A Sociological Study of the Optically Emitting Isolated Neutron Stars
  Although less than 1% of all radio pulsars are detected at optical wavelengths, their optical emission can yield a wealth of information that is either very difficult or plainly impossible to obtain at other wavelengths. 
P.A. Caraveo (Istituto di Fisica Cosmica del CNR, Milano, Italy, also   Istituto Astronomico, Roma, Italy)