# Patching IMB-FITS at read time

MRTCAL is divided in 2 main libraries:

1. the libimbfits reads the IMB-FITS files and fills a memory structure from it,
2. the libmrtcal gets the libimbfits memory structure, and use it to index the files and calibrate them.

When MRTCAL has to deal with several versions of the IMB-FITS (e.g. V1.35 for HERA and V2 for EMIR), the variations are hidden in the libimfits so that the libmrtcal does not have to worry about those details. This means that the memory structure is unique while the disk structure differ7.1. There are also other cases when we want to fill memory structures that differ from the file on disk. All this is described hereafter.

Note that each patch is saved as a warning in the comment field of each element. Those warning are visible when using the command MDUMP.

Subsections

- Added elements when missing
- Modified elements