diff --git a/msaexp/utils.py b/msaexp/utils.py
index 6265b4a..604637f 100644
--- a/msaexp/utils.py
+++ b/msaexp/utils.py
@@ -1822,7 +1822,12 @@ def drizzled_hdu_figure(
             lw=2,
         )
         ap.step(
-            ptab["pfit"] / pmax, xpr, color="coral", where="pre", alpha=0.5, lw=1
+            ptab["pfit"] / pmax,
+            xpr,
+            color="coral",
+            where="pre",
+            alpha=0.5,
+            lw=1,
         )
         ap.fill_betweenx(
             xpr + 0.5, xpr * 0.0, ptab["pfit"] / pmax, color="coral", alpha=0.2
@@ -2614,6 +2619,231 @@ def get_prism_wave_bar_correction(
     return bar, is_wrapped
 
 
+def slit_shutter_scale(slit):
+    """
+    Pixel scale of the ``slit_frame`` coordinate
+
+    Parameters
+    ----------
+    slit : `~jwst.datamodels.SlitModel`
+        Slitlet data object
+
+    Returns
+    -------
+    pix_scale : float
+        delta ``slit_frame`` / delta pixel.
+
+    """
+    sh = slit.data.shape
+    wcs = slit.meta.wcs
+    d2s = wcs.get_transform("detector", "slit_frame")
+
+    x0 = d2s(sh[1] // 2, sh[0] // 2)
+    x1 = d2s(sh[1] // 2, sh[0] // 2 + 1)
+
+    dx = np.array(x1) - np.array(x0)
+    pix_scale = np.sqrt(dx[0] ** 2 + dx[1] ** 2)
+
+    return pix_scale
+
+
+def get_slit_coordinates(slit, trace_with_ypos=False, **kwargs):
+    """
+    Get wavelength and cross-dispersion coordinate arrays for a 2D slitlet
+
+    Parameters
+    ----------
+    slit : `~jwst.datamodels.SlitModel`
+        Slitlet data object
+
+    trace_with_ypos : bool
+        Include source y position in trace calculation
+
+    Returns
+    -------
+    wave : array-like
+        Wavelengths, microns
+
+    slit_frame_y : array-like
+        ``slit_frame`` cross dispersion coordinate
+
+    yslit : array-like
+        Pixel offset relative to the trace center
+    """
+    sh = slit.data.shape
+    yp, xp = np.indices(sh)
+
+    _res = slit_trace_center(
+        slit,
+        with_source_xpos=False,
+        with_source_ypos=trace_with_ypos,
+        index_offset=0.0,
+    )
+
+    _xtr, _ytr, _wtr, slit_ra, slit_dec = _res
+
+    xslit = xp
+    yslit = yp - _ytr
+    ypix = yp
+
+    wcs = slit.meta.wcs
+    d2w = wcs.get_transform("detector", "world")
+
+    _ypi, _xpi = np.indices(slit.data.shape)
+    _ras, _des, _wave = d2w(_xpi, _ypi)
+
+    d2s = wcs.get_transform("detector", "slit_frame")
+    _sx, _sy, _slam = np.array(d2s(_xpi, _ypi))
+    slit_frame_y = _sy
+
+    return _wave, slit_frame_y, yslit
+
+
+def get_slit_data(slit, wrap=True, **kwargs):
+    """
+    Parse slit coordinates and attributes
+
+    Parameters
+    ----------
+    slit : `~jwst.datamodels.SlitModel`
+        Slitlet data object
+
+    wrap : bool
+        Parameter for `msaexp.utils.get_prism_wave_bar_correction`
+
+    Returns
+    -------
+    data : dict
+        Slit data
+
+    """
+    slx = slice(slit.xstart - 1, slit.xstart - 1 + slit.xsize)
+    sly = slice(slit.ystart - 1, slit.ystart - 1 + slit.ysize)
+
+    wave, slit_frame_y, yslit = get_slit_coordinates(slit, **kwargs)
+    sh = wave.shape
+
+    shutter_scale = slit_shutter_scale(slit)
+
+    dy = 0.0
+
+    shutter_y = (slit_frame_y / shutter_scale + dy) / 5.0
+
+    if slit.meta.exposure.type == "NRS_FIXEDSLIT":
+        bar = np.ones_like(wave).reshape(sh)
+        bar_wrapped = False
+    else:
+        bar, bar_wrapped = get_prism_wave_bar_correction(
+            shutter_y.flatten(),
+            wave.flatten(),
+            num_shutters=np.minimum(len(slit.shutter_state), 3),
+            wrap=wrap,
+        )
+        bar = bar.reshape(sh)
+
+    corr = slit.data * 1 / bar
+    msk = ~np.isfinite(corr + wave + bar)
+
+    corr[msk] = 0
+    wave[msk] = 0
+    bar[msk] = 0
+
+    data = {
+        "wave": wave,
+        "slit_frame_y": slit_frame_y,
+        "shutter_y": shutter_y,
+        "yslit": yslit,
+        "shape": sh,
+        "corr": corr,
+        "bar": bar,
+        "slx": slx,
+        "sly": sly,
+        "shutter_state": slit.shutter_state,
+        "num_shutters": len(slit.shutter_state),
+        "bar_wrapped": bar_wrapped,
+    }
+
+    return data
+
+
+def fixed_slit_flat_field(
+    slit, apply=True, verbose=True, force=False, **kwargs
+):
+    """
+    Fixed slit cross-dispersion profile flat field
+
+    Parameters
+    ----------
+    slit : `~jwst.datamodels.SlitModel`
+        Slitlet data object
+
+    apply : bool
+        Apply to ``slit.data`` attribute
+
+    force : bool
+        Apply even if ``slit.flat_profile`` already found
+
+    Returns
+    -------
+    flat_profile : array-like
+        2D flat-field profile
+
+    """
+    import yaml
+
+    if slit.meta.exposure.type != "NRS_FIXEDSLIT":
+        return None
+
+    profile_file = os.path.join(
+        os.path.dirname(__file__),
+        "data/extended_sensitivity/",
+        "fixed_slit_flat_profile_{0}.yaml".format(slit.name.lower()),
+    )
+
+    if not os.path.exists(profile_file):
+        msg = f"fixed_slit_flat_field: {profile_file} not found"
+        grizli.utils.log_comment(grizli.utils.LOGFILE, msg, verbose=verbose)
+        return None
+
+    msg = f"fixed_slit_flat_field: {profile_file}  (apply={apply})"
+    grizli.utils.log_comment(grizli.utils.LOGFILE, msg, verbose=verbose)
+
+    with open(profile_file) as fp:
+        fs_data = yaml.load(fp, Loader=yaml.Loader)
+
+    slit_data = get_slit_data(slit)
+
+    bspl = grizli.utils.bspline_templates(
+        slit_data["yslit"].flatten(),
+        df=fs_data["ydf"],
+        minmax=fs_data["minmax"],
+        get_matrix=True,
+    )
+
+    coeffs = np.array(fs_data["coeffs"])
+    flat_profile = bspl.dot(coeffs).reshape(slit.data.shape)
+    flat_profile[flat_profile < 0] = 0.0
+
+    if apply:
+        if (not hasattr(slit, "flat_profile")) | force:
+            if hasattr(slit, "flat_profile"):
+                slit.flat_profile *= flat_profile
+            else:
+                slit.flat_profile = flat_profile
+
+            slit.data /= flat_profile
+            slit.err /= flat_profile
+            slit.var_rnoise /= flat_profile**2
+            slit.var_poisson /= flat_profile**2
+        else:
+            msg = f"fixed_slit_flat_field: existing flat_profile found"
+            grizli.utils.log_comment(
+                grizli.utils.LOGFILE, msg, verbose=verbose
+            )
+
+    return flat_profile
+
+
 def slit_extended_flux_calibration(
     slit,
     sens_file=None,