Use obstime from observer if obstime not otherwise provided

sunpy/coordinates/frames.py

@@ -133,6 +133,10 @@ def __init__(self, *args, **kwargs):
         if not kwargs.pop('wrap_longitude', True):
             self._wrap_angle = None
 
+        # If obstime is not provided but observer has an obstime, use that as the obstime
+        if 'obstime' not in kwargs and 'observer' in kwargs and kwargs['observer'].obstime is not None:
+            kwargs['obstime'] = kwargs['observer'].obstime
+
         super().__init__(*args, **kwargs)
 
         # If obstime is specified, treat the default observer (None) as explicitly set

sunpy/coordinates/tests/test_frames.py

@@ -231,6 +231,14 @@ def test_hpc_low_precision_float_warning():
         hpc.make_3d()
 
 
+def test_hpc_obstime_from_observer():
+    # Test that observer.obstime is used for obstime if obstime is not provided
+    observer = HeliographicStonyhurst(obstime='2023-09-08')
+    hpc = Helioprojective(observer=observer)
+
+    assert hpc.obstime == observer.obstime
+
+
 # ==============================================================================
 # ## Heliographic Tests
 # ==============================================================================