How “HISTORY” and “COMMENT” FITS Keys are Handled in sunpy#
In FITS files, there are often multiple entries for both the “HISTORY” and “COMMENT” keys.
For example, when applying a prep routine to an image, a “HISTORY” entry may be added to the FITS header for every step in the prep pipeline.
Because the metadata associated with each GenericMap acts like a dictionary, where each key must be unique, these repeated “HISTORY” and “COMMENT” keys cannot be represented as separate entries in MetaDict.
Thus, when a FITS file with multiple “HISTORY” keys is read into a Map object, all the values corresponding to “HISTORY” are joined together with newline characters \n and stored as a single “history” entry in MetaDict.
When writing the resulting Map to a FITS file, “history” is split along the \n characters and each entry is written to a separate “HISTORY” key in the resulting FITS header.
The same is true for “COMMENT” keys.