diff --git a/examples/demo11.py b/examples/demo11.py
index 4a62a742517..d6178b3faa6 100644
--- a/examples/demo11.py
+++ b/examples/demo11.py
@@ -61,6 +61,7 @@
 import logging
 import time
 import galsim
+import astropy.units as u
 
 def main(argv):
     """
@@ -95,7 +96,10 @@ def main(argv):
                                       # (This corresponds to 8 galaxies / arcmin^2)
     grid_spacing = 90.0               # The spacing between the samples for the power spectrum
                                       # realization (arcsec)
-    tel_diam = 4                      # Let's figure out the flux for a 4 m class telescope
+    tel_diam = 4*u.m                  # Let's figure out the flux for a 4 m class telescope.  We'll
+                                      # also show how astropy units can be used here (see especially
+                                      # the eval_variables section of the yaml version of this
+                                      # demo).
     exp_time = 300                    # exposing for 300 seconds.
     center_ra = 19.3*galsim.hours     # The RA, Dec of the center of the image on the sky
     center_dec = -33.1*galsim.degrees
@@ -106,7 +110,7 @@ def main(argv):
     # is 2.4 but there is obscuration (a linear factor of 0.33).  Here, we assume that the telescope
     # we're simulating effectively has no obscuration factor.  We're also ignoring the pi/4 factor
     # since it appears in the numerator and denominator, so we use area = diam^2.
-    hst_eff_area = 2.4**2 * (1.-0.33**2)
+    hst_eff_area = (2.4*u.m)**2 * (1.-0.33**2)
     flux_scaling = (tel_diam**2/hst_eff_area) * exp_time
 
     # random_seed is used for both the power spectrum realization and the random properties
diff --git a/examples/demo11.yaml b/examples/demo11.yaml
index 2441c1dd5d4..d2449a00930 100644
--- a/examples/demo11.yaml
+++ b/examples/demo11.yaml
@@ -44,6 +44,7 @@
 # - image.noise : symmetrize
 # - wcs type : Tan(dudx, dudy, dvdx, dvdy, units, origin, ra, dec)
 # - top level field eval_variables
+# - quantity letter type for eval variable.
 #
 # - Power spectrum shears and magnifications for non-gridded positions.
 # - Reading a compressed FITS image (using BZip2 compression).
@@ -74,8 +75,10 @@ eval_variables :
     # An unusual prefix: a = Angle.
     atheta : &theta 0.17 degrees
 
-    # tel_diam = the telescope diameter in meters.
-    ftel_diam : &tel_diam 4
+    # tel_diam = the telescope diameter.  Prefixing this with a 'q' means it is an
+    # astropy.units.Quantity.  We'll specify centimeters here, note the interaction with
+    # meters down below when we use it to calculate the scale_flux.
+    qtel_diam : &tel_diam 4 m
 
     # exp_time = the exposure time in seconds.
     fexp_time : &exp_time 300
@@ -167,7 +170,7 @@ gal :
     #     noise_pad_size = 40 * sqrt(2) * 0.2 arcsec/pixel = 11.3 arcsec
     noise_pad_size : 11.3
 
-    # We will select a galaxy at random from the catalog. One could easily do this by setting 
+    # We will select a galaxy at random from the catalog. One could easily do this by setting
     # index : { type : Random }
     # but we will instead allow the catalog to choose a random galaxy for us.  It will remove any
     # selection effects involved in postage stamp creation using weights that are stored in the
@@ -192,7 +195,7 @@ gal :
         # second exposures.  The COSMOS galaxies have their flux set to be appropriate for HST
         # (a 2.4 m telescope with a linear obscuration factor of 0.33) with 1 second exposures.
         # So the flux should be scaled by (4**2/(2.4*(1-0.33**2))) * 300
-        "$(tel_diam**2 / (2.4**2*(1.-0.33**2))) * exp_time"
+        "$(tel_diam**2 / ((2.4*u.m)**2*(1.-0.33**2))) * exp_time"
 
 
 # Define some other information about the images
diff --git a/examples/json/demo11.json b/examples/json/demo11.json
index 16b856fa3ab..5736f3b52cc 100644
--- a/examples/json/demo11.json
+++ b/examples/json/demo11.json
@@ -19,7 +19,7 @@
 "eval_variables" : {
     "fpixel_scale" : 0.2,
     "atheta" : "0.17 degrees",
-    "ftel_diam" : 4,
+    "qtel_diam" : "4 m",
     "fexp_time" : 300,
     "fimage_size" : 2048,
     "inobjects" : 288,
@@ -44,7 +44,7 @@
     "shear" : { "type" : "PowerSpectrumShear" },
     "magnification" : { "type" : "PowerSpectrumMagnification" },
     "rotation" : { "type" : "Random" },
-    "scale_flux" : "$(tel_diam**2 / (2.4**2*(1.-0.33**2))) * exp_time"
+    "scale_flux" : "$(tel_diam**2 / ((2.4*u.m)**2*(1.-0.33**2))) * exp_time"
 },
 
 "image" : {