Details of H2O and dust production in P/Halley are mainly derived from amateur observations of cometary tails, leading to a gas/dust mass-ratio. The rotational period (2.21 day) is also found from amateur observations. Velocity and acceleration of the plasma and velocity and direction of the solar wind are also derived, together with size and indications of the mass-distribution function for dust particles.

These techniques have been applied to other comets, giving interesting results on differences between various comets. There is a lot of information available in cometary photographs. The coordination and archival storage of such information is urgently required.