What criteria are used to define mosaics? What is the naming convention?
Unlike visits, which are defined by the HST scheduling process and do not change, the definition of mosaics can change as more data are obtained in the same region of the sky. The definition of mosaics is based on “pointings”, or groupings of image data that mutually overlap to cover a connected area of the sky. Pointings are instrument-specific; for simplicity, at this time they are identified by a single number. For this preliminary release, additional constraints have been imposed to simplify the processing: the data must be taken within a 4-month interval, individual visits must contain enough images to ensure that cosmic-ray identification can be carried out on for each visit separately, and all exposures must overlap each other to simplify the alignment. A list of the mosaics for which data are available at the time of this release can be obtained by doing an all-sky search for Level 3 data for ACS (selection available under the Advanced Search options). All mosaic data contain t
