regrid-an-image-or-cube-using-casa.html

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         title="Permalink to Regrid an image (or cube) using CASA">Regrid an image (or cube) using CASA</a></h2>
 
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    <abbr class="published" title="2013-12-18T17:52:00-07:00">
      2013/12/18
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    <address class="vcard author">
      By <a class="url fn" href="https://keflavich.github.io/blog/author/adam-ginsburg.html">Adam Ginsburg <adam.g.ginsburg@gmail.com></a>
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  <div class="entry-content">
    <p>In order to regrid an image, you need some target reprojection.  The simplest way to acquire such a reprojection
is by extracting it from another image, e.g. a <a class="reference external" href="casaguides.nrao.edu/index.php?title=FITStoImage">FITS image</a></p>
<p>(I use <cite>&gt;&gt;&gt;</cite> to indicate the <cite>CASA &lt;#&gt;:</cite> prompt here because it allows you to copy &amp; paste the code into CASA)</p>
<div class="highlight"><pre><span></span><span class="o">&gt;&gt;&gt;</span> <span class="n">header</span> <span class="o">=</span> <span class="n">imregrid</span><span class="p">(</span><span class="n">imagename</span><span class="o">=</span><span class="s1">&#39;file.image&#39;</span><span class="p">,</span> <span class="n">template</span><span class="o">=</span><span class="s1">&#39;get&#39;</span><span class="p">)</span>
</pre></div>
<p>This header will be a python dictionary with two keys, <cite>csys</cite>, which specifies
the coordinate system, and <cite>shap</cite>, which gives the shape.</p>
<p>If you want to write up your own FITS header and you don't have a FITS file
with those exact coordinates, you can use the <a class="reference external" href="https://github.com/keflavich/casa_tools/blob/master/casa_tools/header_to_template.py#L11">header_to_template</a> convenience
tool in <a class="reference external" href="https://github.com/keflavich/casa_tools">casa_tools</a>.</p>
<div class="highlight"><pre><span></span><span class="o">&gt;&gt;&gt;</span> <span class="kn">from</span> <span class="nn">casa_tools</span> <span class="kn">import</span> <span class="n">header_to_template</span>
<span class="o">&gt;&gt;&gt;</span> <span class="kn">from</span> <span class="nn">astropy.io</span> <span class="kn">import</span> <span class="n">fits</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">fits_header</span> <span class="o">=</span> <span class="n">fits</span><span class="o">.</span><span class="n">Header</span><span class="o">.</span><span class="n">fromtextfile</span><span class="p">(</span><span class="s1">&#39;my_header.hdr&#39;</span><span class="p">)</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">header</span> <span class="o">=</span> <span class="n">header_to_template</span><span class="p">(</span><span class="n">fits_header</span><span class="p">)</span>
</pre></div>
<p>In order to regrid the file, you then need to pass in the image name and template to imregrid.</p>
<div class="highlight"><pre><span></span><span class="o">&gt;&gt;&gt;</span> <span class="n">imregrid</span><span class="p">(</span><span class="n">imagename</span><span class="o">=</span><span class="s1">&#39;file.image&#39;</span><span class="p">,</span>
<span class="o">...</span>          <span class="n">template</span><span class="o">=</span><span class="n">header</span><span class="p">,</span>
<span class="o">...</span>          <span class="n">output</span><span class="o">=</span><span class="s1">&#39;file_regrid.image&#39;</span><span class="p">)</span>
</pre></div>
<p>If your input image's header did not include a 'telescope' keyword, CASA will complain:</p>
<blockquote>
2013-12-18 16:46:33 SEVERE  imregrid::ImageRegrid::regrid (file /opt/casa/release-4_1_0-release-1/darwin64/include/casacore/images/Images/ImageRegrid.tcc, line 118)        Cannot find the observatory name UNKNOWN in the CASA</blockquote>
<p>You can add the keyword to the header yourself:</p>
<div class="highlight"><pre><span></span><span class="o">&gt;&gt;&gt;</span> <span class="n">imhead</span><span class="p">(</span><span class="s1">&#39;file.image&#39;</span><span class="p">,</span>
<span class="o">...</span>        <span class="n">mode</span><span class="o">=</span><span class="s1">&#39;put&#39;</span><span class="p">,</span>
<span class="o">...</span>        <span class="n">hdkey</span><span class="o">=</span><span class="s1">&#39;telescope&#39;</span><span class="p">,</span>
<span class="o">...</span>        <span class="n">hdvalue</span><span class="o">=</span><span class="s1">&#39;Arecibo&#39;</span><span class="p">)</span>
</pre></div>
<div class="section" id="references">
<h2>References</h2>
<blockquote>
<ul class="simple">
<li><a class="reference external" href="http://www.eso.org/projects/alma/arc/tw/pub/External/EUARCCASATutorialJan2012/ImageAnalysis-CASA.pdf">Sebastian Muller's presentation</a> (page 16)</li>
<li><a class="reference external" href="http://casa.nrao.edu/docs/casaref/image.regrid.html">imregrid docs</a></li>
</ul>
</blockquote>
</div>

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