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# Chunks and chunksets

For the processing of the IMBFITS files, MRTCAL introduces the concept of chunks (see Fig ). One dump of the IMBFITS `DATA` column is a collection of such chunks. MRTCAL can make no assumption on the way they are ordered (this comes from ???). The `PART` column indicates to which spectrum each chunk belongs (spectra are identified with integer numbers). Each ``set of chunks belonging to the same final spectrum'' is called a chunkset in the MRTCAL nomenclature. From one chunkset, MRTCAL will produce one spectrum1.1.

The chunks in each `DATA` row can belong to several spectra (e.g., 8 for FTS), and by definition to as many chunksets. MRTCAL maps those chunksets internally as a `chunkset_1D` Fortran structure, i.e., an array of chunksets.

The `DATA` column has as many rows as the number of pixels of the receiver times the number of time dumps. This introduces 2 new dimensions which are used to map all1.2, the `DATA` column into a `chunkset_3D` Fortran structure. The 2 new dimensions here are pixels and time (see Fig ).

For its internal needs, MRTCAL has to refer to a single time dump in the 3rd dimension. It can also time-average this 3rd dimension. In those 2 use-cases, the final product is then available through a `chunkset_2D` Fortran structure.

Figure: How one dump in the IMBFITS `DATA` column is divided into chunks, gathered by family (chunksets), and reordered to produce the spectra.

Figure: How all the dumps in the IMBFITS `DATA` column is divided into 3D chunksets. After this, they can be time-averaged, or can be accessed one by one.

Subsections

- Chunks slicing
- Polarimetry

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