diff --git a/.travis.yml b/.travis.yml
index 61064188..a6c8cd7b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -49,10 +49,10 @@ matrix:
         - python: 3.6
           env: SETUP_CMD='test' ASTROPY_VERSION=development
 
-        # Try older numpy versions
-        - python: 3.4
+        # Try minimum supported versions
+        - python: 3.5
           env: SETUP_CMD='test' NUMPY_VERSION=1.12
-        - python: 3.4
+        - python: 3.5
           env: SETUP_CMD='test' NUMPY_VERSION=1.11
 
         # Try released POPPY
@@ -99,7 +99,6 @@ script:
 
 after_success:
     # coveralls.io integration
-    # - if [[ $SETUP_CMD == 'test --coverage' && TRAVIS_REPO_SLUG == "mperrin/webbpsf" ]]; then coveralls --rcfile='webbpsf/tests/coveragerc'; fi
     - if [[ $SETUP_CMD == 'test --coverage' ]]; then coveralls --rcfile='webbpsf/tests/coveragerc'; fi
 
 notifications:
diff --git a/setup.py b/setup.py
index b319ef5d..8f0a718d 100755
--- a/setup.py
+++ b/setup.py
@@ -1,6 +1,6 @@
 #!/usr/bin/env python
+# Based on astropy affiliated package template's setup.py
 # Licensed under a 3-clause BSD style license - see LICENSE.rst
-# --based on setup.py from astropy--
 from __future__ import print_function
 
 import glob
@@ -9,6 +9,15 @@
 import imp
 import ast
 
+__packagename__ = 'webbpsf'
+__minimum_python_version__ = "3.5"
+
+# Enforce Python version check - this is the same check as in __init__.py but
+# this one has to happen before importing ah_bootstrap.
+if sys.version_info < tuple((int(val) for val in __minimum_python_version__.split('.'))):
+    sys.stderr.write("ERROR: {} requires Python {} or later\n".format(__packagename__, __minimum_python_version__))
+    sys.exit(1)
+
 try:
     import numpy
 except ImportError:
@@ -109,16 +118,15 @@
       version=VERSION,
       description=DESCRIPTION,
       scripts=scripts,
+      python_requires='>=' + __minimum_python_version__,
       install_requires=[
           'numpy>=1.10.0',
           'matplotlib>=1.5.0',
           'scipy>=0.16.0',
           'poppy>=0.7.0',
-          'astropy>=1.2.0',
+          'astropy>=1.3.0',
           'jwxml>=0.3.0',
           'pysiaf>=0.1.8', 'six',
-          'pytest'  # unlisted requirement for pysiaf - see https://github.com/spacetelescope/pysiaf/issues/16
-                    # Remove this requirement once that issue is addressed.
       ],
       provides=[PACKAGENAME],
       author=AUTHOR,
diff --git a/webbpsf/__init__.py b/webbpsf/__init__.py
index 89303f7c..1848fd00 100644
--- a/webbpsf/__init__.py
+++ b/webbpsf/__init__.py
@@ -1,5 +1,4 @@
 # Licensed under a 3-clause BSD style license - see LICENSE.rst
-from __future__ import division, print_function, absolute_import, unicode_literals
 """
 WebbPSF: Simulated Point Spread Functions for the James Webb Space Telescope
 ----------------------------------------------------------------------------
@@ -9,15 +8,32 @@
 four science instruments plus the fine guidance sensor, including both direct
 imaging and coronagraphic modes.
 
-Developed by Marshall Perrin and contributors at STScI, 2010-2015.
+Developed by Marshall Perrin and collaborators at STScI, 2010-2018.
+
+Documentation can be found online at https://webbpsf.readthedocs.io/
 """
 
 # Affiliated packages may add whatever they like to this file, but
 # should keep this content at the top.
 # ----------------------------------------------------------------------------
+# make use of astropy affiliate framework to set __version__, __githash__, and
+# add the test() helper function
 from ._astropy_init import *
 # ----------------------------------------------------------------------------
 
+# Enforce Python version check during package import.
+# This is the same check as the one at the top of setup.py
+import sys
+
+__minimum_python_version__ = "3.5"
+
+class UnsupportedPythonError(Exception):
+    pass
+
+if sys.version_info < tuple((int(val) for val in __minimum_python_version__.split('.'))):
+    raise UnsupportedPythonError("webbpsf does not support Python < {}".format(__minimum_python_version__))
+
+
 # This tuple gives the *minimum* version of the WebbPSF data package
 # required. If changes to the code and data mean WebbPSF won't work
 # properly with an old data package, increment this version number.
@@ -27,11 +43,11 @@
 import astropy
 from astropy import config as _config
 
+
 class Conf(_config.ConfigNamespace):
     """
     Configuration parameters for `webbpsf`.
     """
-# Should probably be science state in astropy>=0.4 schema:
 
     default_oversampling = _config.ConfigItem(4, 'Default '+
             'oversampling factor: number of times more finely sampled than '+
diff --git a/webbpsf/constants.py b/webbpsf/constants.py
index 7ae0f90a..9be74041 100644
--- a/webbpsf/constants.py
+++ b/webbpsf/constants.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 # This file contains constants and data that do not fit in the data
 # package for one reason or another. It could be that they are in a
 # data structure that doesn't serialize well or they're too small to
diff --git a/webbpsf/distortion.py b/webbpsf/distortion.py
index 6785a700..f5b75a26 100644
--- a/webbpsf/distortion.py
+++ b/webbpsf/distortion.py
@@ -1,12 +1,9 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
-
 import copy
 
 import astropy.convolution
 import astropy.io.fits as fits
 import numpy as np
 import pysiaf
-import six
 from scipy.interpolate import griddata
 from scipy.ndimage.interpolation import rotate
 
@@ -48,7 +45,7 @@ def apply_distortion(hdulist_or_filename=None, fill_value=0):
     """
 
     # Read in input PSF
-    if isinstance(hdulist_or_filename, six.string_types):
+    if isinstance(hdulist_or_filename, str):
         hdu_list = fits.open(hdulist_or_filename)
     elif isinstance(hdulist_or_filename, fits.HDUList):
         hdu_list = hdulist_or_filename
@@ -166,7 +163,7 @@ def apply_rotation(hdulist_or_filename=None, rotate_value=None, crop=True):
 
     """
     # Read in input PSF
-    if isinstance(hdulist_or_filename, six.string_types):
+    if isinstance(hdulist_or_filename, str):
         hdu_list = fits.open(hdulist_or_filename)
     elif isinstance(hdulist_or_filename, fits.HDUList):
         hdu_list = hdulist_or_filename
@@ -270,7 +267,7 @@ def apply_miri_scattering(hdulist_or_filename=None, kernel_amp=None):
     """
 
     # Read in input PSF
-    if isinstance(hdulist_or_filename, six.string_types):
+    if isinstance(hdulist_or_filename, str):
         hdu_list = fits.open(hdulist_or_filename)
     elif isinstance(hdulist_or_filename, fits.HDUList):
         hdu_list = hdulist_or_filename
diff --git a/webbpsf/jupyter_gui.py b/webbpsf/jupyter_gui.py
index 4fa5d5f7..ffa985a6 100644
--- a/webbpsf/jupyter_gui.py
+++ b/webbpsf/jupyter_gui.py
@@ -1,8 +1,6 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import logging
 import matplotlib
 import astropy.io.fits as fits
-import six
 
 import poppy
 from . import webbpsf_core
@@ -235,7 +233,7 @@ def show_notebook_interface_jwst(instrument):
     from matplotlib import pyplot as plt
 
 
-    if isinstance(instrument, six.string_types):
+    if isinstance(instrument, str):
         instrument = Instrument(instrument)
 
     try:
diff --git a/webbpsf/obssim.py b/webbpsf/obssim.py
index 5b711c06..505c11c6 100755
--- a/webbpsf/obssim.py
+++ b/webbpsf/obssim.py
@@ -1,5 +1,4 @@
 #!/usr/bin/env python
-from __future__ import division, print_function, absolute_import, unicode_literals
 """
 obssim.py
 
diff --git a/webbpsf/opds.py b/webbpsf/opds.py
index d34a07ce..aacd87ef 100644
--- a/webbpsf/opds.py
+++ b/webbpsf/opds.py
@@ -27,8 +27,6 @@
 #
 ###############################################################################
 
-from __future__ import division
-
 import os
 import numpy as np
 import matplotlib.pyplot as plt
diff --git a/webbpsf/optics.py b/webbpsf/optics.py
index 8855081b..19c4d6f4 100644
--- a/webbpsf/optics.py
+++ b/webbpsf/optics.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import os
 import poppy
 import poppy.utils
diff --git a/webbpsf/tests/__init__.py b/webbpsf/tests/__init__.py
index 5e6606e1..ef2abc96 100644
--- a/webbpsf/tests/__init__.py
+++ b/webbpsf/tests/__init__.py
@@ -1,5 +1,4 @@
 # Licensed under a 3-clause BSD style license - see LICENSE.rst
-from __future__ import division, print_function, absolute_import, unicode_literals
 """
 This package contains automated software tests for WebbPSF.
 """
diff --git a/webbpsf/tests/test_errorhandling.py b/webbpsf/tests/test_errorhandling.py
index bc9aeeea..bfeb1a38 100644
--- a/webbpsf/tests/test_errorhandling.py
+++ b/webbpsf/tests/test_errorhandling.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 # This file contains code for testing various error handlers and user interface edge cases,
 # as opposed to testing the main body of functionality of the code.
 
diff --git a/webbpsf/tests/test_fgs.py b/webbpsf/tests/test_fgs.py
index c3ffbeab..10d8f914 100644
--- a/webbpsf/tests/test_fgs.py
+++ b/webbpsf/tests/test_fgs.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import sys, os
 import numpy as np
 import matplotlib.pyplot as plt
diff --git a/webbpsf/tests/test_miri.py b/webbpsf/tests/test_miri.py
index 8d7f5a76..16684df9 100644
--- a/webbpsf/tests/test_miri.py
+++ b/webbpsf/tests/test_miri.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import sys, os
 import numpy as np
 import matplotlib.pyplot as plt
diff --git a/webbpsf/tests/test_nircam.py b/webbpsf/tests/test_nircam.py
index dcaec7b5..ec6a3a62 100644
--- a/webbpsf/tests/test_nircam.py
+++ b/webbpsf/tests/test_nircam.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import sys, os
 import numpy as np
 import matplotlib.pyplot as plt
diff --git a/webbpsf/tests/test_niriss.py b/webbpsf/tests/test_niriss.py
index 5ab63dac..661b4e79 100644
--- a/webbpsf/tests/test_niriss.py
+++ b/webbpsf/tests/test_niriss.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import sys, os
 import numpy as np
 import matplotlib.pyplot as plt
diff --git a/webbpsf/tests/test_nirspec.py b/webbpsf/tests/test_nirspec.py
index c899ba71..f52dbf8d 100644
--- a/webbpsf/tests/test_nirspec.py
+++ b/webbpsf/tests/test_nirspec.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import sys, os
 import numpy as np
 import matplotlib.pyplot as plt
diff --git a/webbpsf/tests/test_utils.py b/webbpsf/tests/test_utils.py
index 4b48a263..78af15c8 100644
--- a/webbpsf/tests/test_utils.py
+++ b/webbpsf/tests/test_utils.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import sys, os
 import numpy as np
 import matplotlib.pyplot as plt
diff --git a/webbpsf/tests/test_webbpsf.py b/webbpsf/tests/test_webbpsf.py
index b884269b..b178d531 100644
--- a/webbpsf/tests/test_webbpsf.py
+++ b/webbpsf/tests/test_webbpsf.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import sys, os
 import numpy as np
 import matplotlib.pyplot as plt
diff --git a/webbpsf/tests/test_wfirst.py b/webbpsf/tests/test_wfirst.py
index 7c9526c7..979a4e92 100644
--- a/webbpsf/tests/test_wfirst.py
+++ b/webbpsf/tests/test_wfirst.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import pytest
 from webbpsf import wfirst
 
diff --git a/webbpsf/tests/validate_webbpsf.py b/webbpsf/tests/validate_webbpsf.py
index 05ba72aa..c041977e 100644
--- a/webbpsf/tests/validate_webbpsf.py
+++ b/webbpsf/tests/validate_webbpsf.py
@@ -1,4 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import os
 from astropy.io import fits
 import numpy as N
diff --git a/webbpsf/tkgui.py b/webbpsf/tkgui.py
index 9cb7613d..f6972c74 100755
--- a/webbpsf/tkgui.py
+++ b/webbpsf/tkgui.py
@@ -1,22 +1,21 @@
 #!/usr/bin/env python
-from __future__ import division, print_function, absolute_import, unicode_literals
 import os
 import numpy as np
 import matplotlib.pyplot as plt
 import matplotlib
 import astropy.io.fits as fits
-from six.moves import tkinter as tk
-from six.moves import tkinter_tkfiledialog as tkFileDialog
-from six.moves import tkinter_messagebox as tkMessageBox
+import tkinter as tk
+import tkinter.filedialog as tkFileDialog
+import tkinter.messagebox as tkMessageBox
 import logging
 import logging
 _log = logging.getLogger('webbpsf')
 
 
 try:
-    from six.moves import tkinter_ttk as ttk
+    import tkinter.ttk as ttk
 except ImportError:
-    raise RuntimeError("Python 2.7.0 (or newer) with ttk widget support is required")
+    raise RuntimeError("Python with ttk widget support is required")
 
 
 
diff --git a/webbpsf/utils.py b/webbpsf/utils.py
index 8635f65f..441d60de 100644
--- a/webbpsf/utils.py
+++ b/webbpsf/utils.py
@@ -1,6 +1,4 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
 import os, sys
-import six
 import astropy.io.fits as fits
 import numpy as np
 import matplotlib.pyplot as plt
@@ -439,7 +437,7 @@ def measure_strehl(HDUlist_or_filename=None, ext=0, slice=0, center=None, displa
     from .webbpsf_core import Instrument
     from poppy import display_psf
 
-    if isinstance(HDUlist_or_filename, six.string_types):
+    if isinstance(HDUlist_or_filename, str):
         HDUlist = fits.open(HDUlist_or_filename)
     elif isinstance(HDUlist_or_filename, fits.HDUList):
         HDUlist = HDUlist_or_filename
diff --git a/webbpsf/webbpsf_core.py b/webbpsf/webbpsf_core.py
index 2146ab85..ec15d0bd 100644
--- a/webbpsf/webbpsf_core.py
+++ b/webbpsf/webbpsf_core.py
@@ -1,5 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
-
 """
 ============
 WebbPSF Core
@@ -24,7 +22,6 @@
 import os
 import glob
 import time
-import six
 import copy
 from collections import namedtuple, OrderedDict
 import numpy as np
@@ -403,10 +400,10 @@ def _get_optical_system(self, fft_oversample=2, detector_oversample=None,
             optsys.source_offset_theta = options['source_offset_theta']
 
         # ---- set pupil OPD
-        if isinstance(self.pupilopd, six.string_types):  # simple filename
+        if isinstance(self.pupilopd, str):  # simple filename
             opd_map = self.pupilopd if os.path.exists(self.pupilopd) else \
                       os.path.join(self._datapath, "OPD", self.pupilopd)
-        elif hasattr(self.pupilopd, '__getitem__') and isinstance(self.pupilopd[0], six.string_types):
+        elif hasattr(self.pupilopd, '__getitem__') and isinstance(self.pupilopd[0], str):
             # tuple with filename and slice
             opd_map = (self.pupilopd[0] if os.path.exists(self.pupilopd[0])
                        else os.path.join(self._datapath, "OPD", self.pupilopd[0]),
@@ -427,7 +424,7 @@ def _get_optical_system(self, fft_oversample=2, detector_oversample=None,
             pupil_optic = optsys.add_pupil(self.pupil)
         else:
             # wrap in an optic and supply to POPPY
-            if isinstance(self.pupil, six.string_types):  # simple filename
+            if isinstance(self.pupil, str):  # simple filename
                 if os.path.exists(self.pupil):
                     pupil_transmission = self.pupil
                 else:
diff --git a/webbpsf/wfirst.py b/webbpsf/wfirst.py
index 6bc8e67c..3dbc2bdd 100644
--- a/webbpsf/wfirst.py
+++ b/webbpsf/wfirst.py
@@ -1,5 +1,3 @@
-from __future__ import division, print_function, absolute_import, unicode_literals
-
 """
 ==============================
 WFIRST Instruments (pre-alpha)
diff --git a/webbpsf/wxgui.py b/webbpsf/wxgui.py
index 9f5f9e7c..e4fa4011 100755
--- a/webbpsf/wxgui.py
+++ b/webbpsf/wxgui.py
@@ -1,5 +1,4 @@
 #!/usr/bin/env python
-from __future__ import division, print_function, absolute_import, unicode_literals
 
 import astropy.io.fits as fits
 import matplotlib