Fix some things found in pyuvsim testing

pyuvdata/uvdata/initializers.py

@@ -329,7 +329,7 @@ def new_uvdata(
         telescope name and location, x_orientation and antenna names, numbers
         and positions.
     antpairs : sequence of 2-tuples of int or 2D array of int, optional
-        Antenna pairs in the data. If an ndarray, must have shape (Nants, 2).
+        Antenna pairs in the data. If an ndarray, must have shape (N antpairs, 2).
         These may be the *unique* antpairs of the data if
         each antpair observes the same set of times, otherwise they should be an
         Nblts-length array of each antpair at each time. It is recommended
@@ -522,8 +522,11 @@ def new_uvdata(
     )
 
     if antpairs is None:
+        bl_order = ("ant1", "ant2")
         antpairs = list(combinations_with_replacement(telescope.antenna_numbers, 2))
         do_blt_outer = True
+    else:
+        bl_order = None
 
     (
         nbls,
@@ -541,6 +544,19 @@ def new_uvdata(
         time_axis_faster_than_bls=time_axis_faster_than_bls,
         time_sized_arrays=(lst_array, integration_time),
     )
+
+    if not do_blt_outer:
+        if time_array.size != antpairs.shape[0]:
+            raise ValueError("Length of time array must match the number of antpairs.")
+
+    if bl_order is not None and blts_are_rectangular:
+        if time_axis_faster_than_bls:
+            blt_order = ("time", "ant1")
+        else:
+            blt_order = ("ant1", "ant2")
+    else:
+        blt_order = None
+
     baseline_array = get_baseline_params(
         antenna_numbers=telescope.antenna_numbers, antpairs=antpairs
     )
@@ -595,6 +611,7 @@ def new_uvdata(
     obj.spw_array = spw_array
     obj.flex_spw_id_array = flex_spw_id_array
     obj.integration_time = integration_time
+    obj.blt_order = blt_order
 
     set_phase_params(
         obj,

pyuvdata/uvdata/tests/test_initializers.py

@@ -130,6 +130,13 @@ def test_lunar_simple_new_uvdata(lunar_simple_params: dict[str, Any], selenoid:
             },
             "instrument must be set on the Telescope object passed to `telescope`.",
         ],
+        [
+            {
+                "antpairs": np.array([(0, 1), (0, 2), (1, 2), (0, 3)]),
+                "do_blt_outer": False,
+            },
+            "Length of time array must match the number of antpairs.",
+        ],
     ],
 )
 def test_bad_inputs(simplest_working_params: dict[str, Any], update_dict, err_msg):
@@ -138,6 +145,28 @@ def test_bad_inputs(simplest_working_params: dict[str, Any], update_dict, err_ms
         UVData.new(**simplest_working_params)
 
 
+@pytest.mark.parametrize(
+    ["update_dict", "blt_order"],
+    [
+        [{}, ("ant1", "ant2")],
+        [{"time_axis_faster_than_bls": True}, ("time", "ant1")],
+        [
+            {
+                "antpairs": np.array([(0, 1), (0, 2), (1, 2), (0, 1)]),
+                "times": np.array([2459855, 2459855, 2459855.5, 2459855.5]),
+                "integration_time": np.full((4,), 12.0, dtype=float),
+                "do_blt_outer": False,
+            },
+            None,
+        ],
+    ],
+)
+def test_blt_order(simplest_working_params, update_dict, blt_order):
+    simplest_working_params.update(update_dict)
+    uvd = UVData.new(**simplest_working_params)
+    assert uvd.blt_order == blt_order
+
+
 def test_bad_time_inputs(simplest_working_params: dict[str, Any]):
     with pytest.raises(ValueError, match="time_array must be a numpy array"):
         get_time_params(

pyuvdata/uvdata/uvfits.py

@@ -930,7 +930,9 @@ def write_uvfits(
                 start_freq_array += [freq_array_use[chan_mask][0]]
                 # Need the array direction here since channel_width is always supposed
                 # to be > 0, but channels can be in decending freq order
-                freq_dir = np.sign(np.median(np.diff(freq_array_use[chan_mask])))
+                freq_dir = 1.0
+                if nchan_list[-1] > 1:
+                    freq_dir = np.sign(np.median(np.diff(freq_array_use[chan_mask])))
                 delta_freq_array += [
                     np.median(self.channel_width[chan_mask]) * freq_dir
                 ]