Allow un-setting aux WCS parameters

CHANGES.rst

@@ -70,7 +70,7 @@ astropy.tests
 astropy.time
 ^^^^^^^^^^^^
 
-- Add new time format ``unix_ptp`` (Precision Time Protocol) which provides 
+- Add new time format ``unix_ptp`` (Precision Time Protocol) which provides
   the number of SI seconds since ``1970-01-01T00:00:00 TAI``. [#11143]
 
 astropy.timeseries
@@ -1152,6 +1152,9 @@ astropy.visualization
 astropy.wcs
 ^^^^^^^^^^^
 
+- Allow "un-setting" of auxiliary WCS parameters in the ``aux`` attribute of
+  ``Wcsprm``. [#11166]
+
 
 Other Changes and Additions
 ---------------------------

astropy/wcs/src/wcslib_auxprm_wrap.c

@@ -101,6 +101,9 @@ static PyObject* PyAuxprm_get_rsun_ref(PyAuxprm* self, void* closure) {
 static int PyAuxprm_set_rsun_ref(PyAuxprm* self, PyObject* value, void* closure) {
   if(self->x == NULL) {
     return -1;
+  } else if (value == Py_None) {
+    self->x->rsun_ref = UNDEFINED;
+    return 0;
   } else {
     return set_double("rsun_ref", value, &self->x->rsun_ref);
   }
@@ -117,6 +120,9 @@ static PyObject* PyAuxprm_get_dsun_obs(PyAuxprm* self, void* closure) {
 static int PyAuxprm_set_dsun_obs(PyAuxprm* self, PyObject* value, void* closure) {
   if(self->x == NULL) {
     return -1;
+  } else if (value == Py_None) {
+    self->x->dsun_obs = UNDEFINED;
+    return 0;
   } else {
     return set_double("dsun_obs", value, &self->x->dsun_obs);
   }
@@ -133,6 +139,9 @@ static PyObject* PyAuxprm_get_crln_obs(PyAuxprm* self, void* closure) {
 static int PyAuxprm_set_crln_obs(PyAuxprm* self, PyObject* value, void* closure) {
   if(self->x == NULL) {
     return -1;
+  } else if (value == Py_None) {
+    self->x->crln_obs = UNDEFINED;
+    return 0;
   } else {
     return set_double("crln_obs", value, &self->x->crln_obs);
   }
@@ -149,6 +158,9 @@ static PyObject* PyAuxprm_get_hgln_obs(PyAuxprm* self, void* closure) {
 static int PyAuxprm_set_hgln_obs(PyAuxprm* self, PyObject* value, void* closure) {
   if(self->x == NULL) {
     return -1;
+  } else if (value == Py_None) {
+    self->x->hgln_obs = UNDEFINED;
+    return 0;
   } else {
     return set_double("hgln_obs", value, &self->x->hgln_obs);
   }
@@ -165,6 +177,9 @@ static PyObject* PyAuxprm_get_hglt_obs(PyAuxprm* self, void* closure) {
 static int PyAuxprm_set_hglt_obs(PyAuxprm* self, PyObject* value, void* closure) {
   if(self->x == NULL) {
     return -1;
+  } else if (value == Py_None) {
+    self->x->hglt_obs = UNDEFINED;
+    return 0;
   } else {
     return set_double("hglt_obs", value, &self->x->hglt_obs);
   }

astropy/wcs/tests/test_auxprm.py

@@ -139,3 +139,36 @@ def test_set_aux_on_empty():
     assert_allclose(header['CRLN_OBS'], 10.)
     assert_allclose(header['HGLN_OBS'], 30.)
     assert_allclose(header['HGLT_OBS'], 40.)
+
+
+def test_unset_aux():
+    w = WCS(HEADER_SOLAR)
+
+    assert w.wcs.aux.rsun_ref is not None
+    w.wcs.aux.rsun_ref = None
+    assert w.wcs.aux.rsun_ref is None
+
+    assert w.wcs.aux.dsun_obs is not None
+    w.wcs.aux.dsun_obs = None
+    assert w.wcs.aux.dsun_obs is None
+
+    assert w.wcs.aux.crln_obs is not None
+    w.wcs.aux.crln_obs = None
+    assert w.wcs.aux.crln_obs is None
+
+    assert w.wcs.aux.hgln_obs is not None
+    w.wcs.aux.hgln_obs = None
+    assert w.wcs.aux.hgln_obs is None
+
+    assert w.wcs.aux.hglt_obs is not None
+    w.wcs.aux.hglt_obs = None
+    assert w.wcs.aux.hglt_obs is None
+
+    assert str(w.wcs.aux) == 'rsun_ref:\ndsun_obs:\ncrln_obs:\nhgln_obs:\nhglt_obs:'
+
+    header = w.to_header()
+    assert 'RSUN_REF' not in header
+    assert 'DSUN_OBS' not in header
+    assert 'CRLN_OBS' not in header
+    assert 'HGLN_OBS' not in header
+    assert 'HGLT_OBS' not in header