(survey using lengthy observation to identify distant galaxies)
The phrase deep survey is generally used for
surveys carrying out lengthy observations
(long exposure time)
to identify very distant phenomena, e.g., galaxies and quasars
at high redshift. The observation time can be days or more,
typically focused on a region (often referred to as a deep field)
generally of no more than a square degree, often much less.
A field may be devised for the survey or the survey may reuse
an earlier-established deep field, e.g., to image it in more depth
or in a different part of the EMR spectrum,
or for spectrography.
Example deep surveys: