In the visible bands, adaptive optics can only correct a relatively small field of view, whereas HST can conduct high-resolution optical imaging over a wide field. Only a small fraction of astronomical objects are accessible to high-resolution ground-based imaging; in contrast Hubble can perform high-resolution observations of any part of the night sky, and on objects that are extremely faint.[edit] UsageThe Hubble Space Telescope as seen from Space Shuttle Discovery during its second servicing mission (STS-82).To commemorate Hubble Telescope's 20th Birthday, NASA, with ESA and STI, released this Wide Field Camera 3 shot of a portion of Carina Nebula, dubbed 'mystic mountain'. Taken in February 2010, oxygen is colored blue, hydrogen and nitrogen green, and sulfur with redAnyone can apply for time on the telescope; there are no restrictions on nationality or academic affiliation.[106] Competition for time on the telescope is intense, and the ratio of time requested to time available (the oversubscription ratio) typically ranges between 6 and 9.[107]Calls for proposals are issued roughly annually, with time allocated for a cycle lasting approximately one year. Proposals are divided into several categories; 'general observer' proposals are the most common, covering routine observations. 'Snapshot observations' are those in which targets require only 45 minutes or less of telescope time, including overheads such as acquiring the target; snapshot observations are used to fill in gaps in the telescope schedule that cannot be filled by regular GO programs.[108]Astronomers may make 'Target of Opportunity' proposals, in which observations are scheduled if a transient event covered by the proposal occurs during the scheduling cycle. In addition, up to 10% of the telescope time is designated Director's Discretionary (DD) Time. Astronomers can apply to use DD time at any time of year, and it is typically awarded for study of unexpected transient phenomena such as supernovae.[109] Other uses of DD time have included the observations that led to the production of the Hubble Deep Field and Hubble Ultra Deep Field, and in the first four cycles of telescope time, observations carried out by amateur astronomers.[edit] Amateur observationsThe first director of STScI, Riccardo Giacconi, announced in 1986 that he intended to devote some of his Director Discretionary time to allowing amateur astronomers to use the telescope. The total time to be allocated was only a few hours per cycle, but excited great interest among amateur astronomers.[110]Proposals for amateur time were stringently peer reviewed by a committee of leading amateur astronomers, and time was awarded only to proposals that were deemed to have genuine scientific merit, did not duplicate proposals made by professionals, and required the unique capabilities of the space telescope. In total, 13 amateur astronomers were awarded time on the telescope, with observations being carried out between 1990 and 1997. One such study was Transition Comets UV Search for OH Emissions in Asteroids. The very first proposal, A Hubble Space Telescope Study of Post Eclipse Brightening and Albedo Changes on Io, was published in Icarus,[111] a journal devoted to solar system studies. After that time, however, budget reductions at STScI made the support of work by amateur astronomers