diff --git a/pyproject.toml b/pyproject.toml
index c5bc6ed05..42c0b3f70 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -33,7 +33,7 @@ dependencies = [
     "crds >=13.0.2",
     "drizzle >= 2.1.1,<2.2",
     "gwcs >=0.25.2,<0.27.0",
-    "stcal>=1.15.1,<1.16.0",
+    "stcal>=1.15.2,<1.16.0",
     "stpipe >=0.11.0,<0.12.0",
     "spherical-geometry >=1.3.3,<1.4",
     "stsci.imagestats >=1.8.3,<1.9",  # remove when stcal fixes this missing dep
diff --git a/romancal/tweakreg/tests/conftest.py b/romancal/tweakreg/tests/conftest.py
new file mode 100644
index 000000000..14503a999
--- /dev/null
+++ b/romancal/tweakreg/tests/conftest.py
@@ -0,0 +1,128 @@
+import numpy as np
+import pytest
+from astropy import coordinates as coord
+from astropy import units as u
+from astropy.modeling.models import RotationSequence3D, Scale, Shift
+from astropy.table import Table
+from astropy.time import Time
+from gwcs import coordinate_frames as cf
+from gwcs import wcs
+from gwcs.geometry import CartesianToSpherical, SphericalToCartesian
+from roman_datamodels import datamodels as rdm
+from stcal.tweakreg.astrometric_utils import get_catalog
+
+from romancal.tweakreg.tweakreg_step import DEFAULT_ABS_REFCAT
+
+
+@pytest.fixture(scope="session")
+def gaia_coords():
+    gaia_cat = get_catalog(
+        right_ascension=270,
+        declination=66,
+        search_radius=100 / 3600.0,
+        catalog=DEFAULT_ABS_REFCAT,
+        timeout=120,
+    )
+    return [(ra, dec) for ra, dec in zip(gaia_cat["ra"], gaia_cat["dec"], strict=False)]
+
+
+@pytest.fixture
+def tweakreg_image(tmp_path, gaia_coords):
+    """
+    Create a base image with a realistic WCSInfo and a WCS.
+
+    Notes
+    -----
+    The size of the image needs to be relatively large in order for
+    the source catalog step to find a reasonable number of sources in the image.
+
+    shift_1 and shift_2 (units in pixel) are used to shift the WCS projection plane.
+    """
+
+    def _tweakreg_image(shift_1=0, shift_2=0, catalog_filename="catalog.ecsv"):
+        l2 = rdm.ImageModel.create_fake_data(shape=(2000, 2000))
+        l2.meta.filename = "none"
+        l2.meta.cal_step = {}
+        for step_name in l2.schema_info("required")["roman"]["meta"]["cal_step"][
+            "required"
+        ].info:
+            l2.meta.cal_step[step_name] = "INCOMPLETE"
+        l2.meta.cal_logs = []
+
+        # to match GAIADR3 epoch
+        l2.meta.exposure.start_time = Time("2016-01-01T00:00:00")
+
+        # update wcsinfo
+        l2.meta.wcsinfo.v2_ref = 0.42955128282521254
+        l2.meta.wcsinfo.v3_ref = -0.2479976768255853
+        l2.meta.wcsinfo.vparity = -1
+        l2.meta.wcsinfo.v3yangle = -999999
+        l2.meta.wcsinfo.ra_ref = 270.0
+        l2.meta.wcsinfo.dec_ref = 66.0
+        l2.meta.wcsinfo.roll_ref = 60
+        l2.meta.wcsinfo.s_region = (
+            "POLYGON ICRS "
+            "269.3318903230621 65.56866666048172 "
+            "269.32578768154605 65.69246311613287 "
+            "269.02457173246125 65.69201346248587 "
+            "269.0333096074621 65.56870823657276 "
+        )
+
+        # create necessary transformations
+        distortion = Shift(-shift_1) & Shift(-shift_2)
+        v2_ref = l2.meta.wcsinfo.v2_ref / 3600
+        v3_ref = l2.meta.wcsinfo.v3_ref / 3600
+        roll_ref = l2.meta.wcsinfo.roll_ref
+        ra_ref = l2.meta.wcsinfo.ra_ref
+        dec_ref = l2.meta.wcsinfo.dec_ref
+
+        angles = np.array([v2_ref, -v3_ref, roll_ref, dec_ref, -ra_ref])
+        axes = "zyxyz"
+        rot = RotationSequence3D(angles, axes_order=axes)
+
+        # The sky rotation expects values in deg.
+        tel2sky = (
+            (Scale(0.1 / 3600) & Scale(0.1 / 3600))
+            | SphericalToCartesian(wrap_lon_at=180)
+            | rot
+            | CartesianToSpherical(wrap_lon_at=360)
+        )
+        tel2sky.name = "v23tosky"
+
+        # create required frames
+        detector = cf.Frame2D(name="detector", axes_order=(0, 1), unit=(u.pix, u.pix))
+        v2v3 = cf.Frame2D(
+            name="v2v3",
+            axes_order=(0, 1),
+            axes_names=("v2", "v3"),
+            unit=(u.arcsec, u.arcsec),
+        )
+        world = cf.CelestialFrame(reference_frame=coord.ICRS(), name="world")
+
+        # create pipeline
+        pipeline = [
+            wcs.Step(detector, distortion),
+            wcs.Step(v2v3, tel2sky),
+            wcs.Step(world, None),
+        ]
+
+        wcs_obj = wcs.WCS(pipeline)
+        wcs_obj.bounding_box = (
+            (-0.5, l2.data.shape[-2] + 0.5),
+            (-0.5, l2.data.shape[-1] + 0.5),
+        )
+
+        l2.meta.wcs = wcs_obj
+
+        catalog_path = tmp_path / catalog_filename
+        catalog_data = np.array(
+            [wcs_obj.world_to_pixel_values(ra, dec) for ra, dec in gaia_coords]
+        )
+        Table(catalog_data, names=("x", "y")).write(catalog_path, format="ascii.ecsv")
+        l2.meta["source_catalog"] = {
+            "tweakreg_catalog_name": str(catalog_path),
+        }
+
+        return l2
+
+    return _tweakreg_image
diff --git a/romancal/tweakreg/tests/test_parse_catfile.py b/romancal/tweakreg/tests/test_parse_catfile.py
new file mode 100644
index 000000000..f810596a1
--- /dev/null
+++ b/romancal/tweakreg/tests/test_parse_catfile.py
@@ -0,0 +1,56 @@
+import pytest
+
+from romancal.tweakreg.tweakreg_step import _parse_catfile
+
+
+def test_parse_catfile_raises_error_on_invalid_content(tmp_path):
+    """
+    Test that _parse_catfile raises an error if catfile contains more than
+    two columns (i.e. image name and corresponding catalog path).
+    """
+    # create custom catalog file and input datamodels
+    catfile = str(tmp_path / "catfile.txt")
+    with open(catfile, "w") as f:
+        f.write("img1.asdf column1 column2 column3")
+
+    with pytest.raises(ValueError):
+        _parse_catfile(catfile)
+
+
+def test_parse_catfile_valid_catalog(tmp_path):
+    """
+    Test that _parse_catfile can parse a custom catalog with valid format.
+    """
+    catfile = str(tmp_path / "catfile.txt")
+    with open(catfile, mode="w") as f:
+        f.write("img1.asdf cat1\nimg2.asdf cat2")
+    catdict = _parse_catfile(catfile)
+
+    assert catdict == {
+        "img1.asdf": str(tmp_path / "cat1"),
+        "img2.asdf": str(tmp_path / "cat2"),
+    }
+
+
+@pytest.mark.parametrize("catfile", (None, ""))
+def test_parse_catfile_returns_none(catfile):
+    """
+    Test that _parse_catfile returns None when catfile = None or catfile = "".
+    """
+    catdict = _parse_catfile(catfile=catfile)
+
+    assert catdict is None
+
+
+def test_parse_catfile_returns_none_on_invalid_content(tmp_path):
+    """
+    Test that _parse_catfile returns a dict where all the values are None
+    if only filename is present in catfile (i.e. no associated catalog).
+    """
+    catfile = str(tmp_path / "catfile.txt")
+    with open(catfile, mode="w") as f:
+        f.write("img1.asdf\nimg2.asdf\nimg3.asdf")
+
+    catdict = _parse_catfile(catfile)
+
+    assert not all(catdict.values())
diff --git a/romancal/tweakreg/tests/test_prepare_input.py b/romancal/tweakreg/tests/test_prepare_input.py
new file mode 100644
index 000000000..ce15e2645
--- /dev/null
+++ b/romancal/tweakreg/tests/test_prepare_input.py
@@ -0,0 +1,57 @@
+import pytest
+
+from romancal.datamodels import ModelLibrary
+from romancal.tweakreg.tweakreg_step import TweakRegStep
+
+
+@pytest.fixture()
+def datamodel(tweakreg_image):
+    model = tweakreg_image()
+    model.meta.filename = "test.asdf"
+    return model
+
+
+@pytest.fixture()
+def datamodel_filename(datamodel, tmp_path):
+    fn = tmp_path / "test.asdf"
+    datamodel.save(fn)
+    return fn
+
+
+@pytest.fixture()
+def list_of_models(datamodel):
+    return [datamodel]
+
+
+@pytest.fixture()
+def list_of_filenames(datamodel_filename):
+    return [datamodel_filename]
+
+
+@pytest.fixture()
+def library(list_of_models):
+    return ModelLibrary(list_of_models)
+
+
+@pytest.fixture()
+def association(library, tmp_path):
+    fn = tmp_path / "asn.json"
+    library._save(fn.parent)
+    return fn
+
+
+@pytest.mark.parametrize(
+    "init",
+    [
+        "datamodel",
+        "datamodel_filename",
+        "list_of_models",
+        "list_of_filenames",
+        "library",
+        "association",
+    ],
+)
+def test_tweakreg_handle_input(init, request):
+    assert isinstance(
+        TweakRegStep()._prepare_input(request.getfixturevalue(init)), ModelLibrary
+    )
diff --git a/romancal/tweakreg/tests/test_tweakreg.py b/romancal/tweakreg/tests/test_tweakreg.py
index 1eace7d6e..5e309ed6f 100644
--- a/romancal/tweakreg/tests/test_tweakreg.py
+++ b/romancal/tweakreg/tests/test_tweakreg.py
@@ -2,678 +2,44 @@
 import json
 import os
 import shutil
-from io import StringIO
 
 import numpy as np
 import pytest
-import requests
 from astropy import coordinates as coord
 from astropy import units as u
 from astropy.modeling import models
-from astropy.modeling.models import RotationSequence3D, Scale, Shift
 from astropy.table import Table
-from astropy.time import Time
-from gwcs import coordinate_frames as cf
-from gwcs import wcs
-from gwcs.geometry import CartesianToSpherical, SphericalToCartesian
 from numpy.random import default_rng
-from roman_datamodels import datamodels as rdm
-from stcal.tweakreg.astrometric_utils import get_catalog
 
 from romancal.datamodels import ModelLibrary
-from romancal.tweakreg import tweakreg_step as trs
-from romancal.tweakreg.tweakreg_step import _validate_catalog_columns
+from romancal.tweakreg.tweakreg_step import TweakRegStep, _validate_catalog_columns
 
 
-class MockConnectionError:
-    def __init__(self, *args, **kwargs):
-        raise requests.exceptions.ConnectionError
-
-
-def create_custom_catalogs(tmp_path, base_image, catalog_format="ascii.ecsv"):
-    """
-    Creates a custom catalog with three datamodels and its respective catalog data.
-    """
-    img1 = base_image(shift_1=1000, shift_2=1000)
-    img2 = base_image(shift_1=1010, shift_2=1010)
-    img3 = base_image(shift_1=1020, shift_2=1020)
-
-    img1.meta.filename = "img1.asdf"
-    img2.meta.filename = "img2.asdf"
-    img3.meta.filename = "img3.asdf"
-
-    # create valid custom catalog data to be used with each input datamodel
-    catalog_data1 = get_catalog_data(img1)
-    catalog_data2 = get_catalog_data(img2)
-    catalog_data3 = get_catalog_data(img3)
-
-    custom_catalog_map = [
-        {
-            "cat_filename": "ref_catalog_1",
-            "cat_datamodel": img1.meta.filename,
-            "cat_data": catalog_data1,
-        },
-        {
-            "cat_filename": "ref_catalog_2",
-            "cat_datamodel": img2.meta.filename,
-            "cat_data": catalog_data2,
-        },
-        {
-            "cat_filename": "ref_catalog_3",
-            "cat_datamodel": img3.meta.filename,
-            "cat_data": catalog_data3,
-        },
-    ]
-
-    # create catfile
-    catfile = str(tmp_path / "catfile.txt")
-    catfile_content = StringIO()
-    for x in custom_catalog_map:
-        # write line to catfile
-        catfile_content.write(f"{x.get('cat_datamodel')} {x.get('cat_filename')}\n")
-        # write out the catalog data
-        t = Table(x.get("cat_data"), names=("x", "y"))
-        t.write(tmp_path / x.get("cat_filename"), format=catalog_format)
-    with open(catfile, mode="w") as f:
-        print(catfile_content.getvalue(), file=f)
-
-    return {"catfile": catfile, "datamodels": [img1, img2, img3]}
-
-
-def create_asn_file(tmp_path, members_mapping=None):
-    asn_content = """
-        {
-            "asn_type": "None",
-            "asn_rule": "DMS_ELPP_Base",
-            "version_id": null,
-            "code_version": "0.9.1.dev28+ge987cc9.d20230106",
-            "degraded_status": "No known degraded exposures in association.",
-            "program": "noprogram",
-            "constraints": "No constraints",
-            "asn_id": "a3001",
-            "target": "none",
-            "asn_pool": "test_pool_name",
-            "products": [
-                {
-                    "name": "files.asdf",
-                    "members": [
-                        {
-                            "expname": "img_1.asdf",
-                            "exptype": "science"
-                        },
-                        {
-                            "expname": "img_2.asdf",
-                            "exptype": "science"
-                        }
-                    ]
-                }
-            ]
-        }
-    """
-    if members_mapping is not None:
-        asn_dict = json.loads(asn_content)
-        asn_dict["products"][0]["members"] = []
-        for x in members_mapping:
-            asn_dict["products"][0]["members"].append(x)
-        asn_content = json.dumps(asn_dict)
-
-    asn_file_path = str(tmp_path / "sample_asn.json")
-    asn_file = StringIO()
-    asn_file.write(asn_content)
-    with open(asn_file_path, mode="w") as f:
-        print(asn_file.getvalue(), file=f)
-
-    return asn_file_path
-
-
-def update_wcsinfo(input_dm):
-    """
-    Update WCSInfo with realistic data (i.e. obtained from romanisim simulations).
-
-    Parameters
-    ----------
-    input_dm : roman_datamodels.ImageModel
-        A Roman image datamodel.
-    """
-    input_dm.meta.wcsinfo.v2_ref = 0.42955128282521254
-    input_dm.meta.wcsinfo.v3_ref = -0.2479976768255853
-    input_dm.meta.wcsinfo.vparity = -1
-    input_dm.meta.wcsinfo.v3yangle = -999999
-    input_dm.meta.wcsinfo.ra_ref = 270.0
-    input_dm.meta.wcsinfo.dec_ref = 66.0
-    input_dm.meta.wcsinfo.roll_ref = 60
-    input_dm.meta.wcsinfo.s_region = (
-        "POLYGON ICRS "
-        "269.3318903230621 65.56866666048172 "
-        "269.32578768154605 65.69246311613287 "
-        "269.02457173246125 65.69201346248587 "
-        "269.0333096074621 65.56870823657276 "
-    )
-
-
-def _create_tel2sky_model(input_dm):
-    """
-    Create the transform from telescope to sky.
-
-    The transform is defined with a reference point in a Frame
-    associated with the telescope (V2, V3) in arcsec, the corresponding
-    reference point on sky (RA_REF, DEC_REF) in deg, and the position angle
-    at the center of the aperture, ROLL_REF in deg.
-
-    Parameters
-    ----------
-    input_dm : roman_datamodels.ImageModel
-        A Roman image datamodel.
-
-    Returns
-    -------
-    astropy.modeling.core.CompoundModel
-        An astropy compound model with the transformations to map
-        the telescope reference system onto the world reference systems.
-    """
-    v2_ref = input_dm.meta.wcsinfo.v2_ref / 3600
-    v3_ref = input_dm.meta.wcsinfo.v3_ref / 3600
-    roll_ref = input_dm.meta.wcsinfo.roll_ref
-    ra_ref = input_dm.meta.wcsinfo.ra_ref
-    dec_ref = input_dm.meta.wcsinfo.dec_ref
-
-    angles = np.array([v2_ref, -v3_ref, roll_ref, dec_ref, -ra_ref])
-    axes = "zyxyz"
-    rot = RotationSequence3D(angles, axes_order=axes)
-
-    # The sky rotation expects values in deg.
-    # This should be removed when models work with quantities.
-    model = (
-        (Scale(0.1 / 3600) & Scale(0.1 / 3600))
-        | SphericalToCartesian(wrap_lon_at=180)
-        | rot
-        | CartesianToSpherical(wrap_lon_at=360)
-    )
-    model.name = "v23tosky"
-    return model
-
-
-def create_basic_wcs(
-    img_shape: tuple = (100, 100),
-    ref_pix: tuple = (0, 0),
-    ref_val: tuple[u.Quantity, u.Quantity] = (
-        u.Quantity("10 deg"),
-        u.Quantity("0 deg"),
-    ),
-    pix_scale: tuple[u.Quantity, u.Quantity] = (
-        u.Quantity("0.1 arcsec"),
-        u.Quantity("0.1 arcsec"),
-    ),
-    theta: u.Quantity | None = None,
-):
-    """
-    Creates a basic WCS (no distortion) to map pixel coordinates
-    onto celestial spherical coordinates.
-
-    Parameters
-    ----------
-    img_shape : tuple, optional
-        Dimension of the pixel coordinates, by default (100, 100).
-    ref_pix : tuple, optional
-        Projection plane coordinates of the reference pixel, by default (0, 0)
-    ref_val : tuple[u.Quantity, u.Quantity], optional
-        Celestial coordinates at the reference pixel in degrees,
-        by default (10, 0)*u.degree
-    pix_scale : tuple[u.Quantity, u.Quantity], optional
-        Size of a pixel side in arcsec, by default (0.1, 0.1)*u.arcsec
-    theta : u.Quantity, optional
-        CCW rotation angle of the projection plane, by default 0*u.deg
-
-    Returns
-    -------
-    type: gwcs.wcs.WCS object
-        A gwcs.wcs.WCS object with a model f : (x, y) |-> (ra, dec).
-
-    Notes
-    -----
-    With the default parameters, the result will be a WCS for an image
-    that's 100x100 pixels (@0.1"/pix in both axis) and reference
-    coordinates (ra=10 deg, dec=0 deg, ICRS) sitting at the origin of the
-    pixel coordinates frame (0,0).
-
-    The WCS object created by this method generates a pipeline that
-    does not contain the required steps to validate against the
-    TweakReg pipeline.
-    """
-    theta = u.Quantity("0 deg") if theta is None else theta
-
-    # linear transformations
-    shift_pixel_coords = models.Shift(-ref_pix[0]) & models.Shift(-ref_pix[1])
-    scaling_matrix = np.array(
-        [[pix_scale.to("deg")[0].value, 0], [0, pix_scale.to("deg")[1].value]]
-    )
-    rot_matrix = np.array(
-        [
-            [np.cos(theta), -np.sin(theta)],
-            [np.sin(theta), np.cos(theta)],
-        ]
-    )
-    matrix = np.dot(rot_matrix, scaling_matrix)
-    affine_transf = models.AffineTransformation2D(matrix)
-    affine_transf.inverse = models.AffineTransformation2D(np.linalg.inv(matrix))
-    tan = models.Pix2Sky_TAN()
-    celestial_rotation = models.RotateNative2Celestial(
-        ref_val[0].value, ref_val[1].value, 180
-    )
-    det2sky = shift_pixel_coords | affine_transf | tan | celestial_rotation
-    det2sky.name = "linear_transform"
-
-    # frames
-    detector_frame = cf.Frame2D(
-        name="detector", unit=(u.pix, u.pix), axes_names=("x", "y")
-    )
-    cf.Frame2D(
-        name="v2v3",
-        axes_order=(0, 1),
-        axes_names=("v2", "v3"),
-        unit=(u.arcsec, u.arcsec),
-    )
-    sky_frame = cf.CelestialFrame(
-        name="sky", unit=(u.deg, u.deg), reference_frame=coord.ICRS()
-    )
-
-    pipeline = [(detector_frame, det2sky), (sky_frame, None)]
-    wcsobj = wcs.WCS(pipeline)
-    wcsobj.bounding_box = (
-        (-0.5, img_shape[0] + 0.5),
-        (-0.5, img_shape[1] + 0.5),
-    )
-
-    return wcsobj
-
-
-def create_wcs_for_tweakreg_pipeline(input_dm, shift_1=0, shift_2=0):
-    """
-    Create a basic WCS object (with optional shift) and
-    append it to the input_dm.meta attribute.
-
-    The WCS object will have a pipeline with the required
-    steps to validate against the TweakReg pipeline.
-
-    Parameters
-    ----------
-    input_dm : roman_datamodels.ImageModel
-        A Roman image datamodel.
-    shift_1 : int, optional
-        Shift to be applied in the direction of the first axis, by default 0
-    shift_2 : int, optional
-        Shift to be applied in the direction of the second axis, by default 0
-    """
-
-    shape = input_dm.data.shape
-
-    # create necessary transformations
-    distortion = Shift(-shift_1) & Shift(-shift_2)
-    tel2sky = _create_tel2sky_model(input_dm)
-
-    # create required frames
-    detector = cf.Frame2D(name="detector", axes_order=(0, 1), unit=(u.pix, u.pix))
-    v2v3 = cf.Frame2D(
-        name="v2v3",
-        axes_order=(0, 1),
-        axes_names=("v2", "v3"),
-        unit=(u.arcsec, u.arcsec),
-    )
-    world = cf.CelestialFrame(reference_frame=coord.ICRS(), name="world")
-
-    # create pipeline
-    pipeline = [
-        wcs.Step(detector, distortion),
-        wcs.Step(v2v3, tel2sky),
-        wcs.Step(world, None),
-    ]
-
-    wcs_obj = wcs.WCS(pipeline)
-    wcs_obj.bounding_box = ((-0.5, shape[-2] + 0.5), (-0.5, shape[-1] + 0.5))
-
-    input_dm.meta["wcs"] = wcs_obj
-
-
-def get_catalog_data(input_dm, **kwargs):
-    ra = kwargs.get("ra", 270)
-    dec = kwargs.get("dec", 66)
-    sr = kwargs.get("sr", 100 / 3600)
-    add_shifts = kwargs.get("add_shifts", False)
-    gaia_cat = get_catalog(right_ascension=ra, declination=dec, search_radius=sr)
-    gaia_source_coords = [
-        (ra, dec) for ra, dec in zip(gaia_cat["ra"], gaia_cat["dec"], strict=False)
-    ]
-    catalog_data = np.array(
-        [
-            input_dm.meta.wcs.world_to_pixel_values(ra, dec)
-            for ra, dec in gaia_source_coords
-        ]
-    )
-    if add_shifts:
-        rng = np.random.default_rng(seed=int(ra + dec))
-        shifts = rng.uniform(-1, 1, size=catalog_data.shape)
-        catalog_data += shifts
-    return catalog_data
-
-
-def create_base_image_source_catalog(
-    tmp_path,
-    output_filename,
-    catalog_data,
-    catalog_format: str = "ascii.ecsv",
-    save_catalogs=True,
-):
-    """
-    Write a temp CSV file to be used as source catalog, similar to what
-    is produced by the previous pipeline step, source detection.
-
-    Parameters
-    ----------
-    tmp_path : pathlib.PosixPath
-        A path-like object representing the path where to save the file.
-    output_filename : string
-        The output filename (with extension).
-    catalog_data : numpy.ndarray
-        A numpy array with the (x, y) coordinates of the
-        "detected" sources
-    catalog_format : str, optional
-        A string indicating the catalog format.
-    save_catalogs : boolean, optional
-        A boolean indicating whether the source catalog should be saved to disk.
-    """
-    src_detector_coords = catalog_data
-    output = os.path.join(tmp_path, output_filename)
-    t = Table(src_detector_coords, names=("x", "y"))
-    if save_catalogs:
-        t.write((tmp_path / output), format=catalog_format)
-    # mimic the same output format from SourceCatalogStep
-    t.add_column([i for i in range(len(t))], name="id", index=0)
-    t.add_column([np.float64(i) for i in range(len(t))], name="flux")
-    t.rename_columns(["x", "y"], ["xcentroid", "ycentroid"])
-    return t.as_array()
-
-
-def add_tweakreg_catalog_attribute(
-    tmp_path,
-    input_dm,
-    catalog_filename="base_image_sources",
-    catalog_data=None,
-    catalog_format: str = "ascii.ecsv",
-    save_catalogs=True,
-    **kwargs,
-):
-    """
-    Add tweakreg_catalog attribute to the meta, which is a mandatory
-    attribute for TweakReg. The tweakreg_catalog attribute contains
-    a table with the coordinates of the sources detected by the
-    previous step. The coordinates will be used in the process of
-    correcting the original WCS.
-
-    Parameters
-    ----------
-    tmp_path : pathlib.PosixPath
-        A path-like object representing the path where to save the file.
-    input_dm : roman_datamodels.ImageModel
-        A Roman image datamodel.
-    catalog_filename : str
-        Filename to be used when saving the catalog to disk.
-    catalog_data : numpy.ndarray, optional
-        A numpy array with the (x, y) coordinates of the
-        "detected" sources, by default None (see note below).
-    catalog_format : str, optional
-        A string indicating the catalog format.
-    save_catalogs : boolean, optional
-        A boolean indicating whether the source catalog should be saved to disk.
-
-    Notes
-    ----
-    - kwargs will be passed on to get_catalog_data();
-    - if no catalog_data is provided, a default catalog will be created
-    by fetching data from Gaia within a search radius of 100 arcsec
-    centered at RA=270, Dec=66.
-    """
-    tweakreg_catalog_filename = catalog_filename
-    if catalog_data is None:
-        catalog_data = get_catalog_data(input_dm, **kwargs)
-
-    source_catalog = create_base_image_source_catalog(
-        tmp_path,
-        tweakreg_catalog_filename,
-        catalog_data=catalog_data,
-        catalog_format=catalog_format,
-        save_catalogs=save_catalogs,
-    )
-
-    input_dm.meta["source_catalog"] = {}
-
-    if save_catalogs:
-        # SourceCatalogStep adds the catalog path+filename
-        input_dm.meta.source_catalog["tweakreg_catalog_name"] = os.path.join(
-            tmp_path, tweakreg_catalog_filename
-        )
-    else:
-        # SourceCatalogStep attaches the catalog data as a structured array
-        input_dm.meta.source_catalog["tweakreg_catalog"] = source_catalog
-
-
-@pytest.fixture
-def base_image():
-    """
-    Create a base image with a realistic WCSInfo and a WCS.
-
-    Notes
-    -----
-    The size of the image needs to be relatively large in order for
-    the source catalog step to find a reasonable number of sources in the image.
-
-    shift_1 and shift_2 (units in pixel) are used to shift the WCS projection plane.
-    """
-
-    def _base_image(shift_1=0, shift_2=0):
-        l2 = rdm.ImageModel.create_fake_data(shape=(2000, 2000))
-        l2.meta.filename = "none"
-        l2.meta.cal_step = {}
-        for step_name in l2.schema_info("required")["roman"]["meta"]["cal_step"][
-            "required"
-        ].info:
-            l2.meta.cal_step[step_name] = "INCOMPLETE"
-        l2.meta.cal_logs = []
-        l2.meta.exposure.start_time = Time("2016-01-01T00:00:00")
-        # update wcsinfo
-        update_wcsinfo(l2)
-        # add a dummy WCS object
-        create_wcs_for_tweakreg_pipeline(l2, shift_1=shift_1, shift_2=shift_2)
-        return l2
-
-    return _base_image
-
-
-@pytest.mark.parametrize(
-    "input, error_type",
-    [
-        (list(), (Exception,)),
-        ([""], (Exception,)),
-        (["", ""], (Exception,)),
-        ("", (Exception,)),
-        ([1, 2, 3], (Exception,)),
-    ],
-)
-def test_tweakreg_raises_error_on_invalid_input(input, error_type):
-    # sourcery skip: list-literal
-    """Test that TweakReg raises an error when an invalid input is provided."""
-    with pytest.raises(error_type):
-        trs.TweakRegStep.call(input)
-
-
-def test_tweakreg_raises_attributeerror_on_missing_tweakreg_catalog(base_image):
+def test_tweakreg_raises_attributeerror_on_missing_tweakreg_catalog(tweakreg_image):
     """
     Test that TweakReg raises an AttributeError if meta.tweakreg_catalog is missing.
     """
-    img = base_image()
+    img = tweakreg_image()
     # make sure tweakreg_catalog_name doesn't exist
     img.meta.source_catalog = {}
     assert "tweakreg_catalog_name" not in img.meta.source_catalog
-    with pytest.raises(AttributeError):
-        trs.TweakRegStep.call([img])
-
-
-def test_tweakreg_returns_modellibrary_on_roman_datamodel_as_input(
-    tmp_path, base_image
-):
-    """Test that TweakReg always returns a ModelLibrary when processing an open Roman DataModel as input."""
-
-    img = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img, catalog_filename="img_1")
+    with pytest.raises(
+        AttributeError,
+        match=r"Attribute 'meta.source_catalog.tweakreg_catalog' is missing",
+    ):
+        TweakRegStep.call([img])
 
-    test_input = img
 
-    res = trs.TweakRegStep.call(test_input)
-    assert isinstance(res, ModelLibrary)
-    with res:
-        model = res.borrow(0)
-        assert model.meta.cal_step.tweakreg == "COMPLETE"
-        res.shelve(model, 0, modify=False)
-
-
-def test_tweakreg_returns_modellibrary_on_modellibrary_as_input(tmp_path, base_image):
-    """Test that TweakReg always returns a ModelLibrary when processing a ModelLibrary as input."""
-
-    img = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img, catalog_filename="img_1")
-
-    test_input = ModelLibrary([img])
-
-    res = trs.TweakRegStep.call(test_input)
-    assert isinstance(res, ModelLibrary)
-    with res:
-        model = res.borrow(0)
-        assert model.meta.cal_step.tweakreg == "COMPLETE"
-        res.shelve(model, 0, modify=False)
-
-
-def test_tweakreg_returns_modellibrary_on_association_file_as_input(
-    tmp_path, base_image
-):
-    """Test that TweakReg always returns a ModelLibrary when processing an association file as input."""
-
-    img_1 = base_image(shift_1=1000, shift_2=1000)
-    img_2 = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img_1, catalog_filename="img_1")
-    add_tweakreg_catalog_attribute(tmp_path, img_2, catalog_filename="img_2")
-    img_1.save(tmp_path / "img_1.asdf")
-    img_2.save(tmp_path / "img_2.asdf")
-    asn_filepath = create_asn_file(tmp_path)
-
-    test_input = asn_filepath
-
-    res = trs.TweakRegStep.call(test_input)
-    assert isinstance(res, ModelLibrary)
-    with res:
-        for i, model in enumerate(res):
-            assert model.meta.cal_step.tweakreg == "COMPLETE"
-            res.shelve(model, i, modify=False)
-
-
-def test_tweakreg_returns_modellibrary_on_list_of_asdf_file_as_input(
-    tmp_path, base_image
-):
-    """Test that TweakReg always returns a ModelLibrary when processing a list of ASDF files as input."""
-
-    img_1 = base_image(shift_1=1000, shift_2=1000)
-    img_2 = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img_1, catalog_filename="img_1")
-    add_tweakreg_catalog_attribute(tmp_path, img_2, catalog_filename="img_2")
-    img_1.save(tmp_path / "img_1.asdf")
-    img_2.save(tmp_path / "img_2.asdf")
-
-    tmp_path_str = tmp_path.as_posix()
-    test_input = [
-        f"{tmp_path_str}/img_1.asdf",
-        f"{tmp_path_str}/img_2.asdf",
-    ]
-
-    res = trs.TweakRegStep.call(test_input)
-    assert isinstance(res, ModelLibrary)
-    with res:
-        for i, model in enumerate(res):
-            assert model.meta.cal_step.tweakreg == "COMPLETE"
-            res.shelve(model, i, modify=False)
-
-
-def test_tweakreg_returns_modellibrary_on_list_of_roman_datamodels_as_input(
-    tmp_path, base_image
-):
-    """Test that TweakReg always returns a ModelLibrary when processing a list of open Roman datamodels as input."""
-    img_1 = base_image(shift_1=1000, shift_2=1000)
-    img_2 = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img_1, catalog_filename="img_1")
-    add_tweakreg_catalog_attribute(tmp_path, img_2, catalog_filename="img_2")
-    img_1.save(tmp_path / "img_1.asdf")
-    img_2.save(tmp_path / "img_2.asdf")
-
-    test_input = [img_1, img_2]
-
-    res = trs.TweakRegStep.call(test_input)
-    assert isinstance(res, ModelLibrary)
-    with res:
-        for i, model in enumerate(res):
-            assert model.meta.cal_step.tweakreg == "COMPLETE"
-            res.shelve(model, i, modify=False)
-
-
-def test_tweakreg_updates_cal_step(tmp_path, base_image):
-    """Test that TweakReg updates meta.cal_step with tweakreg = COMPLETE."""
-    img = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img)
-    res = trs.TweakRegStep.call([img])
-
-    with res:
-        model = res.borrow(0)
-        assert hasattr(model.meta.cal_step, "tweakreg")
-        assert model.meta.cal_step.tweakreg == "COMPLETE"
-        res.shelve(model, 0, modify=False)
-
-
-@pytest.mark.parametrize(
-    "abs_refcat",
-    (
-        "GAIADR1",
-        "GAIADR2",
-        "GAIADR3",
-    ),
-)
-def test_tweakreg_save_valid_abs_refcat(tmp_path, abs_refcat, request):
+def test_tweakreg_save_valid_abs_refcat(tmp_path, tweakreg_image):
     """Test that TweakReg saves the catalog used for absolute astrometry."""
+    abs_refcat = "GAIADR3"
 
-    img = request.getfixturevalue("base_image")(shift_1=1000, shift_2=1000)
-    catalog_filename = "ref_catalog.ecsv"
-    abs_refcat_filename = f"fit_{abs_refcat.lower()}_ref.ecsv"
-    add_tweakreg_catalog_attribute(tmp_path, img, catalog_filename=catalog_filename)
-
-    trs.TweakRegStep.call(
-        [img],
-        save_abs_catalog=True,
-        abs_refcat=abs_refcat,
-        catalog_path=str(tmp_path),
-        output_dir=str(tmp_path),
+    img = tweakreg_image(
+        shift_1=1000, shift_2=1000, catalog_filename="ref_catalog.ecsv"
     )
+    abs_refcat_filename = f"fit_{abs_refcat.lower()}_ref.ecsv"
 
-    assert os.path.exists(tmp_path / abs_refcat_filename)
-
-
-@pytest.mark.parametrize(
-    "abs_refcat",
-    (None, ""),
-)
-def test_tweakreg_defaults_to_valid_abs_refcat(tmp_path, abs_refcat, request):
-    """Test that TweakReg defaults to DEFAULT_ABS_REFCAT on invalid values."""
-
-    img = request.getfixturevalue("base_image")(shift_1=1000, shift_2=1000)
-    catalog_filename = "ref_catalog.ecsv"
-    abs_refcat_filename = f"fit_{trs.DEFAULT_ABS_REFCAT.lower()}_ref.ecsv"
-    add_tweakreg_catalog_attribute(tmp_path, img, catalog_filename=catalog_filename)
-
-    trs.TweakRegStep.call(
+    TweakRegStep.call(
         [img],
         save_abs_catalog=True,
         abs_refcat=abs_refcat,
@@ -684,13 +50,12 @@ def test_tweakreg_defaults_to_valid_abs_refcat(tmp_path, abs_refcat, request):
     assert os.path.exists(tmp_path / abs_refcat_filename)
 
 
-def test_tweakreg_raises_error_on_invalid_abs_refcat(tmp_path, base_image):
+def test_tweakreg_raises_error_on_invalid_abs_refcat(tmp_path, tweakreg_image):
     """Test that TweakReg raises an error when an invalid abs_refcat is provided."""
-    img = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img)
+    img = tweakreg_image(shift_1=1000, shift_2=1000)
 
-    with pytest.raises(TypeError):
-        trs.TweakRegStep.call(
+    with pytest.raises(ValueError, match="Invalid 'abs_refcat'"):
+        TweakRegStep.call(
             [img],
             save_abs_catalog=True,
             abs_refcat="my_ref_cat",
@@ -699,92 +64,77 @@ def test_tweakreg_raises_error_on_invalid_abs_refcat(tmp_path, base_image):
         )
 
 
-def test_tweakreg_combine_custom_catalogs_and_asn_file(tmp_path, base_image):
+def test_tweakreg_combine_custom_catalogs_and_asn_file(
+    tmp_path, tweakreg_image, gaia_coords
+):
     """
     Test that TweakRegStep can handle a custom catalog for the members of an ASN file.
     In this case, the user can create a custom catalog file (catfile) for each of the
     members of an ASN file.
     """
-    # create custom catalog and input datamodels
-    catalog_format = "ascii.ecsv"
-    res_dict = create_custom_catalogs(
-        tmp_path, base_image, catalog_format=catalog_format
+    catfile = str(tmp_path / "catfile.txt")
+
+    # generate and save model
+    img = tweakreg_image()
+    img.meta.filename = "img.asdf"
+    img.save(tmp_path / img.meta.filename)
+
+    # generate and save custom catalog
+    catalog_data = np.array(
+        [img.meta.wcs.world_to_pixel_values(ra, dec) for ra, dec in gaia_coords]
     )
-    catfile = res_dict.get("catfile")
-    img1, img2, img3 = res_dict.get("datamodels")
-    add_tweakreg_catalog_attribute(tmp_path, img1, catalog_filename="img1")
-    add_tweakreg_catalog_attribute(tmp_path, img2, catalog_filename="img2")
-    add_tweakreg_catalog_attribute(tmp_path, img3, catalog_filename="img3")
-    img1.save(tmp_path / "img1.asdf")
-    img2.save(tmp_path / "img2.asdf")
-    img3.save(tmp_path / "img3.asdf")
+    custom_catalog_fn = str(tmp_path / "custom_catalog")
+    Table(catalog_data, names=("x", "y")).write(custom_catalog_fn, format="ascii.ecsv")
+    # record custom catalog
+    with open(catfile, "w") as f:
+        f.write(f"{img.meta.filename} {custom_catalog_fn}")
 
     # create ASN file
-    asn_filepath = create_asn_file(
-        tmp_path,
-        members_mapping=[
-            {"expname": img1.meta.filename, "exptype": "science"},
-            {"expname": img2.meta.filename, "exptype": "science"},
-            {"expname": img3.meta.filename, "exptype": "science"},
+    asn_filepath = str(tmp_path / "test_asn.json")
+    asn_dict = {
+        "asn_id": "a3001",
+        "products": [
+            {
+                "name": "test.asdf",
+                "members": [{"expname": img.meta.filename, "exptype": "science"}],
+            }
         ],
-    )
+    }
+    with open(asn_filepath, "w") as f:
+        json.dump(asn_dict, f)
 
-    res = trs.TweakRegStep.call(
+    res = TweakRegStep.call(
         asn_filepath,
         use_custom_catalogs=True,
-        catalog_format=catalog_format,
         catfile=catfile,
     )
 
     assert isinstance(res, ModelLibrary)
-
     with res:
-        for i, (model, target) in enumerate(zip(res, [img1, img2, img3], strict=False)):
-            assert hasattr(model.meta, "asn")
-
-            assert model.meta.filename == target.meta.filename
-
-            assert type(model) is type(target)
-
-            assert (model.data == target.data).all()
-
-            res.shelve(model, i, modify=False)
+        m = res.borrow(0)
+        assert m.meta.cal_step.tweakreg == "COMPLETE"
+        assert m.meta.source_catalog.tweakreg_catalog_name.endswith("custom_catalog")
+        res.shelve(m, modify=False)
 
 
 @pytest.mark.parametrize(
     "theta, offset_x, offset_y",
     [
-        (0 * u.deg, 0, 0),
-        (0 * u.deg, 1, 0),
-        (0 * u.deg, 0, 1),
-        (0 * u.deg, 1, 1),
-        (0.05 * u.deg, 0, 0),
-        (0.05 * u.deg, 1, 0),
-        (0.05 * u.deg, 0, 1),
-        (0.05 * u.deg, 1, 1),
-        (0.1 * u.deg, 0, 0),
-        (0.1 * u.deg, 1, 0),
-        (0.1 * u.deg, 0, 1),
         (0.1 * u.deg, 1, 1),
     ],
 )
-def test_tweakreg_rotated_plane(tmp_path, theta, offset_x, offset_y, request):
+def test_tweakreg_rotated_plane(
+    tmp_path, tweakreg_image, theta, offset_x, offset_y, gaia_coords
+):
     """
     Test that TweakReg returns accurate results.
     """
-    gaia_cat = get_catalog(
-        right_ascension=270, declination=66, search_radius=100 / 3600
-    )
-    gaia_source_coords = [
-        (ra, dec) for ra, dec in zip(gaia_cat["ra"], gaia_cat["dec"], strict=False)
-    ]
-
-    img = request.getfixturevalue("base_image")(shift_1=1000, shift_2=1000)
+    img = tweakreg_image(shift_1=1000, shift_2=1000)
     original_wcs = copy.deepcopy(img.meta.wcs)
 
     # calculate original (x,y) for Gaia sources
     original_xy_gaia_sources = np.array(
-        [original_wcs.world_to_pixel_values(ra, dec) for ra, dec in gaia_source_coords]
+        [original_wcs.world_to_pixel_values(ra, dec) for ra, dec in gaia_coords]
     )
     # move Gaia sources around by applying linear transformations
     # to their coords in the projected plane (same as a "wrong WCS")
@@ -797,11 +147,13 @@ def test_tweakreg_rotated_plane(tmp_path, theta, offset_x, offset_y, request):
     )
     # save modified catalog to meta.tweakreg_catalog
     # (coords in the projection plane)
-    add_tweakreg_catalog_attribute(
-        tmp_path, img, catalog_data=transformed_xy_gaia_sources
+    Table(transformed_xy_gaia_sources, names=("x", "y")).write(
+        img.meta.source_catalog.tweakreg_catalog_name,
+        format="ascii.ecsv",
+        overwrite=True,
     )
 
-    trs.TweakRegStep.call([img], abs_minobj=3)
+    TweakRegStep.call([img], abs_minobj=3)
 
     # get world coords for Gaia sources using "wrong WCS"
     original_ref_source = [
@@ -813,8 +165,7 @@ def test_tweakreg_rotated_plane(tmp_path, theta, offset_x, offset_y, request):
     ]
     # celestial coordinates for Gaia sources
     gaia_ref_source = [
-        coord.SkyCoord(ra * u.deg, dec * u.deg, frame="icrs")
-        for ra, dec in gaia_source_coords
+        coord.SkyCoord(ra * u.deg, dec * u.deg, frame="icrs") for ra, dec in gaia_coords
     ]
     # calculate distance between "wrong WCS" result and Gaia
     # (rounded to the 10th decimal place to avoid floating point issues)
@@ -834,52 +185,14 @@ def test_tweakreg_rotated_plane(tmp_path, theta, offset_x, offset_y, request):
     ).all()
 
 
-def test_tweakreg_parses_asn_correctly(tmp_path, base_image):
-    """Test that TweakReg can parse an ASN file properly."""
-
-    img_1 = base_image(shift_1=1000, shift_2=1000)
-    img_2 = base_image(shift_1=1000, shift_2=1000)
-    img_1.meta["filename"] = "img_1.asdf"
-    img_2.meta["filename"] = "img_2.asdf"
-    add_tweakreg_catalog_attribute(tmp_path, img_1, catalog_filename="img_1")
-    add_tweakreg_catalog_attribute(tmp_path, img_2, catalog_filename="img_2")
-    img_1.save(tmp_path / "img_1.asdf")
-    img_2.save(tmp_path / "img_2.asdf")
-    asn_filepath = create_asn_file(tmp_path)
-    with open(asn_filepath) as f:
-        asn_content = json.load(f)
-
-    res = trs.TweakRegStep.call(asn_filepath)
-
-    assert isinstance(res, ModelLibrary)
-
-    with res:
-        models = list(res)
-        assert hasattr(models[0].meta, "asn")
-        assert models[0].meta.asn["pool_name"] == asn_content["asn_pool"]
-        assert models[1].meta.asn["pool_name"] == asn_content["asn_pool"]
-
-        assert models[0].meta.filename == img_1.meta.filename
-        assert models[1].meta.filename == img_2.meta.filename
-
-        assert type(models[0]) is type(img_1)
-        assert type(models[1]) is type(img_2)
-
-        assert (models[0].data == img_1.data).all()
-        assert (models[1].data == img_2.data).all()
-
-        [res.shelve(m, i, modify=False) for i, m in enumerate(models)]
-
-
-def test_fit_results_in_meta(tmp_path, base_image):
+def test_fit_results_in_meta(tmp_path, tweakreg_image):
     """
     Test that the WCS fit results from tweakwcs are available in the meta tree.
     """
 
-    img = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img)
+    img = tweakreg_image(shift_1=1000, shift_2=1000)
 
-    res = trs.TweakRegStep.call([img])
+    res = TweakRegStep.call([img])
 
     assert isinstance(res, ModelLibrary)
     with res:
@@ -889,15 +202,13 @@ def test_fit_results_in_meta(tmp_path, base_image):
             res.shelve(model, i, modify=False)
 
 
-def test_tweakreg_handles_multiple_groups(tmp_path, base_image):
+def test_tweakreg_handles_multiple_groups(tmp_path, tweakreg_image):
     """
     Test that TweakRegStep can perform relative alignment for all images in the groups
     before performing absolute alignment.
     """
-    img1 = base_image(shift_1=1000, shift_2=1000)
-    img2 = base_image(shift_1=990, shift_2=990)
-    add_tweakreg_catalog_attribute(tmp_path, img1, catalog_filename="img1")
-    add_tweakreg_catalog_attribute(tmp_path, img2, catalog_filename="img2")
+    img1 = tweakreg_image(shift_1=1000, shift_2=1000, catalog_filename="img1")
+    img2 = tweakreg_image(shift_1=990, shift_2=990, catalog_filename="img2")
 
     img1.meta.observation.program = 1
     img1.meta.observation["observation_id"] = "1"
@@ -907,89 +218,15 @@ def test_tweakreg_handles_multiple_groups(tmp_path, base_image):
     img1.meta["filename"] = "file1.asdf"
     img2.meta["filename"] = "file2.asdf"
 
-    res = trs.TweakRegStep.call([img1, img2])
+    res = TweakRegStep.call([img1, img2])
 
     assert len(res.group_names) == 2
 
 
-def test_parse_catfile_valid_catalog(tmp_path, base_image):
-    """
-    Test that _parse_catfile can parse a custom catalog with valid format.
-    """
-    # create custom catalog file and input datamodels
-    catalog_format = "ascii.ecsv"
-    res_dict = create_custom_catalogs(
-        tmp_path, base_image, catalog_format=catalog_format
-    )
-    catfile = res_dict.get("catfile")
-    catdict = trs._parse_catfile(catfile)
-
-    assert all(
-        x.meta.filename == y
-        for x, y in zip(res_dict.get("datamodels"), catdict.keys(), strict=False)
-    )
-
-
-@pytest.mark.parametrize("catfile", (None, ""))
-def test_parse_catfile_returns_none(catfile):
-    """
-    Test that _parse_catfile returns None when catfile = None or catfile = "".
-    """
-    catdict = trs._parse_catfile(catfile=catfile)
-
-    assert catdict is None
-
-
-@pytest.mark.parametrize(
-    "catfile_line_content",
-    ["img1.asdf\nimg2.asdf\nimg3.asdf"],
-)
-def test_parse_catfile_returns_none_on_invalid_content(tmp_path, catfile_line_content):
-    """
-    Test that _parse_catfile returns a dict where all the values are None
-    if only filename is present in catfile (i.e. no associated catalog).
-    """
-    # create custom catalog file and input datamodels
-    catfile = str(tmp_path / "catfile.txt")
-    catfile_content = StringIO()
-    # write empty line to catfile
-    catfile_content.write(catfile_line_content)
-    # write StringIO object to disk
-    with open(catfile, mode="w") as f:
-        print(catfile_content.getvalue(), file=f)
-
-    catdict = trs._parse_catfile(catfile)
-
-    assert not all(catdict.values())
-
-
-@pytest.mark.parametrize(
-    "catfile_line_content",
-    ["img1.asdf column1 column2 column3"],
-)
-def test_parse_catfile_raises_error_on_invalid_content(tmp_path, catfile_line_content):
-    """
-    Test that _parse_catfile raises an error if catfile contains more than
-    two columns (i.e. image name and corresponding catalog path).
-    """
-    # create custom catalog file and input datamodels
-    catfile = str(tmp_path / "catfile.txt")
-    catfile_content = StringIO()
-    # write empty line to catfile
-    catfile_content.write(catfile_line_content)
-    # write StringIO object to disk
-    with open(catfile, mode="w") as f:
-        print(catfile_content.getvalue(), file=f)
-
-    with pytest.raises(ValueError):
-        trs._parse_catfile(catfile)
-
-
-def test_update_source_catalog_coordinates(function_jail, base_image):
+def test_update_source_catalog_coordinates(function_jail, tweakreg_image):
     """Test that TweakReg updates the catalog coordinates with the tweaked WCS."""
 
-    img = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(function_jail, img, catalog_filename="img_1")
+    img = tweakreg_image(shift_1=1000, shift_2=1000, catalog_filename="img_1")
 
     # create ImageSourceCatalogModel
     source_catalog = setup_source_catalog(img)
@@ -999,7 +236,7 @@ def test_update_source_catalog_coordinates(function_jail, base_image):
     img.meta.source_catalog.tweakreg_catalog_name = "img_1_cat.parquet"
 
     # run TweakRegStep
-    res = trs.TweakRegStep.call([img])
+    res = TweakRegStep.call([img])
 
     # tweak the current WCS using TweakRegStep and save the updated cat file
     with res:
@@ -1028,11 +265,10 @@ def test_update_source_catalog_coordinates(function_jail, base_image):
     np.testing.assert_array_equal(cat_dec_psf, expected_psf[1])
 
 
-def test_source_catalog_coordinates_have_changed(function_jail, base_image):
+def test_source_catalog_coordinates_have_changed(function_jail, tweakreg_image):
     """Test that the original catalog file content is different from the updated file."""
 
-    img = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(function_jail, img, catalog_filename="img_1")
+    img = tweakreg_image(shift_1=1000, shift_2=1000, catalog_filename="img_1")
 
     # create ImageSourceCatalogModel
     source_catalog = setup_source_catalog(img)
@@ -1045,7 +281,7 @@ def test_source_catalog_coordinates_have_changed(function_jail, base_image):
     img.meta.source_catalog.tweakreg_catalog_name = "img_1_cat.parquet"
 
     # run TweakRegStep
-    res = trs.TweakRegStep.call([img])
+    res = TweakRegStep.call([img])
 
     # tweak the current WCS using TweakRegStep and save the updated cat file
     with res:
@@ -1210,23 +446,6 @@ def update_catalog_coordinates(tweakreg_catalog_name, tweaked_wcs):
     catalog.write(tweakreg_catalog_name, overwrite=True)
 
 
-@pytest.mark.parametrize("exposure_type", ["WFI_FLAT", "WFI_WFSC"])
-def test_tweakreg_skips_invalid_exposure_types(exposure_type, tmp_path, base_image):
-    """Test that TweakReg updates meta.cal_step with tweakreg = COMPLETE."""
-    img1 = base_image(shift_1=1000, shift_2=1000)
-    img1.meta.exposure.type = exposure_type
-    img2 = base_image(shift_1=1000, shift_2=1000)
-    img2.meta.exposure.type = exposure_type
-    res = trs.TweakRegStep.call([img1, img2])
-
-    assert isinstance(res, ModelLibrary)
-    with res:
-        for i, model in enumerate(res):
-            assert hasattr(model.meta.cal_step, "tweakreg")
-            assert model.meta.cal_step.tweakreg == "SKIPPED"
-            res.shelve(model, i, modify=False)
-
-
 @pytest.mark.parametrize(
     "catalog_data, expected_colnames, flags_step_as_failed",
     [
@@ -1262,69 +481,64 @@ def test_validate_catalog_columns(
         assert set(catalog.colnames) == set(expected_colnames)
 
 
-def test_tweakreg_flags_failed_step_on_invalid_catalog_columns(base_image):
+def test_tweakreg_flags_failed_step_on_invalid_catalog_columns(tweakreg_image):
     """Test that TweakRegStep raises ValueError when catalog columns are invalid."""
-    import pytest
-
-    class FakeSourceCatalog(dict):
-        """Create a fake source catalog with both attribute and item access."""
-
-        def __getattr__(self, name):
-            return self[name]
-
-        def __setattr__(self, name, value):
-            self[name] = value
-
-    img = base_image(shift_1=1000, shift_2=1000)
+    img = tweakreg_image(shift_1=1000, shift_2=1000)
     # Add a tweakreg catalog with missing required columns
-    bad_catalog = Table({"a": [1, 2, 3], "b": [4, 5, 6]})
-    img.meta["source_catalog"] = FakeSourceCatalog()
-    img.meta.source_catalog.tweakreg_catalog = bad_catalog.as_array()
+    img.meta.source_catalog = {
+        "tweakreg_catalog": Table({"a": [1, 2, 3], "b": [4, 5, 6]}).as_array()
+    }
 
     # Should raise ValueError due to invalid catalog columns
-    with pytest.raises(ValueError):
-        trs.TweakRegStep.call([img])
+    with pytest.raises(ValueError, match=r"'tweakreg' source catalogs must contain"):
+        TweakRegStep.call([img])
 
 
-def test_tweakreg_handles_mixed_exposure_types(tmp_path, base_image):
+def test_tweakreg_handles_mixed_exposure_types(tmp_path, tweakreg_image):
     """Test that TweakReg can handle mixed exposure types
     (non-WFI_IMAGE data will be marked as SKIPPED only and won't be processed)."""
-    img1 = base_image(shift_1=1000, shift_2=1000)
-    img1.meta.exposure.type = "WFI_IM_DARK"
-
-    img2 = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img2, catalog_filename="img2")
-    img2.meta.exposure.type = "WFI_IMAGE"
-
-    img3 = base_image(shift_1=1000, shift_2=1000)
-    img3.meta.exposure.type = "WFI_SP_DARK"
+    invalid_types = [
+        "WFI_SPECTRAL",
+        "WFI_IM_DARK",
+        "WFI_SP_DARK",
+        "WFI_FLAT",
+        "WFI_LOLO",
+        "WFI_WFSC",
+        "WFI_DARK",
+        "WFI_GRISM",
+        "WFI_PRISM",
+    ]
 
-    img4 = base_image(shift_1=1000, shift_2=1000)
-    add_tweakreg_catalog_attribute(tmp_path, img4, catalog_filename="img4")
-    img4.meta.exposure.type = "WFI_IMAGE"
+    # start with 1 valid type
+    img = tweakreg_image(catalog_filename="img0")
+    imgs = [img]
 
-    img5 = base_image(shift_1=1000, shift_2=1000)
-    img5.meta.exposure.type = "WFI_IM_DARK"
+    # add one of each invalid type
+    for i, invalid_type in enumerate(invalid_types):
+        img = tweakreg_image(catalog_filename=f"img{i + 1}")
+        img.meta.exposure.type = invalid_type
+        imgs.append(img)
 
-    res = trs.TweakRegStep.call([img1, img2, img3, img4, img5])
+    res = TweakRegStep.call(imgs)
 
-    assert len(res) == 5
-    assert img1.meta.cal_step.tweakreg == "SKIPPED"
-    assert img2.meta.cal_step.tweakreg == "COMPLETE"
-    assert img3.meta.cal_step.tweakreg == "SKIPPED"
-    assert img4.meta.cal_step.tweakreg == "COMPLETE"
-    assert img5.meta.cal_step.tweakreg == "SKIPPED"
+    assert len(res) == len(imgs)
+    with res:
+        for i, m in enumerate(res):
+            if i == 0:
+                assert m.meta.cal_step.tweakreg == "COMPLETE"
+            else:
+                assert m.meta.cal_step.tweakreg == "SKIPPED"
+            res.shelve(m, modify=False)
 
 
-def test_tweakreg_updates_s_region(tmp_path, base_image):
+def test_tweakreg_updates_s_region(tmp_path, tweakreg_image):
     """Test that the TweakRegStep updates the s_region attribute."""
-    img = base_image(shift_1=1000, shift_2=1000)
+    img = tweakreg_image(shift_1=1000, shift_2=1000, catalog_filename="img")
     old_fake_s_region = "POLYGON ICRS 1.0000000000000 2.0000000000000 3.0000000000000 4.0000000000000 5.0000000000000 6.0000000000000 7.0000000000000 8.0000000000000 "
     img.meta.wcsinfo["s_region"] = old_fake_s_region
-    add_tweakreg_catalog_attribute(tmp_path, img, catalog_filename="img")
 
     # call TweakRegStep to update WCS & S_REGION
-    res = trs.TweakRegStep.call([img])
+    res = TweakRegStep.call(img)
 
     with res:
         for i, model in enumerate(res):
@@ -1333,12 +547,11 @@ def test_tweakreg_updates_s_region(tmp_path, base_image):
 
 
 @pytest.mark.parametrize("save_results", [True, False])
-def test_tweakreg_produces_output(tmp_path, base_image, save_results):
+def test_tweakreg_produces_output(tmp_path, tweakreg_image, save_results):
     """With save_results and output_dir set confirm expected files are in the output directory"""
-    img = base_image()
-    add_tweakreg_catalog_attribute(tmp_path, img, catalog_filename="img")
+    img = tweakreg_image(catalog_filename="img")
     base_filename = img.meta.filename
-    trs.TweakRegStep.call([img], save_results=save_results, output_dir=str(tmp_path))
+    TweakRegStep.call([img], save_results=save_results, output_dir=str(tmp_path))
 
     fns = [p.name for p in tmp_path.iterdir()]
     # the files should exist only if save_results was True
diff --git a/romancal/tweakreg/tweakreg_step.py b/romancal/tweakreg/tweakreg_step.py
index 1386ae8e1..47b92be61 100644
--- a/romancal/tweakreg/tweakreg_step.py
+++ b/romancal/tweakreg/tweakreg_step.py
@@ -13,11 +13,7 @@
 from astropy.table import Table
 from roman_datamodels import datamodels as rdm
 from stcal.tweakreg import tweakreg
-from stcal.tweakreg.tweakreg import (
-    _SINGLE_GROUP_REFCAT_STR,
-    SINGLE_GROUP_REFCAT,
-    TweakregError,
-)
+from stcal.tweakreg.tweakreg import TweakregError
 
 from romancal.assign_wcs.utils import add_s_region
 from romancal.lib.save_wcs import save_wfiwcs
@@ -29,7 +25,7 @@
 if TYPE_CHECKING:
     from typing import ClassVar
 
-DEFAULT_ABS_REFCAT = SINGLE_GROUP_REFCAT[0]
+DEFAULT_ABS_REFCAT = "GAIAREFCAT"
 
 __all__ = ["TweakRegStep"]
 
@@ -59,8 +55,7 @@ class TweakRegStep(RomanStep):
         fitgeometry = option('shift', 'rshift', 'rscale', 'general', default='rshift') # Fitting geometry
         nclip = integer(min=0, default=3) # Number of clipping iterations in fit
         sigma = float(min=0.0, default=3.0) # Clipping limit in sigma units
-        abs_refcat = string(default='{DEFAULT_ABS_REFCAT}')  # Absolute reference
-        # catalog. Options: {_SINGLE_GROUP_REFCAT_STR}
+        abs_refcat = string(default='{DEFAULT_ABS_REFCAT}')  # Absolute reference catalog
         save_abs_catalog = boolean(default=False)  # Write out used absolute astrometric reference catalog as a separate product
         abs_minobj = integer(default=15) # Minimum number of objects acceptable for matching when performing absolute astrometry
         abs_searchrad = float(default=6.0) # The search radius in arcsec for a match when performing absolute astrometry
@@ -79,25 +74,17 @@ class TweakRegStep(RomanStep):
 
     reference_file_types: ClassVar = []
 
-    def process(self, input):
-        # properly handle input
-        try:
-            if isinstance(input, rdm.DataModel):
-                images = ModelLibrary([input])
-            elif str(input).endswith(".asdf"):
-                images = ModelLibrary([rdm.open(input)])
-            elif isinstance(input, ModelLibrary):
-                images = input
-            else:
-                images = ModelLibrary(input)
-        except TypeError as e:
-            e.args = (
-                "Input to tweakreg must be a list of DataModels, an "
-                "association, or an already open ModelLibrary "
-                "containing one or more DataModels.",
-                *e.args[1:],
-            )
-            raise e
+    def _prepare_input(self, init):
+        if isinstance(init, rdm.DataModel):
+            return ModelLibrary([init])
+        if isinstance(init, ModelLibrary):
+            return init
+        if str(init).endswith(".asdf"):
+            return ModelLibrary([rdm.open(init)])
+        return ModelLibrary(init)
+
+    def process(self, init):
+        images = self._prepare_input(init)
 
         if not images:
             raise ValueError("Input must contain at least one image model.")
@@ -187,11 +174,9 @@ def process(self, input):
 
                     # validate catalog columns
                     if not _validate_catalog_columns(catalog):
-                        raise ValueError("""
-                        'tweakreg' source catalogs must contain a header
-                        with columns named either 'x' and 'y' or
-                        'x_psf' and 'y_psf'. Neither were found in the
-                        catalog provided.""")
+                        raise ValueError(
+                            "'tweakreg' source catalogs must contain a header with columns named either 'x' and 'y' or 'x_psf' and 'y_psf'. Neither were found in the catalog provided."
+                        )
 
                     catalog = tweakreg.filter_catalog_by_bounding_box(
                         catalog, image_model.meta.wcs.bounding_box
@@ -240,12 +225,11 @@ def process(self, input):
                 except TweakregError as e:
                     log.warning(str(e))
 
-            if self.abs_refcat in SINGLE_GROUP_REFCAT:
-                try:
-                    self.do_absolute_alignment(ref_image, imcats)
-                except TweakregError as e:
-                    log.warning(str(e))
-                    return images
+            try:
+                self.do_absolute_alignment(ref_image, imcats)
+            except TweakregError as e:
+                log.warning(str(e))
+                return images
 
             # finalize step
             with images:
@@ -498,8 +482,7 @@ def do_absolute_alignment(self, ref_image, imcats):
             abs_use2dhist=self.abs_use2dhist,
             abs_separation=self.abs_separation,
             abs_tolerance=self.abs_tolerance,
-            save_abs_catalog=self.save_abs_catalog,
-            abs_catalog_output_dir=self.output_dir,
+            save_abs_catalog=False,
             clip_accum=True,
             timeout=self.vo_timeout,
         )