Handedness of the orthonormal frame whose basis vectors are used to 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 ($\phi$ increases eastward, i.e. with increasing RA). This enum does not change how the angle is measured; it only selects the basis onto which the spin-2 ellipticity components are projected:

• NC_WL_ELLIPTICITY_FRAME_CELESTIAL: the holonomic frame of the celestial

  convention. The position angle $\phi_C$ increases eastward. Taken as the reference frame (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 (decreasing RA) — the opposite handedness. This is the “Euclidean” image-plane view (the legacy name was EUCLIDEAN).

The two frames differ by a parity (handedness) 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, i.e. its second/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 -> CARTESIAN and CARTESIAN -> CELESTIAL alike.

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