Keywords: class-filler

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# The Sic API

The historical way (and may be easiest for a CLASS user) to produce spectra is to do it directly in the CLASS program itself. The appendix reproduces a demonstration procedure provided with CLASS. It can be called at the CLASS prompt by typing:

    LAS90> @ gag_demo:demo-telwrite.class

This procedure performs 4 main tasks:

1. it opens a new output CLASS file. The `SINGLE` mode is intended to be used when writing independent observations (e.g. at the telescope),
2. it disables all sections in the R buffer header, except the ones written in this example. The sections General, Position and Spectroscopy (resp. Drift) are the 3 minimum to be filled when writing a spectroscopic observation (resp. a continuum drift).
3. if fills the data and desired sections. The command `MODEL` is used to transfer the channel intensities to the R buffer, and the command `LET` is used to set up the sections.
4. finally it writes the content of the R buffer from the memory to the output file thanks to the command `WRITE`.

A loop is also performed in order to write a hundred of observations to the file. Useful help can be found in the the online HELP of the following commands:

- `FILE`: open a CLASS file for writing.
- `SET VARIABLE`: turn the requested section read-write. See `HELP SET VARIABLE` for a detailed description of each section.
- `MODEL`: store a spectrum in the R buffer,
- `WRITE`: write the R spectrum to the output file.

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