Chained WCS -

Model spectrograph plus model fit to the residuals (GWCS).

Support for chaining additional co-ordinate transformations such as detector mosaicking would allow determination of wavelengths in the raw data, which is useful for various purposes such as accurate flat fielding, normalization of QE variations between detectors and Kelson-style sky subtraction

The issue names no files, tests, or entry points. Start by clarifying the required GWCS chaining design and how detector mosaicking fits with spectrograph and residual model transforms. Done means additional coordinate transformations can be chained and wavelengths can be determined in raw data.

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