add more descriptive text

Author: keflavich

tutorials/spectral-cube-reprojection/SpectralCubeReprojectExample.ipynb

@@ -19,7 +19,14 @@
     "spectral cube, radio astronomy, astroquery, units, dask\n",
     "\n",
     "## Summary\n",
-    "This tutorial shows how to take two spectral cubes observed toward the same part of the sky, but different frequencies, and put them onto the same grid using [spectral-cube](spectral-cube.readthedocs.io).  It uses [astroquery](https://astroquery.readthedocs.io/) to obtain line frequencies from [splatalogue](https://astroquery.readthedocs.io/en/latest/splatalogue/splatalogue.html).  Finally, it shows how to do the reprojection using [dask](https://dask.org) to enable parallelization."
+    "Spectroscopic cube observations taken at different wavelength can trace the motion of gas or stars using spectral lines, but often lines at different wavelengths give different information.\n",
+    "For example, one might observe a galaxy in the 21cm line of HI and the 115 GHz line of CO, or a protoplanetary disk in a line of N2H+ and a line of CO, or a galactic disk in the H-alpha and H-beta lines (in absorption or emission).\n",
+    "In order to compare these data sets pixel-by-pixel, they must be placed onto a common grid with common resolution.\n",
+    "\n",
+    "This tutorial shows how to take two spectral cubes observed toward the same part of the sky, but different frequencies, and put them onto the same grid using [spectral-cube](spectral-cube.readthedocs.io).\n",
+    "\n",
+    "It uses [astroquery](https://astroquery.readthedocs.io/) to obtain line frequencies from [splatalogue](https://astroquery.readthedocs.io/en/latest/splatalogue/splatalogue.html); this example uses radio-wavelength data for which Splatalogue's molecular line lists are appropriate.\n",
+    "Finally, it shows how to do the reprojection using [dask](https://dask.org) to enable parallelization."
    ]
   },
   {
@@ -179,7 +186,9 @@
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    "metadata": {},
    "source": [
-    "## Step 3: Convert cubes from frequency to velocity"
+    "## Step 3: Convert cubes from frequency to velocity\n",
+    "\n",
+    "To compare the kinematic structure of the target, we need to convert from the observed frequency (which must be in a common reference frame; in this case, it already is) to the doppler velocity."
    ]
   },
   {
@@ -268,7 +277,8 @@
    "source": [
     "# Step 4. Spectral Interpolation\n",
     "\n",
-    "We can do the spatial or spectral step first.  In this case, we choose the spectral step first because the H$_2$CS cube is narrower in velocity (`cube1vel`) and this will reduce the number of channels we need to spatially interpolate over in the next step.\n",
+    "We can choose to do either the spatial or spectral step first.  \n",
+    "In this case, we choose the spectral step first because the H$_2$CS cube is narrower in velocity (`cube1vel`) and this will reduce the number of channels we need to spatially interpolate over in the next step.\n",
     "\n",
     "We need to match resolution to the cube with the largest channel width:"
    ]
@@ -562,6 +572,7 @@
     "# Dask\n",
     "\n",
     "All of the above can be done using `dask` as the underlying framework to parallelize the operations.\n",
+    "See [the spectral-cube documentation on dask integration](https://spectral-cube.readthedocs.io/en/latest/dask.html) or the [dask documentation](https://dask.org/) for further details.\n",
     "\n",
     "The dask approach can be made more memory-efficient (avoid using too much RAM) by writing intermediate steps to disk.  The non-dask approach used above will generally need to read the whole cube into memory.  Depending on the situation, either approach may be faster, but `dask` may be needed if the cube is larger than memory."
    ]
@@ -618,6 +629,11 @@
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@@ -627,7 +643,8 @@
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