The current ability of HST to reveal morphological structure in medium-redshift galaxies

A pair of FOC f/48 images was taken on a compact galaxy with z∼0.3; and a neighboring field, including a number of medium-redshift galaxies, was covered with the WFC. The conclusions are as follows: (1) The FOC48 (whose field is unlikely to include more than the single galaxy that is targeted) gives a good resolved image of a compact galaxy at blue magnitude J = 20.5 in a single-orbit exposure; longer exposures would (a) add detail to compact galaxies, or (b) allow morphological classification of objects of lower surface brightness. (2) WFC images have a survey capability that can include many galaxies per field, with sufficient resolving power to distinguish clearly between galaxies and stars down to the level of 0.″2 (FWHM), depending on signal-to-noise ratio, and a reasonable capacity for morphology. (3) Although some morphological detail can be discerned in even the aberrated images, deconvolutions very much enhance the ability to see structural detail. Even at the low S/N that is provided by single-orbit exposures the more sophisticated restoration methods offer some advantage over simple Fourier or Lucy techniques. (4) Where image restorations are involved, it is of great importance to have more than one image of each field, so as to compare results and see what is real, and also to facilitate the removal of cosmic rays.