This is the GMT time, to the nearest second, of the start of the observation. If the day is omitted, the first day of the month is assumed. If a time is omitted, then any time for that day will match. This operator is inclusive on the first date and exclusive on the second. Finally, you can search on a list of dates or date ranges.
Null entries imply a raw density image was obtained.
Note that for each observation, several dat sets may be produced representing different stages of processing. The final linearized undistorted dat set may be stored using a file name such as fuv2349g.fit. The UIT instrument included two six-position filter wheels. Your choice of what columns you want to see in the output. There are two radio buttons that let you select either the default set of columns, or choose your own set of columns.