The first generation of imaging cameras FOC, WFPC I, and spectrographs FOS and GHRS suffer presently from OTA spherical aberration at the primary mirror. After the Servicing Repair Mission of December 1993 and the installation of the corrector system COSTAR, HST should become diffraction limited at wavelengths throughout the visible spectrum.
The second generation wide-field CCD camera WFPC II has essentially the same field of view (FOV) and pixel size as WFPC I with an overall improved efficiency due to the lower read noise, improved charge transfer efficiency, and intrinsically uniform and stable flat field. The other second generation instruments, to be installed in the mid-90's, are the infrared imaging camera NICMOS and the imaging spectrograph STIS.
Finally, a third generation imaging instrument, the Advanced Camera (AC), is being proposed for operation around 1999 to replace the existing cameras, and to offer the benefits of new technologies - megasize CCD arrays of high sensitivity from UV to near IR wavelengths. Towards the end of this decade, very large aperture (8 to 10 meter) ground-based telescopes will become available, working with active and/or adaptive optics systems that correct some of the image blur caused by atmospheric distortion. In this paper, we examine how these advanced instruments compare for the imaging of large extended faint objects.