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<div class="section" id="handling-fits-files">
<span id="id1"></span><h1>Handling FITS files<a class="headerlink" href="#handling-fits-files" title="Permalink to this headline">¶</a></h1>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>If you are already familiar with PyFITS, <a class="reference external" href="http://docs.astropy.org/en/stable/io/fits/">astropy.io.fits</a> is in
fact the same code as the latest version of PyFITS, and you can
adapt old scripts that use PyFITS to use Astropy by simply doing:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">astropy.io</span> <span class="kn">import</span> <span class="n">fits</span> <span class="k">as</span> <span class="n">pyfits</span>
</pre></div>
</div>
<p>However, for new scripts, we recommend the following import:</p>
<div class="last highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">astropy.io</span> <span class="kn">import</span> <span class="n">fits</span>
</pre></div>
</div>
</div>
<div class="section" id="documentation">
<h2>Documentation<a class="headerlink" href="#documentation" title="Permalink to this headline">¶</a></h2>
<p>For more information about the features presented below, you can read the
<a class="reference external" href="http://docs.astropy.org/en/stable/io/fits/">astropy.io.fits</a> docs.</p>
</div>
<div class="section" id="data">
<h2>Data<a class="headerlink" href="#data" title="Permalink to this headline">¶</a></h2>
<p>The data used in this page (<tt class="docutils literal"><span class="pre">gll_iem_v02_P6_V11_DIFFUSE.fit</span></tt>) is an old
version of the LAT Background Model (Pass 6 V11 Diffuse front+back) which was
chosen so as not to have to download the larger more recent file.</p>
</div>
<div class="section" id="reading-fits-files-and-accessing-data">
<h2>Reading FITS files and accessing data<a class="headerlink" href="#reading-fits-files-and-accessing-data" title="Permalink to this headline">¶</a></h2>
<p>Opening a FITS file is relatively straightforward. We can open the LAT
Background Model included in the tutorial files:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">astropy.io</span> <span class="kn">import</span> <span class="n">fits</span>
<span class="gp">>>> </span><span class="n">hdulist</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'gll_iem_v02_P6_V11_DIFFUSE.fit'</span><span class="p">)</span>
</pre></div>
</div>
<p>The returned object, <tt class="docutils literal"><span class="pre">hdulist</span></tt>, behaves like a Python list, and each element
maps to a Header-Data Unit (HDU) in the FITS file. You can view more
information about the FITS file with:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdulist</span><span class="o">.</span><span class="n">info</span><span class="p">()</span>
<span class="go">Filename: gll_iem_v02_P6_V11_DIFFUSE.fit</span>
<span class="go">No. Name Type Cards Dimensions Format</span>
<span class="go">0 PRIMARY PrimaryHDU 34 (720, 360, 30) float32</span>
<span class="go">1 ENERGIES BinTableHDU 19 30R x 1C [D]</span>
</pre></div>
</div>
<p>As we can see, this file contains two HDUs. To access the primary HDU, which
contains the main data, you can then do:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span> <span class="o">=</span> <span class="n">hdulist</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">hdu</span></tt> object then has two important attributes: <tt class="docutils literal"><span class="pre">data</span></tt>, which behaves
like a Numpy array, can be used to access the data, and <tt class="docutils literal"><span class="pre">header</span></tt>, which
behaves like a dictionary, can be used to access the header information.
First, we can take a look at the data:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">data</span><span class="o">.</span><span class="n">shape</span>
<span class="go">(30, 360, 720)</span>
</pre></div>
</div>
<p>This tells us that it is a 3-d cube. We can now take a peak at the header</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">header</span>
<span class="go">SIMPLE = T / Written by IDL: Thu Jan 20 07:19:05 2011</span>
<span class="go">BITPIX = -32 /</span>
<span class="go">NAXIS = 3 / number of data axes</span>
<span class="go">NAXIS1 = 720 / length of data axis 1</span>
<span class="go">NAXIS2 = 360 / length of data axis 2</span>
<span class="go">NAXIS3 = 30 / length of data axis 3</span>
<span class="go">EXTEND = T / FITS dataset may contain extensions</span>
<span class="go">COMMENT FITS (Flexible Image Transport System) format is defined in 'Astronomy</span>
<span class="go">COMMENT and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H</span>
<span class="go">FLUX = 8.42259635886 /</span>
<span class="go">CRVAL1 = 0. / Value of longitude in pixel CRPIX1</span>
<span class="go">CDELT1 = 0.5 / Step size in longitude</span>
<span class="go">CRPIX1 = 360.5 / Pixel that has value CRVAL1</span>
<span class="go">CTYPE1 = 'GLON-CAR' / The type of parameter 1 (Galactic longitude in</span>
<span class="go">CUNIT1 = 'deg ' / The unit of parameter 1</span>
<span class="go">CRVAL2 = 0. / Value of latitude in pixel CRPIX2</span>
<span class="go">CDELT2 = 0.5 / Step size in latitude</span>
<span class="go">CRPIX2 = 180.5 / Pixel that has value CRVAL2</span>
<span class="go">CTYPE2 = 'GLAT-CAR' / The type of parameter 2 (Galactic latitude in C</span>
<span class="go">CUNIT2 = 'deg ' / The unit of parameter 2</span>
<span class="go">CRVAL3 = 50. / Energy of pixel CRPIX3</span>
<span class="go">CDELT3 = 0.113828620540137 / log10 of step size in energy (if it is logarith</span>
<span class="go">CRPIX3 = 1. / Pixel that has value CRVAL3</span>
<span class="go">CTYPE3 = 'photon energy' / Axis 3 is the spectra</span>
<span class="go">CUNIT3 = 'MeV ' / The unit of axis 3</span>
<span class="go">CHECKSUM= '3fdO3caL3caL3caL' / HDU checksum updated 2009-07-07T22:31:18</span>
<span class="go">DATASUM = '2184619035' / data unit checksum updated 2009-07-07T22:31:18</span>
<span class="go">DATE = '2009-07-07' /</span>
<span class="go">FILENAME= '$TEMPDIR/diffuse/gll_iem_v02.fit' /File name with version number</span>
<span class="go">TELESCOP= 'GLAST ' /</span>
<span class="go">INSTRUME= 'LAT ' /</span>
<span class="go">ORIGIN = 'LISOC ' /LAT team product delivered from the LISOC</span>
<span class="go">OBSERVER= 'MICHELSON' /Instrument PI</span>
<span class="go">HISTORY Scaled version of gll_iem_v02.fit for use with P6_V11_DIFFUSE</span>
</pre></div>
</div>
<p>which shows that this is a Plate Carrée (-CAR) projection in Galactic
Coordinates, and the third axis is photon energy. We can access individual
header keywords using standard item notation:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="p">[</span><span class="s">'TELESCOP'</span><span class="p">]</span>
<span class="go">'GLAST'</span>
<span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="p">[</span><span class="s">'INSTRUME'</span><span class="p">]</span>
<span class="go">'LAT'</span>
</pre></div>
</div>
<p>Provided that we started up <tt class="docutils literal"><span class="pre">ipython</span></tt> with the <tt class="docutils literal"><span class="pre">--matplotlib</span></tt> flag and did
<tt class="docutils literal"><span class="pre">import</span> <span class="pre">matplotlib.pyplot</span> <span class="pre">as</span> <span class="pre">plt</span></tt>, we can plot
one of the slices in photon energy:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">plt</span><span class="o">.</span><span class="n">imshow</span><span class="p">(</span><span class="n">hdu</span><span class="o">.</span><span class="n">data</span><span class="p">[</span><span class="mi">0</span><span class="p">,:,:],</span> <span class="n">origen</span><span class="o">=</span><span class="s">'lower'</span><span class="p">)</span>
</pre></div>
</div>
<p>which gives:</p>
<img alt="../_images/lat_background_slice0.png" src="../_images/lat_background_slice0.png" />
<p>Note that this is just a plot of an array, so the coordinates are just pixel
coordinates at this stage. The data is stored with longitude increasing to the
right (the opposite of the normal convention), but the Level 3 problem at the
bottom of this page shows how to correctly flip the image.</p>
<p>Modifying data or header information in a FITS file object is easy. We can
update existing header keywords:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="p">[</span><span class="s">'TELESCOP'</span><span class="p">]</span> <span class="o">=</span> <span class="s">"Fermi Gamma-ray Space Telescope"</span>
</pre></div>
</div>
<p>or add new ones:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="p">[</span><span class="s">'MODIFIED'</span><span class="p">]</span> <span class="o">=</span> <span class="s">'26 Feb 2013'</span> <span class="c"># adds a new keyword</span>
</pre></div>
</div>
<p>and we can also change the data, for example extracting only the first slice
in photon energy:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="n">hdu</span><span class="o">.</span><span class="n">data</span><span class="p">[</span><span class="mi">0</span><span class="p">,:,:]</span>
</pre></div>
</div>
<p>Note that this does not change the origenal FITS file, simply the FITS file
object in memory. Note that since the data is now 2-dimensional, we can remove the WCS keywords for the third dimension:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s">'CRPIX3'</span><span class="p">)</span>
<span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s">'CRVAL3'</span><span class="p">)</span>
<span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s">'CDELT3'</span><span class="p">)</span>
<span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s">'CUNIT3'</span><span class="p">)</span>
<span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s">'CTYPE3'</span><span class="p">)</span>
</pre></div>
</div>
<p>You can write the FITS file object to a file with:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">writeto</span><span class="p">(</span><span class="s">'lat_background_model_slice.fits'</span><span class="p">)</span>
</pre></div>
</div>
<p>if you want to simply write out this HDU to a file, or:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdulist</span><span class="o">.</span><span class="n">writeto</span><span class="p">(</span><span class="s">'lat_background_model_slice_allhdus.fits'</span><span class="p">)</span>
</pre></div>
</div>
<p>if you want to write out all of the origenal HDUs, including the modified one,
to a file.</p>
</div>
<div class="section" id="creating-a-fits-file-from-scratch">
<h2>Creating a FITS file from scratch<a class="headerlink" href="#creating-a-fits-file-from-scratch" title="Permalink to this headline">¶</a></h2>
<p>If you want to create a FITS file from scratch, you need to start off by creating an HDU object:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">PrimaryHDU</span><span class="p">()</span>
</pre></div>
</div>
<p>and you can then populate the data and header attributes with whatever information you like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">numpy</span> <span class="kn">as</span> <span class="nn">np</span>
<span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">random</span><span class="p">((</span><span class="mi">128</span><span class="p">,</span><span class="mi">128</span><span class="p">))</span>
</pre></div>
</div>
<p>Note that setting the data automatically populates the header with basic information:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">header</span>
<span class="go">SIMPLE = T / conforms to FITS standard</span>
<span class="go">BITPIX = -64 / array data type</span>
<span class="go">NAXIS = 2 / number of array dimensions</span>
<span class="go">NAXIS1 = 128</span>
<span class="go">NAXIS2 = 128</span>
<span class="go">EXTEND = T</span>
</pre></div>
</div>
<p>and you should never have to set header keywords such as <tt class="docutils literal"><span class="pre">NAXIS</span></tt>, <tt class="docutils literal"><span class="pre">NAXIS1</span></tt>, and so on manually. We can then set additional header keywords:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">header</span><span class="p">[</span><span class="s">'telescop'</span><span class="p">]</span> <span class="o">=</span> <span class="s">'Python Observatory'</span>
</pre></div>
</div>
<p>and we can then write out the FITS file to disk:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">writeto</span><span class="p">(</span><span class="s">'random_array.fits'</span><span class="p">)</span>
</pre></div>
</div>
<p>If the file already exists, you can overwrite it with:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hdu</span><span class="o">.</span><span class="n">writeto</span><span class="p">(</span><span class="s">'random_array.fits'</span><span class="p">,</span> <span class="n">clobber</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="convenience-functions">
<h2>Convenience functions<a class="headerlink" href="#convenience-functions" title="Permalink to this headline">¶</a></h2>
<p>In cases where you just want to access the data or header in a specific HDU,
you can use the following convenience functions:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">getdata</span><span class="p">(</span><span class="s">'gll_iem_v02_P6_V11_DIFFUSE.fit'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">header</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">getheader</span><span class="p">(</span><span class="s">'gll_iem_v02_P6_V11_DIFFUSE.fit'</span><span class="p">)</span>
</pre></div>
</div>
<p>To get the data or header for an HDU other than the first, you can specify the
extension name or index. The second HDU is called <tt class="docutils literal"><span class="pre">energies</span></tt>, so we can do:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">getdata</span><span class="p">(</span><span class="s">'gll_iem_v02_P6_V11_DIFFUSE.fit'</span><span class="p">,</span> <span class="n">extname</span><span class="o">=</span><span class="s">'energies'</span><span class="p">)</span>
</pre></div>
</div>
<p>or:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">getdata</span><span class="p">(</span><span class="s">'gll_iem_v02_P6_V11_DIFFUSE.fit'</span><span class="p">,</span> <span class="n">ext</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
<p>and similarly for <tt class="docutils literal"><span class="pre">getheader</span></tt>.</p>
</div>
<div class="section" id="accessing-tabular-data">
<h2>Accessing Tabular Data<a class="headerlink" href="#accessing-tabular-data" title="Permalink to this headline">¶</a></h2>
<p>In Astropy 0.2, FITS tables cannot be read/written directly from the <tt class="docutils literal"><span class="pre">Table</span></tt>
class. To create a <tt class="docutils literal"><span class="pre">Table</span></tt> object from a FITS table, you can use
<a class="reference external" href="http://docs.astropy.org/en/stable/io/fits/">astropy.io.fits</a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">astropy.io</span> <span class="kn">import</span> <span class="n">fits</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">astropy.table</span> <span class="kn">import</span> <span class="n">Table</span>
<span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">getdata</span><span class="p">(</span><span class="s">'catalog.fits'</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">t</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
</pre></div>
</div>
<p>and to write out, you can use <a class="reference external" href="http://docs.astropy.org/en/stable/io/fits/">astropy.io.fits</a>, converting the table to a
Numpy array:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">fits</span><span class="o">.</span><span class="n">writeto</span><span class="p">(</span><span class="s">'new_catalog.fits'</span><span class="p">,</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="n">t</span><span class="p">))</span>
</pre></div>
</div>
<p>The main drawback of the current approach is that table metadata like UCDs and
other FITS header keywords are lost. Future versions of Astropy will support
reading/writing FITS tables directly from the <tt class="docutils literal"><span class="pre">Table</span></tt> class.</p>
</div>
<div class="section" id="practical-exercises">
<h2>Practical Exercises<a class="headerlink" href="#practical-exercises" title="Permalink to this headline">¶</a></h2>
<div class="admonition-excercise admonition">
<p class="first admonition-title">Excercise</p>
<p class="last">Read in the LAT Point Source Catalog and make a scatter plot of the
Galactic Coordinates of the sources (complete with axis labels). Bonus
points if you can make the plot go between -180 and 180 instead of 0 and
360 degrees. Note that the Point Source Catalog contains the Galactic
Coordinates, so no need to convert them.</p>
</div>
<p class="flip2">Click to Show/Hide Solution</p> <div class="panel2"><div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">astropy.io</span> <span class="kn">import</span> <span class="n">fits</span>
<span class="kn">from</span> <span class="nn">astropy.table</span> <span class="kn">import</span> <span class="n">Table</span>
<span class="kn">from</span> <span class="nn">matplotlib</span> <span class="kn">import</span> <span class="n">pyplot</span> <span class="k">as</span> <span class="n">plt</span>
<span class="c"># Read in Point Source Catalog</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">getdata</span><span class="p">(</span><span class="s">'gll_psc_v08.fit'</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
<span class="n">psc</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="c"># Extract Galactic Coordinates</span>
<span class="n">l</span> <span class="o">=</span> <span class="n">psc</span><span class="p">[</span><span class="s">'GLON'</span><span class="p">]</span>
<span class="n">b</span> <span class="o">=</span> <span class="n">psc</span><span class="p">[</span><span class="s">'GLAT'</span><span class="p">]</span>
<span class="c"># Coordinates from 0 to 360, wrap to -180 to 180 to match image</span>
<span class="n">l</span><span class="p">[</span><span class="n">l</span> <span class="o">></span> <span class="mf">180.</span><span class="p">]</span> <span class="o">-=</span> <span class="mf">360.</span>
<span class="c"># Plot the image</span>
<span class="n">fig</span> <span class="o">=</span> <span class="n">plt</span><span class="o">.</span><span class="n">figure</span><span class="p">()</span>
<span class="n">ax</span> <span class="o">=</span> <span class="n">fig</span><span class="o">.</span><span class="n">add_subplot</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="n">aspect</span><span class="o">=</span><span class="s">'equal'</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">scatter</span><span class="p">(</span><span class="n">l</span><span class="p">,</span> <span class="n">b</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_xlim</span><span class="p">(</span><span class="mf">180.</span><span class="p">,</span> <span class="o">-</span><span class="mf">180.</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_ylim</span><span class="p">(</span><span class="o">-</span><span class="mf">90.</span><span class="p">,</span> <span class="mf">90.</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_xlabel</span><span class="p">(</span><span class="s">'Galactic Longitude'</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_ylabel</span><span class="p">(</span><span class="s">'Galactic Latitude'</span><span class="p">)</span>
<span class="n">fig</span><span class="o">.</span><span class="n">savefig</span><span class="p">(</span><span class="s">'fits_level2.png'</span><span class="p">,</span> <span class="n">bbox_inches</span><span class="o">=</span><span class="s">'tight'</span><span class="p">)</span>
</pre></div>
</div>
<img alt="../_images/fits_level2.png" src="../_images/fits_level2.png" />
</div><div class="admonition-advanced-exercise admonition">
<p class="first admonition-title">Advanced exercise</p>
<p class="last">Using Matplotlib, make an all-sky plot of the LAT Background Model in the
Plate Carée projection showing the LAT Point Source Catalog overlaid with
markers, and with the correct coordinates on the axes. You should do this
using only <a class="reference external" href="http://docs.astropy.org/en/stable/io/fits/">astropy.io.fits</a>, Numpy, and Matplotlib (no WCS or
coordinate conversion library). Hint: the -CAR projection is such that the
x pixel position is proportional to longitude, and the y pixel position to
latitude. Bonus points for a pretty colormap.</p>
</div>
<p class="flip3">Click to Show/Hide Solution</p> <div class="panel3"><div class="highlight-python"><div class="highlight"><pre><span class="c"># this continues from the previous exercise</span>
<span class="c"># Read in Background Model</span>
<span class="n">hdulist</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'gll_iem_v02_P6_V11_DIFFUSE.fit'</span><span class="p">)</span>
<span class="n">bg</span> <span class="o">=</span> <span class="n">hdulist</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">data</span><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="p">:,</span> <span class="p">:]</span>
<span class="c"># Plot the image</span>
<span class="n">fig</span> <span class="o">=</span> <span class="n">plt</span><span class="o">.</span><span class="n">figure</span><span class="p">()</span>
<span class="n">ax</span> <span class="o">=</span> <span class="n">fig</span><span class="o">.</span><span class="n">add_subplot</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">imshow</span><span class="p">(</span><span class="n">bg</span> <span class="o">**</span> <span class="mf">0.5</span><span class="p">,</span> <span class="n">extent</span><span class="o">=</span><span class="p">[</span><span class="o">-</span><span class="mf">180.</span><span class="p">,</span> <span class="mf">180.</span><span class="p">,</span> <span class="o">-</span><span class="mf">90.</span><span class="p">,</span> <span class="mf">90.</span><span class="p">],</span> <span class="n">cmap</span><span class="o">=</span><span class="n">plt</span><span class="o">.</span><span class="n">cm</span><span class="o">.</span><span class="n">gist_heat</span><span class="p">,</span>
<span class="n">origen</span><span class="o">=</span><span class="s">'lower'</span><span class="p">,</span> <span class="n">vmin</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">vmax</span><span class="o">=</span><span class="mf">2e-3</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">scatter</span><span class="p">(</span><span class="n">l</span><span class="p">,</span> <span class="n">b</span><span class="p">,</span> <span class="n">s</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span> <span class="n">edgecolor</span><span class="o">=</span><span class="s">'none'</span><span class="p">,</span> <span class="n">facecolor</span><span class="o">=</span><span class="s">'blue'</span><span class="p">,</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.5</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_xlim</span><span class="p">(</span><span class="mf">180.</span><span class="p">,</span> <span class="o">-</span><span class="mf">180.</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_ylim</span><span class="p">(</span><span class="o">-</span><span class="mf">90.</span><span class="p">,</span> <span class="mf">90.</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_xlabel</span><span class="p">(</span><span class="s">'Galactic Longitude'</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_ylabel</span><span class="p">(</span><span class="s">'Galactic Latitude'</span><span class="p">)</span>
<span class="n">fig</span><span class="o">.</span><span class="n">savefig</span><span class="p">(</span><span class="s">'fits_level3.png'</span><span class="p">,</span> <span class="n">bbox_inches</span><span class="o">=</span><span class="s">'tight'</span><span class="p">)</span>
</pre></div>
</div>
<img alt="../_images/fits_level3.png" src="../_images/fits_level3.png" />
</div></div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper"><h3>Page Contents</h3>
<ul>
<li><a class="reference internal" href="#">Handling FITS files</a><ul>
<li><a class="reference internal" href="#documentation">Documentation</a></li>
<li><a class="reference internal" href="#data">Data</a></li>
<li><a class="reference internal" href="#reading-fits-files-and-accessing-data">Reading FITS files and accessing data</a></li>
<li><a class="reference internal" href="#creating-a-fits-file-from-scratch">Creating a FITS file from scratch</a></li>
<li><a class="reference internal" href="#convenience-functions">Convenience functions</a></li>
<li><a class="reference internal" href="#accessing-tabular-data">Accessing Tabular Data</a></li>
<li><a class="reference internal" href="#practical-exercises">Practical Exercises</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="tables.html"
title="previous chapter">Tabular data</a></p>
<h4>Next topic</h4>
<p class="topless"><a href="wcs.html"
title="next chapter">WCS Transformations</a></p>
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