Associated with each each image is a set of 100 72-byte header, or label, records. These header records are generated automatically by the IUE operations ground system software during image acquisition and readout. The header contains spacecraft scientific and engineering data, an event "round-robin", information provided by the observer about the object, and comments about the observation. The time-tagged event "round-robin" section documents the sequence of procedures used to slew the spacecraft, acquire the target, start the exposure, and read the image. The label is appended in a sequential manner by IUESIPS to record significant processing parameters. Turnrose and Thompson (1984, Section 9) and Van Steenberg (1989) describe the image header contents. During the first year or so of the IUE mission, the ground system science header software was not fully operational. Consequently, very early IUE images have incomplete headers.
Occasionally a problem occurs with archiving an image during real-time operations and the spectral data must be recovered from the history tapes which record the spacecraft telemetry stream. Unfortunately, since the science header is generated by the ground system and is not contained in the telemetry stream, the image header cannot be recreated in its entirety from the history tapes. Images recovered and archived in this manner have incomplete information in the header and are usually identifiable by the phrase "history replay" or "history tape image recovery" in the comments (the first five to seven lines of the header). Other errors in the header information can occur, so it is important to check critical data (such as exposure time) against other records and the IUE merged log.