def stis_psf(x, a):
    """Evaluate the cross-dispersion PSF at x.

    Parameters
    ----------
    x : float
        offset in pixels from the center of the profile
    a : float
        a measure of the width of the PSF

    Returns
    -------
    val : float
        the PSF evaluated at x

    """
    return (1.0 + (x / float(a)) ** 2) ** -2
