Below is a list of frequently used abbreviations throughout the WFC documentation. FITS - Flexible Image Transport System (standard for file structure) FOT - Final Observation Tape (delivered by the SDC) GTI - Good Time Interval (used for selecting time periods) HEA - High Energy Astrophysics HEAD - HEA Data-handling HK - Housekeeping IFC - In-Flight Calibration (uses Radiation Sources) NFI - Narrow Field Instrument (any of LECS, MECS, HP-GSPS and PDS) NP - Nominal Pointing NPI - Nominal Pointing Interval OBT - On-Board Time OCC - Operations Control Centre (utilizes satellite links) OP - Observing Period (consisting of one or more observations) SAX - Astronomical X-ray Satellite SDC - Science Data Centre SPV - Sensor Performance Verification WFC - Wide Field Camera (any of WFC1 and WFC2)