Worst-case error factor of the reconstructed heliocentric speed, ${\mathrm{err}}_{\alpha }^{max}$, against the opening angle of the detector, α, at a heliocentric distance of r = 1 au (Earth, blue lines) and r = 5.2 au (Jupiter, red lines) when the detector moves antiparallel (↿​⇂, solid lines) or parallel (↿​↾, dashed lines) to the ISD trajectory, and for Ulysses moving perpendicular to the ISD trajectory at its aphelion (black dash-dotted line) or perihelion (black dash-dot-dotted line). The ratio of the local ISD speed to the local escape speed, v_ISD/v_esc (horizontal dotted lines), and to the local circular orbit speed, v_ISD/v_crc (horizontal dash-dot-dotted lines), are given for comparison, taking into account only solar gravitation (β ≈ 0, Q/m ≈ 0). The vertical dotted line marks an opening angle of α_max = 70°. The error factor of the determination of the impact speed is not taken into account. The right panel features a zoomed-in view of the left panel.