diff --git a/observation_sim/PSF/FieldDistortion.py b/observation_sim/psf/FieldDistortion.py
similarity index 100%
rename from observation_sim/PSF/FieldDistortion.py
rename to observation_sim/psf/FieldDistortion.py
diff --git a/observation_sim/PSF/PSFGauss.py b/observation_sim/psf/PSFGauss.py
similarity index 98%
rename from observation_sim/PSF/PSFGauss.py
rename to observation_sim/psf/PSFGauss.py
index ed437d9c15a16697c6d759820dfd554ff36c35bf..a21b0c8963da1aa7b4e22701d06b234b8402a69c 100644
--- a/observation_sim/PSF/PSFGauss.py
+++ b/observation_sim/psf/PSFGauss.py
@@ -3,7 +3,7 @@ import sep
 import numpy as np
 from scipy.interpolate import interp1d
 
-from observation_sim.PSF.PSFModel import PSFModel
+from observation_sim.psf.PSFModel import PSFModel
 
 
 class PSFGauss(PSFModel):
diff --git a/observation_sim/PSF/PSFInterp.py b/observation_sim/psf/PSFInterp.py
similarity index 99%
rename from observation_sim/PSF/PSFInterp.py
rename to observation_sim/psf/PSFInterp.py
index d96d91470425fb5d7928e2c005431c138558372f..97435ad048225969fd5287e1dd01a26ef2eb639f 100644
--- a/observation_sim/PSF/PSFInterp.py
+++ b/observation_sim/psf/PSFInterp.py
@@ -12,7 +12,7 @@ import scipy.spatial as spatial
 import galsim
 import h5py
 
-from observation_sim.PSF.PSFModel import PSFModel
+from observation_sim.psf.PSFModel import PSFModel
 
 
 NPSF = 900  # ***# 30*30
diff --git a/observation_sim/PSF/PSFInterpSLS.py b/observation_sim/psf/PSFInterpSLS.py
similarity index 99%
rename from observation_sim/PSF/PSFInterpSLS.py
rename to observation_sim/psf/PSFInterpSLS.py
index b3b6e7d8c85a915fdac00b25aa19af1b29dfede1..7adfedc5b953339492940d41b08e0f31c062213c 100644
--- a/observation_sim/PSF/PSFInterpSLS.py
+++ b/observation_sim/psf/PSFInterpSLS.py
@@ -14,7 +14,7 @@ import galsim
 import h5py
 
 from observation_sim.instruments import Filter, FilterParam, Chip
-from observation_sim.PSF.PSFModel import PSFModel
+from observation_sim.psf.PSFModel import PSFModel
 from observation_sim.instruments.chip import chip_utils
 import os
 from astropy.io import fits
diff --git a/observation_sim/PSF/PSFModel.py b/observation_sim/psf/PSFModel.py
similarity index 100%
rename from observation_sim/PSF/PSFModel.py
rename to observation_sim/psf/PSFModel.py
diff --git a/observation_sim/PSF/__init__.py b/observation_sim/psf/__init__.py
similarity index 100%
rename from observation_sim/PSF/__init__.py
rename to observation_sim/psf/__init__.py
diff --git a/observation_sim/sim_steps/add_objects.py b/observation_sim/sim_steps/add_objects.py
index d66194adcfcd9cb4907380d94a131691704d6ae0..9f41d3621c8457adbd66db88bac38f2ccd64d505 100644
--- a/observation_sim/sim_steps/add_objects.py
+++ b/observation_sim/sim_steps/add_objects.py
@@ -5,7 +5,7 @@ import traceback
 import numpy as np
 import galsim
 from observation_sim._util import get_shear_field
-from observation_sim.PSF import PSFGauss, FieldDistortion, PSFInterp, PSFInterpSLS
+from observation_sim.psf import PSFGauss, FieldDistortion, PSFInterp, PSFInterpSLS
 
 from astropy.time import Time
 from datetime import datetime, timezone
diff --git a/tests/test_PSFmodule.py b/tests/test_PSFmodule.py
index 0e575f14312b1fcca365672cac85f6e76a604b82..a977dd9960377866e86168452e2c9930ba0a6b52 100644
--- a/tests/test_PSFmodule.py
+++ b/tests/test_PSFmodule.py
@@ -9,7 +9,7 @@ import galsim
 import yaml
 
 from observation_sim.instruments import Chip, Filter, FilterParam, FocalPlane
-from observation_sim.PSF.PSFInterp import PSFInterp
+from observation_sim.psf.PSFInterp import PSFInterp
 
 
 def defineCCD(iccd, config_file):
diff --git a/tests/test_SpecDisperse.py b/tests/test_SpecDisperse.py
index ead08a2635c04dd55928190024791d84f58f5595..4a6553eae3e617f19c173bad1ec61f2c475326f2 100644
--- a/tests/test_SpecDisperse.py
+++ b/tests/test_SpecDisperse.py
@@ -9,7 +9,7 @@ from observation_sim.mock_objects.SpecDisperser import rotate90, SpecDisperser
 from observation_sim.config import ChipOutput
 from observation_sim.instruments import Telescope, Chip, FilterParam, Filter, FocalPlane
 from observation_sim.mock_objects import MockObject, Star
-from observation_sim.PSF import PSFGauss
+from observation_sim.psf import PSFGauss
 
 import numpy as np
 import galsim
diff --git a/tests/test_imaging.py b/tests/test_imaging.py
index 7af8d32156e138b4020ed7da5882c3f2cde8c094..6fa0bca26068c2f52e459a15775f9038c7a1cbd3 100644
--- a/tests/test_imaging.py
+++ b/tests/test_imaging.py
@@ -19,7 +19,7 @@ from observation_sim.mock_objects._util import getObservedSED
 from observation_sim.mock_objects import CatalogBase, Galaxy
 
 from observation_sim.instruments import Chip, Filter, FilterParam, FocalPlane
-from observation_sim.PSF.PSFInterp import PSFInterp
+from observation_sim.psf.PSFInterp import PSFInterp
 from observation_sim.mock_objects._util import integrate_sed_bandpass, getNormFactorForSpecWithABMAG, getABMAG
 
 
diff --git a/tools/get_PSF.py b/tools/get_PSF.py
index c13ea06632233216630fbf9d62f31315c836369c..f861bd5070d7f402e63a89b408d191224f3cf4e5 100644
--- a/tools/get_PSF.py
+++ b/tools/get_PSF.py
@@ -1,6 +1,6 @@
 import os
 import numpy as np
-import observation_sim.PSF.PSFInterp as PSFInterp
+import observation_sim.psf.PSFInterp as PSFInterp
 from observation_sim.instruments import Chip, Filter, FilterParam
 import yaml
 import galsim