def _calculate_PSF_amplitude(mag):
    """
    Returns the amplitude of the PSF for a star of a given magnitude.

    Parameters
    ----------
    `mag`: float
        Input magnitude.

    Returns
    -------
    amp : float
        Corresponding PSF applitude.
    """

    # mag/flux relation constants
    amp = 10**(-0.4*(mag - 12))*1.74e5
    return amp