MRTCAL indexes IMB-FITS files in the so-called index files. A raw index file named index.mrt can be produced with the command INDEX BUILD (see command help for more advanced uses). When one or more index files are then reopened for reading, the command INDEX OPEN builds a memory index named the ``Input indeX'' (IX). At this stage, IX gathers the summary of all entries in all opened index files. From this full set, the command MFIND can be used to make a selection of desired entries: this is the ``Current indeX'' (CX).
While the primary purpose of the index files is to reference IMB-FITS files, their other advantage is to store other products that can be derived from the IMB-FITS files. In particular, after the calibration, the commands CALIBRATE and PIPELINE will save new versions of each calibrated entry. These new versions have a modified calibration status (from none to failed or done), and optionally a calibration section added (storing the calibration results). These modified versions of the entries are themselves saved in an index file (either the original one, or a new one, depending on the user choice), AND are implicitly appended to IX. After the calibration process, it is then easy for the user to select from IX the calibrated entries (or failed, or pending, etc). The figure 4 shows a basic example of these steps.