diff --git a/conftest.py b/conftest.py
index 4c8b48902..c95848a4c 100644
--- a/conftest.py
+++ b/conftest.py
@@ -14,6 +14,8 @@
     have_ray = True
     ray.init(num_cpus=1)  # call required to be here: see ray-project/ray#44087
 
+import jax.numpy as jnp
+
 import scico.numpy as snp
 
 
@@ -24,7 +26,8 @@ def pytest_sessionstart(session):
 
 def pytest_sessionfinish(session, exitstatus):
     """Clean up after end of test session."""
-    ray.shutdown()
+    if have_ray:
+        ray.shutdown()
 
 
 @pytest.fixture(autouse=True)
@@ -33,7 +36,8 @@ def add_modules(doctest_namespace):
 
     Necessary because `np` is used in doc strings for jax functions
     (e.g. `linear_transpose`) that get pulled into `scico/__init__.py`.
-    Also allow `snp` to be used without explicitly importing.
+    Also allow `snp` and `jnp` to be used without explicitly importing.
     """
     doctest_namespace["np"] = np
     doctest_namespace["snp"] = snp
+    doctest_namespace["jnp"] = jnp
diff --git a/requirements.txt b/requirements.txt
index 227948c20..1946b4295 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,8 +4,8 @@ scipy>=1.6.0
 imageio>=2.17
 tifffile
 matplotlib
-jaxlib>=0.4.3,<=0.4.26
-jax>=0.4.3,<=0.4.26
+jaxlib>=0.4.3,<=0.4.28
+jax>=0.4.3,<=0.4.28
 orbax-checkpoint<=0.5.7
-flax>=0.8.0,<=0.8.2
+flax>=0.8.0,<=0.8.3
 pyabel>=0.9.0
diff --git a/scico/linop/xray/astra.py b/scico/linop/xray/astra.py
index 7a270e1f4..4e59cca5f 100644
--- a/scico/linop/xray/astra.py
+++ b/scico/linop/xray/astra.py
@@ -362,10 +362,12 @@ def angle_to_vector(det_spacing: Tuple[float, float], angles: np.ndarray) -> np.
 
 def _ensure_writeable(x):
     """Ensure that `x.flags.writeable` is ``True``, copying if needed."""
-
-    if not x.flags.writeable:
-        try:
-            x.setflags(write=True)
-        except ValueError:
-            x = x.copy()
+    if hasattr(x, "flags"):  # x is a numpy array
+        if not x.flags.writeable:
+            try:
+                x.setflags(write=True)
+            except ValueError:
+                x = x.copy()
+    else:  # x is a jax array (which is immutable)
+        x = np.array(x)
     return x