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<div class="section" id="step-3-subtracting-the-sky">
<span id="subtractsky"></span><h1>Step 3: Subtracting the Sky<a class="headerlink" href="#step-3-subtracting-the-sky" title="Permalink to this headline">¶</a></h1>
<p>This step measures and subtracts the sky from each input image while recording the subtracted value in the image header.</p>
<span class="target" id="module-drizzlepac.sky"></span><p>Function for computing and subtracting the backgroud of
an image. The algorithm employed here uses a sigma
clipped median of each <em>sci</em> image in a data file.
Then the sky value for each detector is compared
and the lowest value is subtracted from all chips
in the detector. Finally, the MDRIZSKY keyword
is updated in the header of the input files.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name" colspan="2">Authors:</th></tr>
<tr class="field-odd field"><td> </td><td class="field-body">Christopher Hanley, Megan Sosey</td>
</tr>
</tbody>
</table>
<dl class="function">
<dt id="drizzlepac.sky.getHelpAsString">
<tt class="descclassname">drizzlepac.sky.</tt><tt class="descname">getHelpAsString</tt><big>(</big><big>)</big><a class="headerlink" href="#drizzlepac.sky.getHelpAsString" title="Permalink to this definition">¶</a></dt>
<dd><p>return useful help from a file in the script directory called module.help</p>
</dd></dl>
<dl class="function">
<dt id="drizzlepac.sky.getreferencesky">
<tt class="descclassname">drizzlepac.sky.</tt><tt class="descname">getreferencesky</tt><big>(</big><em>image</em>, <em>keyval</em><big>)</big><a class="headerlink" href="#drizzlepac.sky.getreferencesky" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="function">
<dt id="drizzlepac.sky.help">
<tt class="descclassname">drizzlepac.sky.</tt><tt class="descname">help</tt><big>(</big><big>)</big><a class="headerlink" href="#drizzlepac.sky.help" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="function">
<dt id="drizzlepac.sky.run">
<tt class="descclassname">drizzlepac.sky.</tt><tt class="descname">run</tt><big>(</big><em>configObj</em>, <em>outExt=None</em><big>)</big><a class="headerlink" href="#drizzlepac.sky.run" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="function">
<dt id="drizzlepac.sky.sky">
<tt class="descclassname">drizzlepac.sky.</tt><tt class="descname">sky</tt><big>(</big><em>input=None</em>, <em>outExt=None</em>, <em>configObj=None</em>, <em>group=None</em>, <em>editpars=False</em>, <em>**inputDict</em><big>)</big><a class="headerlink" href="#drizzlepac.sky.sky" title="Permalink to this definition">¶</a></dt>
<dd><p>Perform sky subtraction on input list of images</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name" colspan="2">Parameters:</th></tr>
<tr class="field-odd field"><td> </td><td class="field-body"><p class="first"><strong id="input">input</strong> : str or list of str</p>
<blockquote>
<div><p>a python list of image filenames, or just a single filename</p>
</div></blockquote>
<p><strong id="configObj">configObj</strong> : configObject</p>
<blockquote>
<div><p>an instance of configObject</p>
</div></blockquote>
<p><strong id="inputDict">inputDict</strong> : dict, optional</p>
<blockquote>
<div><p>an optional list of parameters specified by the user</p>
</div></blockquote>
<p><strong id="outExt">outExt</strong> : str</p>
<blockquote class="last">
<div><p>The extension of the output image. If the output already exists
then the input image is overwritten</p>
</div></blockquote>
</td>
</tr>
</tbody>
</table>
<p class="rubric">Notes</p>
<p>These are parameters that the configObj should contain by default,
they can be altered on the fly using the inputDict</p>
<p>Parameters that should be in configobj:</p>
<table border="1" class="docutils">
<colgroup>
<col width="12%" />
<col width="88%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Name</th>
<th class="head">Definition</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>skyuser</td>
<td>‘KEYWORD in header which indicates a sky subtraction value to use’.</td>
</tr>
<tr class="row-odd"><td>skysub</td>
<td>‘Perform sky subtraction?’</td>
</tr>
<tr class="row-even"><td>skywidth</td>
<td>‘Bin width for sampling sky statistics (in sigma)’</td>
</tr>
<tr class="row-odd"><td>skystat</td>
<td>‘Sky correction statistics parameter’</td>
</tr>
<tr class="row-even"><td>skylower</td>
<td>‘Lower limit of usable data for sky (always in electrons)’</td>
</tr>
<tr class="row-odd"><td>skyupper</td>
<td>‘Upper limit of usable data for sky (always in electrons)’</td>
</tr>
<tr class="row-even"><td>skyclip</td>
<td>‘Number of clipping iterations’</td>
</tr>
<tr class="row-odd"><td>skylsigma</td>
<td>‘Lower side clipping factor (in sigma)’</td>
</tr>
<tr class="row-even"><td>skyusigma</td>
<td>‘Upper side clipping factor (in sigma)’</td>
</tr>
</tbody>
</table>
<p>The output from sky subtraction is a copy of the original input file
where all the science data extensions have been sky subtracted.</p>
</dd></dl>
<dl class="function">
<dt id="drizzlepac.sky.subtractSky">
<tt class="descclassname">drizzlepac.sky.</tt><tt class="descname">subtractSky</tt><big>(</big><em>imageObjList</em>, <em>configObj</em>, <em>saveFile=False</em>, <em>procSteps=None</em><big>)</big><a class="headerlink" href="#drizzlepac.sky.subtractSky" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
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