diff --git a/src/mrinufft/operators/base.py b/src/mrinufft/operators/base.py
index 32921e3d..7689441e 100644
--- a/src/mrinufft/operators/base.py
+++ b/src/mrinufft/operators/base.py
@@ -101,9 +101,11 @@ class or instance of class if args or kwargs are given.
     try:
         available, operator = FourierOperatorBase.interfaces[backend_name]
     except KeyError as exc:
-        raise ValueError("backend is not available") from exc
+        raise ValueError(f"backend {backend_name} is not available") from exc
     if not available:
-        raise ValueError("backend is registered, but dependencies are not met.")
+        raise ValueError(
+            f"backend {backend_name} is registered, but dependencies are not met."
+        )
 
     if args or kwargs:
         operator = operator(*args, **kwargs)
diff --git a/tests/conftest.py b/tests/conftest.py
index 0a585363..d8ca3b1c 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -12,10 +12,15 @@ def pytest_addoption(parser):
         default=[],
         help="NUFFT backend on which the tests are performed.",
     )
+    parser.addoption(
+        "--ref",
+        default="pynfft",
+        help="Reference backend on which the tests are performed.",
+    )
 
 
 def pytest_configure(config):
-    """Configuration hook for pytest."""
+    """Configure hook for pytest."""
     print("Available backends:")
     for backend in list_backends():
         print(f"{backend:<14}: {FourierOperatorBase.interfaces[backend][0]}")
@@ -27,6 +32,15 @@ def pytest_configure(config):
                 backend in selected,
                 FourierOperatorBase.interfaces[backend][1],
             )
+    # ensure the ref backend is available
+    ref_backend = config.getoption("ref")
+    FourierOperatorBase.interfaces[ref_backend] = (
+        True,
+        FourierOperatorBase.interfaces[ref_backend][1],
+    )
+    print("Selected backends:")
+    for backend in list_backends():
+        print(f"{backend:<14}: {FourierOperatorBase.interfaces[backend][0]}")
 
 
 # # for test directly parametrized by a backend
@@ -58,8 +72,13 @@ def pytest_generate_tests(metafunc):
                     if v.argnames[0] == "backend"
                 ][0]
             ]
+            print("backend detected", backend)
             # Only keep the callspec if the backend is available.
             if not check_backend(backend):
                 callspec.marks.append(
                     pytest.mark.skip(f"Backend {backend} not available.")
                 )
+            if backend == metafunc.config.getoption("ref"):
+                callspec.marks.append(
+                    pytest.mark.skip("Not testing ref backend with self.")
+                )
diff --git a/tests/test_cpu.py b/tests/test_cpu.py
deleted file mode 100644
index 8de82610..00000000
--- a/tests/test_cpu.py
+++ /dev/null
@@ -1 +0,0 @@
-"""Test for the CPU interfaces."""
diff --git a/tests/test_interfaces.py b/tests/test_interfaces.py
index d98bc8f7..3e91fd14 100644
--- a/tests/test_interfaces.py
+++ b/tests/test_interfaces.py
@@ -38,11 +38,16 @@ def operator(
     return get_operator(backend)(kspace_locs, shape, n_coils=n_coils, smaps=None)
 
 
+@fixture(scope="session", autouse=True)
+def ref_backend(request):
+    """get the reference backend from the CLI"""
+    return request.config.getoption("ref")
+
+
 @fixture(scope="module")
-@parametrize("backend", ["pynfft"])
-def nfft_ref_op(request, operator, backend="pynfft"):
+def ref_operator(request, operator, ref_backend):
     """Generate a NFFT operator, matching the property of the first operator."""
-    return get_operator(backend)(
+    return get_operator(ref_backend)(
         operator.samples, operator.shape, n_coils=operator.n_coils, smaps=operator.smaps
     )
 
@@ -59,18 +64,18 @@ def kspace_data(operator):
     return kspace_from_op(operator)
 
 
-def test_interfaces_accuracy_forward(operator, image_data, nfft_ref_op):
+def test_interfaces_accuracy_forward(operator, image_data, ref_operator):
     """Compare the interface to the raw NUDFT implementation."""
     kspace_nufft = operator.op(image_data).squeeze()
-    kspace_ref = nfft_ref_op.op(image_data).squeeze()
     # FIXME: check with complex values ail
+    kspace_ref = ref_operator.op(image_data).squeeze()
     assert np.percentile(abs(kspace_nufft - kspace_ref) / abs(kspace_ref), 95) < 1e-1
 
 
-def test_interfaces_accuracy_backward(operator, kspace_data, nfft_ref_op):
+def test_interfaces_accuracy_backward(operator, kspace_data, ref_operator):
     """Compare the interface to the raw NUDFT implementation."""
     image_nufft = operator.adj_op(kspace_data.copy()).squeeze()
-    image_ref = nfft_ref_op.adj_op(kspace_data.copy()).squeeze()
+    image_ref = ref_operator.adj_op(kspace_data.copy()).squeeze()
 
     assert np.percentile(abs(image_nufft - image_ref) / abs(image_ref), 95) < 1e-1
 
diff --git a/tests/test_stacked_gpu.py b/tests/test_stacked_gpu.py
index 6d57c437..0051a19a 100644
--- a/tests/test_stacked_gpu.py
+++ b/tests/test_stacked_gpu.py
@@ -112,16 +112,6 @@ def test_stack_backward(operator, stacked_op, ref_op, kspace_data):
     """Compare the stack interface to the 3D NUFFT implementation."""
     image_nufft = stacked_op.adj_op(kspace_data.copy()).squeeze()
     image_ref = ref_op.adj_op(kspace_data.copy()).squeeze()
-    if stacked_op.n_coils > 1:
-        print(
-            np.max(
-                np.abs(image_nufft - image_ref).reshape(
-                    stacked_op.n_batchs, stacked_op.n_coils, -1
-                ),
-                axis=-1,
-            )
-        )
-        print(image_nufft.shape, image_ref.shape)
     npt.assert_allclose(image_nufft, image_ref, atol=1e-4, rtol=1e-1)