Handedness of the orthonormal frame whose basis vectors resolve a complex spin-2 ellipticity, or reduced shear, into its two components.

Every sky position in NumCosmo is given as right ascension and declination, so the position angle is always measured in the celestial convention, with $\phi$ increasing eastward. This enum does not change how the angle is measured; it selects the basis onto which the components are projected:

• NC_WL_ELLIPTICITY_FRAME_CELESTIAL: the holonomic frame of the celestial

  convention, position angle increasing eastward. Taken as the reference frame, that is the identity.

• NC_WL_ELLIPTICITY_FRAME_CARTESIAN: the frame an observer on the ground

  sees when facing the object with their head pointing to the North celestial pole. The right-hand direction then points East, so the image $x$-axis increases towards the West, that is towards decreasing RA, giving the opposite handedness. This is the image-plane view that NcGalaxyWLObsCoord calls EUCLIDEAN.

The two frames differ by a parity flip. The position angle reverses sense, $$ \phi_E = \pi - \phi_C \quad (\equiv -\phi_C \bmod \pi), $$ and the spin-2 ellipticity transforms by complex conjugation, so its cross component changes sign, $$ \epsilon_E = \epsilon_C^{*} . $$ Since $e^{2 i \phi_E} = e^{-2 i \phi_C}$, flipping $\phi$ and conjugating $\epsilon$ are the same operation. The parity is its own inverse, so a single map converts CELESTIAL to CARTESIAN and CARTESIAN to CELESTIAL.

The integer values match the legacy NcGalaxyWLObsCoord (CELESTIAL = 0, EUCLIDEAN = CARTESIAN = 1), so previously serialized data is read back unchanged.