update CHANGELOG

CHANGELOG.md

@@ -1,5 +1,17 @@
 # HEAD
 
+-   **Breaking change**: Change to the pointing API [#358](https://github.com/litebird/litebird_sim/pull/358), in detail:
+
+    1. DetectorInfo has three new attributes: pol_angle_rad (polarization angle), pol_efficiency (polarization efficiency)and mueller_hwp (mueller matrix of the HWP).
+
+    2. get_pointings() return only the orientation ψ of the detector, the polarization angle is a separate variable stored in the `Observation` class. The same class also handles the mueller_hwp for each detector, and it has a new bool variable `has_hwp` that is set to true if an HWP object is passed to `prepare_pointings()`.
+
+    3. The mock vPTEP IMo has been updated accordingly.
+
+    4. The `HWP` class has a new field called mueller, that contains the mueller matrix of the HWP.
+
+    5. The function `scan_map()` now handles three possible algebras: (i) no HWP, (ii) ideal HWP, (iii) generic optical chain.
+
 -   Implementation of distributing detectors across the MPI processes by grouping them according to given attributes [#334](https://github.com/litebird/litebird_sim/pull/334)
 
 -   **Breaking change**: `PointingSys()` now requires `Observation` as an argument. And several functions to add pointing systematics are merged into `left_multiply_syst_quats()`.

litebird_sim/observations.py

@@ -203,6 +203,9 @@ def __init__(
 
         self.pointing_provider = None  # type: Optional["PointingProvider"]
 
+        # By default this is set to False, prepare_pointings() can change its value
+        self.has_hwp = False
+
     @property
     def sampling_rate_hz(self):
         return self._sampling_rate_hz