def kmax_perpendicular(d_comoving, sigma_beam):
    """Calculate maximum k (across sky) from angular resolution

    Parameters
    ----------
    d_comoving : 
        comoving radial distance (pre-calculated from redshift) [Mpc]
    sigma_beam : 
        arc length of beam width [radians]
    """
    kmax_perp = 1. / (d_comoving * sigma_beam)

    return kmax_perp