# Added elements when missing

The purpose of adding elements in the HDUs when they are missing is that the libmrtcal does not have to worry about this element existing or not, and and doing different things (different code) in one case or the other. The following elements are added:

- POLEX and POLEY are absent under V1.35: they are defined and default to 0,
- SYSOFF, XOFFSET, and YOFFSET columns in the Scan table can have 0, 1, or 2 rows. They are forced to 2 rows providing Nasmyth and projection offsets (default to 0 if they were absent),
- the LINENAME column in the BackEnd HDU is built from the LINENAME column in the FrontEnd HDU for IMB-FITS version lower than 2, or for 4MHz backend,
- MJD_BEG and MJD_END keywords are added as convenient duplicates of DATE-OBS and DATE-END keywords in all HDUs of each subscan. They offer MJD values instead of ISO strings,
- the column TSTAMPED is added to BackendDATA tables in IMB-FITS version lower or equal to 1.35. Its defaults to 1. Remember that this value is sensitive (it introduces a shift on the time stamps) and is actually backend-dependent.
- an IFRONT column is added as a convenient backpointer from the BackEnd table to the FrontEnd table, i.e. each chunk knows easily to which receiver it is associated.