added RA Dec axis demo

Author: MelissaGraham

03_Image_Display_and_Manipulation.ipynb

@@ -49,7 +49,7 @@
    "source": [
     "### 1. Import Common Python Libraries\n",
     "\n",
-    "The [`matplotlib`](https://matplotlib.org/), [`numpy`](http://www.numpy.org/), and [`astropy`](http://www.astropy.org/) libraries are widely used Python libraries for plotting, scientific computing, and astronomical data analysis. We will use these packages below, including the `matplotlib.pyplot` plotting sublibrary. We also import the [`warnings` library](https://docs.python.org/2/library/warnings.html) to prevent some routine warning messages from printing to the screen."
+    "The [`matplotlib`](https://matplotlib.org/), [`numpy`](http://www.numpy.org/), and [`astropy`](http://www.astropy.org/) libraries are widely used Python libraries for plotting, scientific computing, and astronomical data analysis. We will use these packages below, including the `matplotlib.pyplot` plotting sublibrary and the [`astropy.wcs`](https://docs.astropy.org/en/stable/wcs/index.html) package for dealing with World Coordinate Systems (WCS). We also import the [`warnings` library](https://docs.python.org/2/library/warnings.html) to prevent some routine warning messages from printing to the screen."
    ]
   },
   {
@@ -61,7 +61,9 @@
     "# allow for matplotlib to create inline plots in our notebook\n",
     "%matplotlib inline\n",
     "import matplotlib.pyplot as plt      # imports matplotlib.pyplot as plt\n",
-    "import warnings                      # imports the warnings library"
+    "import warnings                      # imports the warnings library\n",
+    "\n",
+    "from astropy.wcs import WCS          # imports astropy's World Coordinate System function WCS"
    ]
   },
   {
@@ -233,6 +235,34 @@
     "plt.show()"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "To see the image axes in sky coordinates instead of pixel coordinates, a simple option is to use astropy's World Coordinate System (WCS) package, along with matplotlib.pyplot's ``subplot``, ``imshow``, and ``grid`` functions. \n",
+    "Recall that we imported ``matplotlib.pyplot`` as ``plt`` already, and that we imported the ``astropy.wcs.WCS`` function as simply ``WCS``.\n",
+    "Find more information about [imshow](https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.imshow.html) and [colormaps](https://matplotlib.org/stable/tutorials/colors/colormaps.html) (``cmap``)."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "plt.figure()\n",
+    "# Set the figure's projection to be the WCS of the calexp\n",
+    "plt.subplot(projection=WCS(calexp.getWcs().getFitsMetadata()))\n",
+    "# Display the calexp image data array using the gray colormap (cmap)\n",
+    "#  and use approximately the same min and max scale values as above\n",
+    "plt.imshow(calexp.image.array, cmap='gray', vmin=-200.0, vmax=400, origin='lower')\n",
+    "# Add solid white grid lines\n",
+    "plt.grid(color='white', ls='solid')\n",
+    "# Label axes\n",
+    "plt.xlabel('Right Ascension')\n",
+    "plt.ylabel('Declination')"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},