def set_gauss(fwhm):
    """
    Compute the sigma value, given Full Width Half Max.
    Returns an operand string

    Parameters
    ----------

    fwhm : float

    Returns
    -------

    op_string : string

    """

    sigma = float(fwhm) / 2.3548

    op_string = "-kernel gauss %f -fmean -mas " % sigma + "%s"

    return op_string