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Sky Matching with Per-Object Errors</a><br>
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<h4><a name="SkyMatchEngine">7.1.1 <code>sky</code>: Sky Matching</a></h4>
<p><pre>matcher=sky values*='<ra/degrees> <dec/degrees>'
params='<max-error/arcsec>'
tuning='<healpix-k>'</pre>
<blockquote>
<dl>
<dt><strong><code>values*</code>:</strong></dt>
<dd>
<ul>
<li><code>ra/degrees</code>: Right Ascension
</li>
<li><code>dec/degrees</code>: Declination
</li>
</ul>
</dd>
<dt><strong><code>params</code>:</strong></dt>
<dd>
<ul>
<li><code>max-error/arcsec</code>: Maximum separation along a great circle
</li>
</ul>
</dd>
<dt><strong><code>tuning</code>:</strong></dt>
<dd>
<ul>
<li><code>healpix-k</code>: Controls sky pixel size. Legal range 0 - 20. 0 is 60deg, 20 is 0.2".
</li>
</ul>
</dd>
</dl>
</blockquote>
</p>
<p>The <code>sky</code> matcher compares positions on the celestial sphere
with a fixed error radius.
Rows are considered to match when the two (<code>ra</code>, <code>dec</code>)
positions are within <code>max-error</code> arcseconds of each other
along a great circle.
</p>
<p>In fact this matching is not restricted to equatorial coordinates -
the <code>ra</code> and <code>dec</code> parameters may represent
any longitude-like and latitude-like coordinates in degrees,
since the spherical geometry for the matching is unchanged under
such transformations.
</p>
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Sky Matching with Per-Object Errors</a><br>
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<hr><i>STILTS - Starlink Tables Infrastructure Library Tool Set<br>Starlink User Note256<br>STILTS web page:
<a href="http://www.starlink.ac.uk/stilts/">http://www.starlink.ac.uk/stilts/</a><br>Author email:
<a href="mailto:m.b.taylor@bristol.ac.uk">m.b.taylor@bristol.ac.uk</a><br>Mailing list:
<a href="mailto:topcat-user@jiscmail.ac.uk">topcat-user@jiscmail.ac.uk</a><br></i></body>
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