diff --git a/.travis.yml b/.travis.yml
index 6621e7de..e1438232 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,7 +7,7 @@ dist: trusty
 compiler: gcc
 
 env:
-  - OS_TYPE=centos OS_VERSION=7 DOCKER_IMAGE=lsstdesc/stack-sims:w_2019_23-sims_w_2019_23
+  - OS_TYPE=centos OS_VERSION=7 DOCKER_IMAGE=lsstdesc/stack-sims:w_2019_37-sims_w_2019_37
 
 services:
   - docker
diff --git a/python/desc/imsim/ImageSimulator.py b/python/desc/imsim/ImageSimulator.py
index 7f1cd728..90a833c5 100644
--- a/python/desc/imsim/ImageSimulator.py
+++ b/python/desc/imsim/ImageSimulator.py
@@ -417,9 +417,6 @@ def __call__(self, gs_objects):
             if nan_fluxes > 0:
                 logger.info("%s objects had nan fluxes", nan_fluxes)
 
-        # Recover the memory devoted to the GalSimCelestialObject instances.
-        gs_objects.reset()
-
         add_cosmic_rays(gs_interpreter, IMAGE_SIMULATOR.phot_params)
         full_well = int(IMAGE_SIMULATOR.config['ccd']['full_well'])
         apply_channel_bleeding(gs_interpreter, full_well)
diff --git a/python/desc/imsim/camera_info.py b/python/desc/imsim/camera_info.py
index 42cf5fae..0b05cf03 100644
--- a/python/desc/imsim/camera_info.py
+++ b/python/desc/imsim/camera_info.py
@@ -2,7 +2,7 @@
 Class to encapsulate info from lsst.obs.lsst.imsim.ImsimMapper().camera.
 """
 import astropy.time
-import lsst.afw.geom as afw_geom
+import lsst.geom as lsst_geom
 from lsst.obs.lsst.imsim import ImsimMapper
 
 __all__ = ['CameraInfo', 'getHourAngle']
@@ -65,15 +65,15 @@ def mosaic_section(amp_info):
 
         Returns
         -------
-        lsst.afw.geom.Box2I
+        lsst.geom.Box2I
         """
         yseg, xseg = (int(x) for x in amp_info.getName()[-2:])
         width = amp_info.getBBox().getWidth()
         height = amp_info.getBBox().getHeight()
         xmin = xseg*width
         ymin = 0 if yseg == 1 else height
-        return afw_geom.Box2I(afw_geom.Point2I(xmin, ymin),
-                              afw_geom.Extent2I(width, height))
+        return lsst_geom.Box2I(lsst_geom.Point2I(xmin, ymin),
+                               lsst_geom.Extent2I(width, height))
 
 
 def getHourAngle(observatory, mjd, ra):
diff --git a/python/desc/imsim/camera_readout.py b/python/desc/imsim/camera_readout.py
index 245d62d2..90d32161 100644
--- a/python/desc/imsim/camera_readout.py
+++ b/python/desc/imsim/camera_readout.py
@@ -23,7 +23,7 @@
 from astropy.io import fits
 import astropy.time
 import galsim
-import lsst.afw.geom as afwGeom
+import lsst.geom as lsst_geom
 import lsst.afw.image as afwImage
 with warnings.catch_warnings():
     warnings.simplefilter('ignore')
@@ -567,7 +567,7 @@ def _noao_section_keyword(bbox, flipx=False, flipy=False):
 
         Parameters
         ----------
-        bbox : lsst.afw.geom.Box2I
+        bbox : lsst.geom.Box2I
             Bounding box.
         flipx : bool
             Flag to indicate that data should be flipped in the x-direction.
@@ -598,16 +598,16 @@ def set_itl_bboxes(amp):
     lsst.afw.table.tableLib.AmpInfoRecord
         The updated AmpInfoRecord.
     """
-    amp.setRawBBox(afwGeom.Box2I(afwGeom.Point2I(0, 0),
-                                 afwGeom.Extent2I(544, 2048)))
-    amp.setRawDataBBox(afwGeom.Box2I(afwGeom.Point2I(3, 0),
-                                     afwGeom.Extent2I(509, 2000)))
-    amp.setRawHorizontalOverscanBBox(afwGeom.Box2I(afwGeom.Point2I(512, 0),
-                                                   afwGeom.Extent2I(48, 2000)))
-    amp.setRawVerticalOverscanBBox(afwGeom.Box2I(afwGeom.Point2I(0, 2000),
-                                                 afwGeom.Extent2I(544, 48)))
-    amp.setRawPrescanBBox(afwGeom.Box2I(afwGeom.Point2I(0, 0),
-                                        afwGeom.Extent2I(3, 2000)))
+    amp.setRawBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 0),
+                                   lsst_geom.Extent2I(544, 2048)))
+    amp.setRawDataBBox(lsst_geom.Box2I(lsst_geom.Point2I(3, 0),
+                                       lsst_geom.Extent2I(509, 2000)))
+    amp.setRawHorizontalOverscanBBox(lsst_geom.Box2I(lsst_geom.Point2I(512, 0),
+                                                     lsst_geom.Extent2I(48, 2000)))
+    amp.setRawVerticalOverscanBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 2000),
+                                                   lsst_geom.Extent2I(544, 48)))
+    amp.setRawPrescanBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 0),
+                                          lsst_geom.Extent2I(3, 2000)))
     return amp
 
 
@@ -626,16 +626,16 @@ def set_e2v_bboxes(amp):
     lsst.afw.table.tableLib.AmpInfoRecord
         The updated AmpInfoRecord.
     """
-    amp.setRawBBox(afwGeom.Box2I(afwGeom.Point2I(0, 0),
-                                 afwGeom.Extent2I(542, 2022)))
-    amp.setRawDataBBox(afwGeom.Box2I(afwGeom.Point2I(10, 0),
-                                     afwGeom.Extent2I(522, 2002)))
-    amp.setRawHorizontalOverscanBBox(afwGeom.Box2I(afwGeom.Point2I(522, 0),
-                                                   afwGeom.Extent2I(20, 2002)))
-    amp.setRawVerticalOverscanBBox(afwGeom.Box2I(afwGeom.Point2I(0, 2002),
-                                                 afwGeom.Extent2I(542, 20)))
-    amp.setRawPrescanBBox(afwGeom.Box2I(afwGeom.Point2I(0, 0),
-                                        afwGeom.Extent2I(10, 2002)))
+    amp.setRawBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 0),
+                                   lsst_geom.Extent2I(542, 2022)))
+    amp.setRawDataBBox(lsst_geom.Box2I(lsst_geom.Point2I(10, 0),
+                                       lsst_geom.Extent2I(522, 2002)))
+    amp.setRawHorizontalOverscanBBox(lsst_geom.Box2I(lsst_geom.Point2I(522, 0),
+                                                     lsst_geom.Extent2I(20, 2002)))
+    amp.setRawVerticalOverscanBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 2002),
+                                                   lsst_geom.Extent2I(542, 20)))
+    amp.setRawPrescanBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 0),
+                                          lsst_geom.Extent2I(10, 2002)))
     return amp
 
 
@@ -654,16 +654,16 @@ def set_phosim_bboxes(amp):
     lsst.afw.table.tableLib.AmpInfoRecord
         The updated AmpInfoRecord.
     """
-    amp.setRawBBox(afwGeom.Box2I(afwGeom.Point2I(0, 0),
-                                 afwGeom.Extent2I(519, 2001)))
-    amp.setRawDataBBox(afwGeom.Box2I(afwGeom.Point2I(4, 1),
-                                     afwGeom.Extent2I(509, 2000)))
-    amp.setRawHorizontalOverscanBBox(afwGeom.Box2I(afwGeom.Point2I(513, 1),
-                                                   afwGeom.Extent2I(6, 2000)))
-    amp.setRawVerticalOverscanBBox(afwGeom.Box2I(afwGeom.Point2I(0, 2001),
-                                                 afwGeom.Extent2I(519, 0)))
-    amp.setRawPrescanBBox(afwGeom.Box2I(afwGeom.Point2I(0, 1),
-                                        afwGeom.Extent2I(4, 2000)))
+    amp.setRawBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 0),
+                                   lsst_geom.Extent2I(519, 2001)))
+    amp.setRawDataBBox(lsst_geom.Box2I(lsst_geom.Point2I(4, 1),
+                                       lsst_geom.Extent2I(509, 2000)))
+    amp.setRawHorizontalOverscanBBox(lsst_geom.Box2I(lsst_geom.Point2I(513, 1),
+                                                     lsst_geom.Extent2I(6, 2000)))
+    amp.setRawVerticalOverscanBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 2001),
+                                                   lsst_geom.Extent2I(519, 0)))
+    amp.setRawPrescanBBox(lsst_geom.Box2I(lsst_geom.Point2I(0, 1),
+                                          lsst_geom.Extent2I(4, 2000)))
     return amp
 
 PixelParameters = namedtuple('PixelParameters',
diff --git a/python/desc/imsim/imSim.py b/python/desc/imsim/imSim.py
index b74e5c0b..df6510a0 100644
--- a/python/desc/imsim/imSim.py
+++ b/python/desc/imsim/imSim.py
@@ -54,7 +54,6 @@
 
 __all__ = ['PhosimInstanceCatalogParseError',
            'photometricParameters', 'phosim_obs_metadata',
-           'sources_from_list',
            'metadata_from_file',
            'read_config', 'get_config', 'get_logger', 'get_image_dirs',
            'get_obs_lsstSim_camera',
@@ -62,7 +61,7 @@
            '_POINT_SOURCE', '_SERSIC_2D', '_RANDOM_WALK', '_FITS_IMAGE',
            'parsePhoSimInstanceFile',
            'add_treering_info', 'airmass', 'FWHMeff', 'FWHMgeom', 'make_psf',
-           'save_psf', 'load_psf', 'TracebackDecorator']
+           'save_psf', 'load_psf', 'TracebackDecorator', 'GsObjectList']
 
 
 class PhosimInstanceCatalogParseError(RuntimeError):
@@ -159,268 +158,6 @@ def uss_mem():
     return my_process.memory_full_info().uss/1024.**3
 
 
-def sources_from_list(object_lines, obs_md, phot_params, file_name,
-                      target_chip=None, log_level='INFO'):
-    """.
-
-    Parse the object lines from a phosim-style instance catalog and
-    repackage as GalSimCelestialObjects after applying on-chip
-    selections and consistency cuts on mag_norm, extinction
-    parameters, and galaxy shape and knot parameters.
-
-    Parameters
-    ----------
-    object_lines: list
-        List of object line entries from the instance catalog.
-    obs_md: ObservationMetaData
-        Visit-specific metadata from the instance catalog.
-    phot_params: PhotometricParameters
-        Visit-specific photometric data needed for computing object
-        fluxes.
-    file_name: str
-        The filename path of the instance catalog, used for inferring
-        the location of the Dynamic SEDs used by the sprinkled objects.
-    target_chip: str [None]
-        For multiprocessing mode, this is the name of the chip
-        (e.g., "R:2,2 S:1,1") being simulated.  If None, then
-        of the object lines are partitioned among all 189 science
-        sensors in the LSST focalplane.  Since this is a costly
-        calculation, doing it only for the target chip saves a lot
-        of compute time.
-    log_level: str ['INFO']
-        Logging level.
-
-    Returns
-    -------
-    list, dict:  A list of all the GalSimCelestialObjects that pass
-        the focalplane-level selections and a dict of those objects keyed
-        by chip name.
-    """
-    camera = get_obs_lsstSim_camera()
-    config = get_config()
-    logger = get_logger(log_level, name=(target_chip if target_chip is
-                                         not None else 'sources_from_list'))
-
-    num_objects = len(object_lines)
-
-    logger.debug('allocating object arrays, %s GB', uss_mem())
-    # RA, Dec in the coordinate system expected by PhoSim
-    ra_phosim = np.zeros(num_objects, dtype=float)
-    dec_phosim = np.zeros(num_objects, dtype=float)
-
-    sed_name = [None]*num_objects
-    mag_norm = 55.0*np.ones(num_objects, dtype=float)
-    gamma1 = np.zeros(num_objects, dtype=float)
-    gamma2 = np.zeros(num_objects, dtype=float)
-    gamma2_sign = config['wl_params']['gamma2_sign']
-    kappa = np.zeros(num_objects, dtype=float)
-
-    internal_av = np.zeros(num_objects, dtype=float)
-    internal_rv = np.zeros(num_objects, dtype=float)
-    galactic_av = np.zeros(num_objects, dtype=float)
-    galactic_rv = np.zeros(num_objects, dtype=float)
-    semi_major_arcsec = np.zeros(num_objects, dtype=float)
-    semi_minor_arcsec = np.zeros(num_objects, dtype=float)
-    position_angle_degrees = np.zeros(num_objects, dtype=float)
-    sersic_index = np.zeros(num_objects, dtype=float)
-    npoints = np.zeros(num_objects, dtype=int)
-    redshift = np.zeros(num_objects, dtype=float)
-    pixel_scale = np.zeros(num_objects, dtype=float)
-    rotation_angle = np.zeros(num_objects, dtype=float)
-    fits_image_file = dict()
-
-    unique_id = [None]*num_objects
-    object_type = np.zeros(num_objects, dtype=int)
-
-    logger.debug('looping over %s objects; %s GB', num_objects, uss_mem())
-    i_obj = -1
-    for line in object_lines:
-        params = line.strip().split()
-        if params[0] != 'object':
-            continue
-        i_obj += 1
-        unique_id[i_obj] = params[1]
-        ra_phosim[i_obj] = float(params[2])
-        dec_phosim[i_obj] = float(params[3])
-        mag_norm[i_obj] = float(params[4])
-        sed_name[i_obj] = params[5]
-        redshift[i_obj] = float(params[6])
-        gamma1[i_obj] = float(params[7])
-        gamma2[i_obj] = gamma2_sign*float(params[8])
-        kappa[i_obj] = float(params[9])
-        if params[12].lower() == 'point':
-            object_type[i_obj] = _POINT_SOURCE
-            i_gal_dust_model = 14
-            if params[13].lower() != 'none':
-                i_gal_dust_model = 16
-                internal_av[i_obj] = float(params[14])
-                internal_rv[i_obj] =float(params[15])
-            if params[i_gal_dust_model].lower() != 'none':
-                galactic_av[i_obj] = float(params[i_gal_dust_model+1])
-                galactic_rv[i_obj] = float(params[i_gal_dust_model+2])
-        elif params[12].lower() == 'sersic2d':
-            object_type[i_obj] = _SERSIC_2D
-            semi_major_arcsec[i_obj] = float(params[13])
-            semi_minor_arcsec[i_obj] = float(params[14])
-            position_angle_degrees[i_obj] = float(params[15])
-            sersic_index[i_obj] = float(params[16])
-            i_gal_dust_model = 18
-            if params[17].lower() != 'none':
-                i_gal_dust_model = 20
-                internal_av[i_obj] = float(params[18])
-                internal_rv[i_obj] = float(params[19])
-            if params[i_gal_dust_model].lower() != 'none':
-                galactic_av[i_obj] = float(params[i_gal_dust_model+1])
-                galactic_rv[i_obj] =float(params[i_gal_dust_model+2])
-        elif params[12].lower() == 'knots':
-            object_type[i_obj] = _RANDOM_WALK
-            semi_major_arcsec[i_obj] = float(params[13])
-            semi_minor_arcsec[i_obj] = float(params[14])
-            position_angle_degrees[i_obj] = float(params[15])
-            npoints[i_obj] = int(params[16])
-            i_gal_dust_model = 18
-            if params[17].lower() != 'none':
-                i_gal_dust_model = 20
-                internal_av[i_obj] = float(params[18])
-                internal_rv[i_obj] = float(params[19])
-            if params[i_gal_dust_model].lower() != 'none':
-                galactic_av[i_obj] = float(params[i_gal_dust_model+1])
-                galactic_rv[i_obj] = float(params[i_gal_dust_model+2])
-        elif (params[12].endswith('.fits') or params[12].endswith('.fits.gz')):
-            object_type[i_obj] = _FITS_IMAGE
-            fits_image_file[i_obj] = find_file_path(params[12], get_image_dirs())
-            pixel_scale[i_obj] = float(params[13])
-            rotation_angle[i_obj] = float(params[14])
-            i_gal_dust_model = 16
-            if params[15].lower() != 'none':
-                i_gal_dust_model = 18
-                internal_av[i_obj] = float(params[16])
-                internal_rv[i_obj] = float(params[17])
-            if params[i_gal_dust_model].lower() != 'none':
-                galactic_av[i_obj] = float(params[i_gal_dust_model+1])
-                galactic_rv[i_obj] = float(params[i_gal_dust_model+2])
-        else:
-            raise RuntimeError("Do not know how to handle "
-                               "object type: %s" % params[12])
-
-    logger.debug("computing pupil coords, %s GB", uss_mem())
-    ra_appGeo, dec_appGeo \
-        = PhoSimAstrometryBase._appGeoFromPhoSim(np.radians(ra_phosim),
-                                                 np.radians(dec_phosim),
-                                                 obs_md)
-
-    ra_obs_rad, dec_obs_rad \
-        = _observedFromAppGeo(ra_appGeo, dec_appGeo,
-                              obs_metadata=obs_md,
-                              includeRefraction=True)
-
-    semi_major_radians = radiansFromArcsec(semi_major_arcsec)
-    semi_minor_radians = radiansFromArcsec(semi_minor_arcsec)
-    # Account for PA sign difference wrt phosim convention.
-    position_angle_radians = np.radians(360. - position_angle_degrees)
-
-    x_pupil, y_pupil = _pupilCoordsFromObserved(ra_obs_rad,
-                                                dec_obs_rad,
-                                                obs_md)
-
-    bp_dict = BandpassDict.loadTotalBandpassesFromFiles()
-
-    object_is_valid = np.array([True]*num_objects)
-
-    invalid_objects = np.where(np.logical_or(np.logical_or(
-                                    mag_norm>50.0,
-                                    np.logical_and(galactic_av==0.0, galactic_rv==0.0)),
-                               np.logical_or(
-                                    np.logical_and(object_type==_SERSIC_2D,
-                                                 semi_major_arcsec<semi_minor_arcsec),
-                                    np.logical_and(object_type==_RANDOM_WALK,npoints<=0))))
-
-    object_is_valid[invalid_objects] = False
-
-    if len(invalid_objects[0]) > 0:
-        message = "\nOmitted %d suspicious objects from " % len(invalid_objects[0])
-        message += "the instance catalog:\n"
-        n_bad_mag_norm = len(np.where(mag_norm>50.0)[0])
-        message += "    %d had mag_norm > 50.0\n" % n_bad_mag_norm
-        n_bad_av = len(np.where(np.logical_and(galactic_av==0.0, galactic_rv==0.0))[0])
-        message += "    %d had galactic_Av == galactic_Rv == 0\n" % n_bad_av
-        n_bad_axes = len(np.where(np.logical_and(object_type==_SERSIC_2D,
-                                                 semi_major_arcsec<semi_minor_arcsec))[0])
-        message += "    %d had semi_major_axis < semi_minor_axis\n" % n_bad_axes
-        n_bad_knots = len(np.where(np.logical_and(object_type==_RANDOM_WALK,npoints<=0))[0])
-        message += "    %d had n_points <= 0 \n" % n_bad_knots
-        warnings.warn(message)
-
-    if target_chip is not None:
-        target_chips = [target_chip]
-    else:
-        # Set target_chips to None, in which case the _chip_downselect
-        # function will use all of the science sensors in the
-        # focalplane.
-        target_chips = None
-
-    on_chip_dict = _chip_downselect(mag_norm, x_pupil, y_pupil, logger,
-                                    target_chips)
-
-    my_sed_dirs = sed_dirs(file_name)
-
-    logger.debug('constructing GalSimCelestialObjects for %s objects; %s GB',
-                 num_objects, uss_mem())
-    gs_object_ids = set()
-    gs_object_arr = []
-    out_obj_dict = defaultdict(list)
-    for chip_name, on_chip in on_chip_dict.items():
-        i_obj_index = np.arange(num_objects)
-        for i_obj in i_obj_index[on_chip]:
-            if not object_is_valid[i_obj]:
-                continue
-
-            fits_file = None
-            if object_type[i_obj] == _POINT_SOURCE:
-                gs_type = 'pointSource'
-            elif object_type[i_obj] == _SERSIC_2D:
-                gs_type = 'sersic'
-            elif object_type[i_obj] == _RANDOM_WALK:
-                gs_type = 'RandomWalk'
-            elif object_type[i_obj] == _FITS_IMAGE:
-                gs_type = 'FitsImage'
-                fits_file = fits_image_file[i_obj]
-
-            sed_obj = SedWrapper(find_file_path(sed_name[i_obj], my_sed_dirs),
-                                 mag_norm[i_obj], redshift[i_obj],
-                                 internal_av[i_obj], internal_rv[i_obj],
-                                 galactic_av[i_obj], galactic_rv[i_obj],
-                                 bp_dict)
-
-            gs_object = GalSimCelestialObject(gs_type,
-                                              x_pupil[i_obj],
-                                              y_pupil[i_obj],
-                                              semi_major_radians[i_obj],
-                                              semi_minor_radians[i_obj],
-                                              semi_major_radians[i_obj],
-                                              position_angle_radians[i_obj],
-                                              sersic_index[i_obj],
-                                              sed_obj,
-                                              bp_dict,
-                                              phot_params,
-                                              npoints[i_obj],
-                                              fits_file,
-                                              pixel_scale[i_obj],
-                                              rotation_angle[i_obj],
-                                              gamma1=gamma1[i_obj],
-                                              gamma2=gamma2[i_obj],
-                                              kappa=kappa[i_obj],
-                                              uniqueId=unique_id[i_obj])
-            if gs_object.uniqueId not in gs_object_ids:
-                gs_object_ids.add(gs_object.uniqueId)
-                gs_object_arr.append(gs_object)
-            out_obj_dict[chip_name].append(gs_object)
-    gs_object_arr = np.array(gs_object_arr)
-    if target_chip is not None:
-        logger.debug('objects remaining %s', len(out_obj_dict[target_chip]))
-    logger.debug("about to return from sources_from_list, %s GB", uss_mem())
-    return gs_object_arr, out_obj_dict
-
 def _chip_downselect(mag_norm, x_pupil, y_pupil, logger, target_chips=None):
     """
     Down-select objects based on focalplane location relative to chip
@@ -611,67 +348,182 @@ def parsePhoSimInstanceFile(fileName, sensor_list, numRows=None,
                               log_level=log_level, sort_magnorm=sort_magnorm)
     gs_object_dict = {detname: GsObjectList(instcats[detname], instcats.obs_md,
                                             phot_params, instcats.instcat_file,
-                                            chip_name=detname,
-                                            log_level=log_level)
+                                            detname, log_level=log_level)
                       for detname in sensor_list}
 
     return PhoSimInstanceCatalogContents(obs_metadata,
                                          phot_params,
                                          ([], gs_object_dict))
 
-
 class GsObjectList:
-    """
-    List-like class to provide access to lists of objects from an
-    instance catalog, deferring creation of GalSimCelestialObjects
-    until items in the list are accessed.
-    """
-    def __init__(self, object_lines, obs_md, phot_params, file_name,
-                 chip_name=None, log_level='INFO'):
-        self.object_lines = object_lines
+    def __init__(self, object_lines, obs_md, phot_params, instcat_file,
+                 chip_name, log_level='INFO'):
         self.obs_md = obs_md
+        self.logger = get_logger(log_level, name=chip_name)
+        self._find_objects_on_chip(object_lines, chip_name)
         self.phot_params = phot_params
-        self.file_name = file_name
-        self.chip_name = chip_name
-        self.log_level = log_level
-        self._gs_objects = None
-
-    @property
-    def gs_objects(self):
-        if self._gs_objects is None:
-            obj_arr, obj_dict \
-                = sources_from_list(self.object_lines, self.obs_md,
-                                    self.phot_params, self.file_name,
-                                    target_chip=self.chip_name,
-                                    log_level=self.log_level)
-            if self.chip_name is not None:
-                try:
-                    self._gs_objects = obj_dict[self.chip_name]
-                except KeyError:
-                    self._gs_objects = []
-            else:
-                self._gs_objects = obj_arr
-        return self._gs_objects
-
-    def reset(self):
-        """
-        Reset the ._gs_objects attribute to None in order to recover
-        memory devoted to the GalSimCelestialObject instances.
-        """
-        self._gs_objects = None
+        self.sed_dirs = sed_dirs(instcat_file)
+        config = get_config()
+        self.gamma2_sign = config['wl_params']['gamma2_sign']
+
+    def _find_objects_on_chip(self, object_lines, chip_name):
+        num_lines = len(object_lines)
+        ra_phosim = np.zeros(num_lines, dtype=float)
+        dec_phosim = np.zeros(num_lines, dtype=float)
+        mag_norm = 55.*np.ones(num_lines, dtype=float)
+        for i, line in enumerate(object_lines):
+            if not line.startswith('object'):
+                raise RuntimeError('Trying to process non-object entry from '
+                                   'the instance catalog.')
+            tokens = line.split()
+            ra_phosim[i] = float(tokens[2])
+            dec_phosim[i] = float(tokens[3])
+            mag_norm[i] = float(tokens[4])
+        ra_appGeo, dec_appGeo \
+            = PhoSimAstrometryBase._appGeoFromPhoSim(np.radians(ra_phosim),
+                                                     np.radians(dec_phosim),
+                                                     self.obs_md)
+        ra_obs_rad, dec_obs_rad \
+            = _observedFromAppGeo(ra_appGeo, dec_appGeo,
+                                  obs_metadata=self.obs_md,
+                                  includeRefraction=True)
+        x_pupil, y_pupil = _pupilCoordsFromObserved(ra_obs_rad, dec_obs_rad,
+                                                    self.obs_md)
+        on_chip_dict = _chip_downselect(mag_norm, x_pupil, y_pupil,
+                                        self.logger, [chip_name])
+        index = on_chip_dict[chip_name]
+
+        self.object_lines = []
+        for i in index[0]:
+            self.object_lines.append(object_lines[i])
+        self.x_pupil = list(x_pupil[index])
+        self.y_pupil = list(y_pupil[index])
+        self.bp_dict = BandpassDict.loadTotalBandpassesFromFiles()
 
     def __len__(self):
-        try:
-            return len(self._gs_objects)
-        except TypeError:
-            return len(self.object_lines)
+        return len(self.object_lines)
 
     def __iter__(self):
-        for gs_obj in self.gs_objects:
-            yield gs_obj
+        return self
 
-    def __getitem__(self, index):
-        return self.gs_objects[index]
+    def __next__(self):
+        try:
+            while True:
+                gs_obj = self._make_gs_object(self.object_lines.pop(0),
+                                              self.x_pupil.pop(0),
+                                              self.y_pupil.pop(0))
+                if gs_obj is not None:
+                    return gs_obj
+        except IndexError:
+            raise StopIteration()
+
+    def _make_gs_object(self, object_line, x_pupil, y_pupil):
+        params = object_line.strip().split()
+        unique_id = params[1]
+        ra_phosim = float(params[2])
+        dec_phosim = float(params[3])
+        mag_norm = float(params[4])
+        sed_name = params[5]
+        redshift = float(params[6])
+        gamma1 = float(params[7])
+        gamma2 = self.gamma2_sign*float(params[8])
+        kappa = float(params[9])
+        internal_av = 0
+        internal_rv = 0
+        galactic_av = 0
+        galactic_rv = 0
+        fits_file = None
+        pixel_scale = 0
+        rotation_angle = 0
+        npoints = 0
+        semi_major_arcsec = 0
+        semi_minor_arcsec = 0
+        position_angle_degrees = 0
+        sersic_index = 0
+        if params[12].lower() == 'point':
+            gs_type = 'pointSource'
+            i_gal_dust_model = 14
+            if params[13].lower() != 'none':
+                i_gal_dust_model = 16
+                internal_av = float(params[14])
+                internal_rv =float(params[15])
+            if params[i_gal_dust_model].lower() != 'none':
+                galactic_av = float(params[i_gal_dust_model+1])
+                galactic_rv = float(params[i_gal_dust_model+2])
+        elif params[12].lower() == 'sersic2d':
+            gs_type = 'sersic'
+            semi_major_arcsec = float(params[13])
+            semi_minor_arcsec = float(params[14])
+            position_angle_degrees = float(params[15])
+            sersic_index = float(params[16])
+            i_gal_dust_model = 18
+            if params[17].lower() != 'none':
+                i_gal_dust_model = 20
+                internal_av = float(params[18])
+                internal_rv = float(params[19])
+            if params[i_gal_dust_model].lower() != 'none':
+                galactic_av = float(params[i_gal_dust_model+1])
+                galactic_rv = float(params[i_gal_dust_model+2])
+        elif params[12].lower() == 'knots':
+            gs_type = 'RandomWalk'
+            semi_major_arcsec = float(params[13])
+            semi_minor_arcsec = float(params[14])
+            position_angle_degrees = float(params[15])
+            npoints = int(params[16])
+            i_gal_dust_model = 18
+            if params[17].lower() != 'none':
+                i_gal_dust_model = 20
+                internal_av = float(params[18])
+                internal_rv = float(params[19])
+            if params[i_gal_dust_model].lower() != 'none':
+                galactic_av = float(params[i_gal_dust_model+1])
+                galactic_rv = float(params[i_gal_dust_model+2])
+        elif (params[12].endswith('.fits') or params[12].endswith('.fits.gz')):
+            gs_type = 'FitsImage'
+            fits_file = find_file_path(params[12], get_image_dirs())
+            pixel_scale = float(params[13])
+            rotation_angle = float(params[14])
+            i_gal_dust_model = 16
+            if params[15].lower() != 'none':
+                i_gal_dust_model = 18
+                internal_av = float(params[16])
+                internal_rv = float(params[17])
+            if params[i_gal_dust_model].lower() != 'none':
+                galactic_av = float(params[i_gal_dust_model+1])
+                galactic_rv = float(params[i_gal_dust_model+2])
+        else:
+            raise RuntimeError("Do not know how to handle "
+                               "object type: %s" % params[12])
+
+        object_is_valid = (mag_norm < 50.0 and
+                           (galactic_av != 0 or galactic_rv != 0) and
+                           not (gs_type == 'sersic' and
+                                semi_major_arcsec < semi_minor_arcsec) and
+                           not (gs_type == 'RandomWalk' and npoints <=0))
+        if not object_is_valid:
+            return None
+        sed_obj = SedWrapper(find_file_path(sed_name, self.sed_dirs),
+                             mag_norm, redshift, internal_av, internal_rv,
+                             galactic_av, galactic_rv, self.bp_dict)
+        position_angle_radians = np.radians(360 - position_angle_degrees)
+        gs_object = GalSimCelestialObject(gs_type, x_pupil, y_pupil,
+                                          radiansFromArcsec(semi_major_arcsec),
+                                          radiansFromArcsec(semi_minor_arcsec),
+                                          radiansFromArcsec(semi_major_arcsec),
+                                          position_angle_radians,
+                                          sersic_index,
+                                          sed_obj,
+                                          self.bp_dict,
+                                          self.phot_params,
+                                          npoints,
+                                          fits_file,
+                                          pixel_scale,
+                                          rotation_angle,
+                                          gamma1=gamma1,
+                                          gamma2=gamma2,
+                                          kappa=kappa,
+                                          uniqueId=unique_id)
+        return gs_object
 
 
 class ImSimConfiguration(object):
diff --git a/python/desc/imsim/trim.py b/python/desc/imsim/trim.py
index 5753d629..b3b0e9aa 100644
--- a/python/desc/imsim/trim.py
+++ b/python/desc/imsim/trim.py
@@ -207,6 +207,8 @@ def _process_objects(self, sensor_list, chunk_size, radius=0.18,
         """
         num_gals = defaultdict(lambda: 0)
         self.update({sensor: [] for sensor in sensor_list})
+        if numRows is not None:
+            chunk_size = min(chunk_size, numRows)
         with desc.imsim.fopen(self.instcat_file, mode='rt') as fd:
             nread = 0
             while numRows is None or nread < numRows:
diff --git a/tests/test_instcat_parser.py b/tests/test_instcat_parser.py
index f4ccffb9..e8e075de 100644
--- a/tests/test_instcat_parser.py
+++ b/tests/test_instcat_parser.py
@@ -10,6 +10,7 @@
 import numpy as np
 import desc.imsim
 from lsst.afw.cameraGeom import WAVEFRONT, GUIDER
+from lsst.sims.coordUtils import lsst_camera
 from lsst.sims.utils import _pupilCoordsFromRaDec
 from lsst.sims.utils import altAzPaFromRaDec
 from lsst.sims.utils import angularSeparation
@@ -20,6 +21,21 @@
 from lsst.sims.coordUtils import chipNameFromPupilCoordsLSST
 from lsst.sims.GalSimInterface import LSSTCameraWrapper
 
+def sources_from_list(lines, obs_md, phot_params, file_name):
+    """Return a two-item tuple containing
+       * a list of GalSimCelestialObjects for each object entry in `lines`
+       * a dictionary of these objects disaggregated by chip_name.
+    """
+    target_chips = [det.getName() for det in lsst_camera()]
+    gs_object_dict = dict()
+    out_obj_dict = dict()
+    for chip_name in target_chips:
+        out_obj_dict[chip_name] \
+            = [_ for _ in desc.imsim.GsObjectList(lines, obs_md, phot_params,
+                                                  file_name, chip_name)]
+        for gsobj in out_obj_dict[chip_name]:
+            gs_object_dict[gsobj.uniqueId] = gsobj
+    return list(gs_object_dict.values()), out_obj_dict
 
 class InstanceCatalogParserTestCase(unittest.TestCase):
     """
@@ -121,16 +137,7 @@ def test_object_extraction_stars(self):
         obs_md = desc.imsim.phosim_obs_metadata(commands)
         phot_params = desc.imsim.photometricParameters(commands)
         with desc.imsim.fopen(self.phosim_file, mode='rt') as input_:
-            lines = [x for x in input_]
-        (gs_object_arr,
-         gs_object_dict) = desc.imsim.sources_from_list(lines,
-                                                        obs_md,
-                                                        phot_params,
-                                                        self.phosim_file)
-
-        id_arr = [None]*len(gs_object_arr)
-        for i_obj in range(len(gs_object_arr)):
-            id_arr[i_obj] = gs_object_arr[i_obj].uniqueId
+            lines = [x for x in input_ if x.startswith('object')]
 
         truth_dtype = np.dtype([('uniqueId', str, 200), ('x_pupil', float), ('y_pupil', float),
                                 ('sedFilename', str, 200), ('magNorm', float),
@@ -143,6 +150,13 @@ def test_object_extraction_stars(self):
                                    dtype=truth_dtype, delimiter=';')
 
         truth_data.sort()
+
+        gs_object_arr, gs_object_dict \
+            = sources_from_list(lines, obs_md, phot_params, self.phosim_file)
+
+        id_arr = [None]*len(gs_object_arr)
+        for i_obj in range(len(gs_object_arr)):
+            id_arr[i_obj] = gs_object_arr[i_obj].uniqueId
         id_arr = sorted(id_arr)
         np.testing.assert_array_equal(truth_data['uniqueId'], id_arr)
 
@@ -226,16 +240,7 @@ def test_object_extraction_galaxies(self):
         obs_md = desc.imsim.phosim_obs_metadata(commands)
         phot_params = desc.imsim.photometricParameters(commands)
         with desc.imsim.fopen(galaxy_phosim_file, mode='rt') as input_:
-            lines = [x for x in input_]
-        (gs_object_arr,
-         gs_object_dict) = desc.imsim.sources_from_list(lines,
-                                                        obs_md,
-                                                        phot_params,
-                                                        galaxy_phosim_file)
-
-        id_arr = [None]*len(gs_object_arr)
-        for i_obj in range(len(gs_object_arr)):
-            id_arr[i_obj] = gs_object_arr[i_obj].uniqueId
+            lines = [x for x in input_ if x.startswith('object')]
 
         truth_dtype = np.dtype([('uniqueId', str, 200), ('x_pupil', float), ('y_pupil', float),
                                 ('sedFilename', str, 200), ('magNorm', float),
@@ -251,6 +256,13 @@ def test_object_extraction_galaxies(self):
                                    dtype=truth_dtype, delimiter=';')
 
         truth_data.sort()
+
+        gs_object_arr, gs_object_dict \
+            = sources_from_list(lines, obs_md, phot_params, galaxy_phosim_file)
+
+        id_arr = [None]*len(gs_object_arr)
+        for i_obj in range(len(gs_object_arr)):
+            id_arr[i_obj] = gs_object_arr[i_obj].uniqueId
         id_arr = sorted(id_arr)
         np.testing.assert_array_equal(truth_data['uniqueId'], id_arr)
 
@@ -349,20 +361,6 @@ def test_object_extraction_galaxies(self):
         self.assertGreater(valid, 10)
         self.assertGreater(len(valid_chip_names), 5)
 
-    def test_parsePhoSimInstanceFile_warning(self):
-        "Test the warnings emitted by the instance catalog parser."
-        commands = desc.imsim.metadata_from_file(self.extra_commands)
-        obs_md = desc.imsim.phosim_obs_metadata(commands)
-        phot_params = desc.imsim.photometricParameters(commands)
-        with desc.imsim.fopen(self.extra_commands, mode='rt') as input_:
-            lines = [x for x in input_]
-        with warnings.catch_warnings():
-            warnings.simplefilter("error")
-            self.assertRaises(UserWarning,
-                              desc.imsim.sources_from_list,
-                              lines, obs_md, phot_params,
-                              self.extra_commands)
-
     def test_photometricParameters(self):
         "Test the photometricParameters function."
         commands = desc.imsim.metadata_from_file(self.phosim_file)
@@ -389,22 +387,6 @@ def test_validate_phosim_object_list(self):
             my_objs = set()
             for sensor in sensors:
                 [my_objs.add(x) for x in instcat_contents.sources[1][sensor]]
-        self.assertGreater(len(wa), 0)
-
-        # we must detect which warning is the warning we are actually
-        # testing, because PALPY keeps raising ERFAWarnings over our
-        # request for dates in the future
-        desired_warning_dex = -1
-        for i_ww, ww in enumerate(wa):
-            if 'Omitted 6 suspicious objects' in ww.message.args[0]:
-                desired_warning_dex = i_ww
-                break
-
-        if desired_warning_dex<0:
-            raise RuntimeError("Expected warning about bad sources "
-                               "not issued")
-
-        message = wa[desired_warning_dex].message.args[0]
 
         # these are the objects that should be omitted
         bad_unique_ids = set([str(x) for x in
@@ -412,12 +394,6 @@ def test_validate_phosim_object_list(self):
                                811883374596, 956090392580,
                                34307989098523, 34304522113056]])
 
-        self.assertIn('Omitted 6 suspicious objects', message)
-        self.assertIn('4 had galactic_Av', message)
-        self.assertIn('1 had mag_norm', message)
-        self.assertIn('1 had semi_major_axis', message)
-        self.assertIn('1 had n_points', message)
-
         self.assertEqual(len(my_objs), 18)
         for obj in instcat_contents.sources[0]:
             self.assertNotIn(obj.uniqueId, bad_unique_ids)
diff --git a/travis_test.sh b/travis_test.sh
index 29d37653..7501fce7 100755
--- a/travis_test.sh
+++ b/travis_test.sh
@@ -1,7 +1,7 @@
 #!/bin/bash
 source scl_source enable devtoolset-6
 source loadLSST.bash
-setup lsst_sims
+setup -t current lsst_sims
 pip install nose
 pip install coveralls
 pip install pylint
@@ -16,6 +16,14 @@ setup -r . -j
 scons lib python shebang examples doc policy python/lsst/obs/lsst/version.py
 
 cd ..
+
+# Clone sims_GalSimInterface and set up master to suppress
+# lsst.afw.geom deprecation warnings.
+git clone https://github.com/lsst/sims_GalSimInterface.git
+cd sims_GalSimInterface
+setup -r . -j
+cd ..
+
 eups declare imsim -r ${TRAVIS_BUILD_DIR} -t current
 setup imsim
 cd ${TRAVIS_BUILD_DIR}