/LSE-40
    @param [in] FWHMeff in arcseconds
       (the width of a single-gaussian that produces correct Neff for typical PSF profile)
    @param [in] platescale in arcseconds per pixel
    @param [out] the effective number of pixels contained in the PSF
    The FWHMeff is a way to represent the equivalent seeing value, if the
    atmosphere could be simply represented as a single gaussian (instead of a more
    complicated von Karman profile for the atmosphere, convolved properly with the
    telescope hardware additional blurring of 0.4").
    A translation from the geometric FWHM to the FWHMeff is provided in FWHMgeom2FWHMeff.
    """
    return 2.266 * (FWHMeff / platescale) ** 2
