Sketch of FilterDefinition and some examples

FilterDefinitionSketch.py

class FilterDefinitionCollection(collections.abc.Sequence):
    """A list of multiple `FilterDefinition`.

    Parameters
    ----------
    filters : `~collections.abc.Sequence` [`FilterDefinition`]
        The filters in this collection.
    """
    def __init__(self, *filters):
        self._filters = list(filters)
 
    def __getitem__(self, key): 
        return self._filters[key] 
 
    def __len__(self): 
        return len(self._filters) 
 
    def __repr__(self): 
        return "FilterDefinitionCollection(" + ', '.join(repr(f) for f in self._filters) + ')'


@dataclasses.dataclass(frozen=True)
class FilterDefinition:
    """The definition of an instrument's physical filters.
    This class is used as the "source of truth" for an instrument's filter
    defintions, including the information stored in `~lsst.afw.image.FilterLabel`,
    in the Gen 2 `~lsst.daf.persistence.CameraMapper`, and the Gen3
    ``physical_filter``/``abstract_filter`` `~lsst.daf.butler.Dimension`.
    """

    physical_filter: str

    band: str = None
    # NOTE: I don't have an immediate use for this, other than providing
    # useful information for those trying to understand what's in a Collection.
    # It could also be handled by inline comments in the FilterDefinitionCollection, but that seems less reliable?
    # Providing `doc` at least gives people a place to write notes?
    doc: str = None
    """Additional reference information about this filter."""

    def __repr__(self):
        labels = []
        if self.physical_filter is not None:
            labels.append(f"physical_filter={self.physical_filter!r}")
        if self.band is not None:
            labels.append(f"band={self.band!r}")
        return f"FilterDefinition({", ".join(labels)})"

    # NOTE: No more `defineFilter()` calls, as there's no longer an afw registry


# Examples

HSC_FILTER_DEFINITIONS = FilterDefinitionCollection(
    FilterDefintion(physical_filter="HSC-G", band="g"),
    FilterDefintion(physical_filter="HSC-R", band="r"),
    FilterDefintion(physical_filter="HSC-R2", band="r"),
    FilterDefintion(physical_filter="HSC-NB0387", band="N387"),
    # These two will require header remapping in astro_metadata_translator, at minimum to prepend "HSC-"
    FilterDefintion(physical_filter="UNRECOGNISED", doc="filter unknown during observation"),
    FilterDefintion(physical_filter="None", band="none", doc="No filter in use"),
)

DECAM_FILTER_DEFINITIONS = FilterDefinitionCollection(
    FilterDefintion(physical_filter="u DECam c0006 3500.0 1000.0", band="u"),
    FilterDefintion(physical_filter="g DECam SDSS c0001 4720.0 1520.0", band="g"),
    FilterDefintion(physical_filter="solid plate 0.0 0.0"),
    FilterDefintion(physical_filter="VR DECam c0007 6300.0 2600.0", band="VR", doc="very wide visible filter"),
)

# These are the names currently in obs_lsst; almost certainly not final
LSST_FILTER_DEFINITIONS = FilterDefinitionCollection(
    FilterDefintion(physical_filter="u", band="u"),
    FilterDefintion(physical_filter="g", band="g"),
    FilterDefintion(physical_filter="r", band="r"),
    FilterDefintion(physical_filter="i", band="i"),
    FilterDefintion(physical_filter="z", band="z"),
    FilterDefintion(physical_filter="y", band="y"),
    FilterDefintion(physical_filter="NONE", band="none", doc="No filter in use"),
)