Lock-in image subtraction: detectability of circumstellar planets with the large space telescope

Faint circumstellar features in the close vicinity of stars or qso's are of considerable astrophysical interest since they can provide important informations on stellar parameters and evolutions: stellar companions give mass information; coronae many of which are likely to be much brighter than the sun's give information on magnetic fields and fast varying phenomena at stellar surfaces; planetary nebulae may provide information on the birth of planets; disk structures similar to Saturn's rings provide checks on theories which predict their frequent occurence; planets themselves of course are worth appreciable detection efforts since they may ultimately provide hints on the existence of extra-terrestrial civilisations. In this article, the authors discuss coronagraphic and lock-in detection techniques applicable in the near future to observations with the Large Space Telescope. Those techniques enhance the visibility of circumstellar features to such a degree that even planets should become marginally detectable in some favorable cases. (Auth.)