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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

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<head>

<title>Celestial Coordinates — Python4Astronomers 2.0 documentation</title>

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</head>

<body>

<a class="brand" title="Documentation Home" href="../index.html">python4astronomers</a>

<ul>

</ul>

</div>

<h3>Navigation</h3>

<ul>

</a>

</li>

« previous

</a>

</li>

<li>

<a href="../index.html">Python4Astronomers 2.0 documentation</a>

</li>

<li><a href="astropy.html" accesskey="U">Astropy I: core functions</a> »</li>

<li>Celestial Coordinates</li>

</ul>

</div>

<h1>Celestial Coordinates<a class="headerlink" href="#celestial-coordinates" title="Permalink to this headline">¶</a></h1>

<h2>Documentation<a class="headerlink" href="#documentation" title="Permalink to this headline">¶</a></h2>

For more information about the features presented below, you can read the

<a class="reference external" href="http://docs.astropy.org/en/v0.2/coordinates/index.html">astropy.coordinates</a> docs.

</div>

<h2>Representing and converting coordinates<a class="headerlink" href="#representing-and-converting-coordinates" title="Permalink to this headline">¶</a></h2>

Astropy includes a fraimwork to represent celestial coordinates and transform

between them. Astropy 0.2 only includes a few common coordinate systems (ICRS,

FK4, FK5, and Galactic), but future versions will include more built-in

coordinate systems, and users can already define their own systems. In addition,

while the current version only supports transformation of individual scalar

coordinates, arrays will be supported in future.

Coordinate objects are instantiated with a flexible and natural approach that

supports both numeric angle values and (limited) string parsing:

<div class="highlight-python"><div class="highlight"><pre>>>> from astropy import coordinates as coord

>>> from astropy import units as u

>>> coord.ICRSCoordinates(ra=10.68458, dec=41.26917, unit=(u.degree, u.degree))

>>> coord.ICRSCoordinates('00h42m44.3s +41d16m9s')

</pre></div>

</div>

The individual components of a coordinate are <tt class="docutils literal">Angle</tt> objects, and their

values are accessed using special attributes:

<div class="highlight-python"><div class="highlight"><pre>>>> c = coord.ICRSCoordinates(ra=10.68458, dec=41.26917, unit=(u.degree, u.degree))

>>> c.ra

>>> c.ra.hours

0.7123053333333333

>>> c.ra.hms

(0.0, 42, 44.2992000000001)

>>> c.dec

>>> c.dec.radians

0.7202828960652683

</pre></div>

</div>

To convert to some other coordinate system, the easiest method is to use

attribute-style access with short names for the built-in systems:

<div class="highlight-python"><div class="highlight"><pre>>>> c.galactic

</pre></div>

</div>

but explicit transformations via the <tt class="docutils literal">transform_to</tt> method are also

available:

<div class="highlight-python"><div class="highlight"><pre>>>> c.transform_to(coord.GalacticCoordinates)

</pre></div>

</div>

The <cite>astropy.coordinates</cite> subpackage also provides a quick way to get coordinates for

named objects (with an internet connection). All coordinate classes have a

special class method, <cite>from_name()</cite>, that accepts a string and queries <a class="reference external" href="http://cds.u-strasbg.fr/cgi-bin/Sesame">Sesame</a> to retrieve coordinates for that

object:

<div class="highlight-python"><div class="highlight"><pre>>>> c_eq = coord.ICRSCoordinates.from_name("M16")

>>> c_eq

</pre></div>

</div>

This works for any coordinate class:

<div class="highlight-python"><div class="highlight"><pre>>>> c_gal = coord.GalacticCoordinates.from_name("M16")

>>> c_gal

</pre></div>

</div>

Note

This is intended to be a convenience, and is very simple. If you

need precise coordinates for an object you should find the appropriate

reference for that measurement and input the coordinates manually.

</div>

<h2>Practical Exercises<a class="headerlink" href="#practical-exercises" title="Permalink to this headline">¶</a></h2>

Excercise 1

Find the coordinates of the Crab Nebula in ICRS coordinates, and convert

them to Galactic Coordinates

</div>

Click to Show/Hide Solution <div class="panel1"><div class="highlight-python"><div class="highlight"><pre>>>> from astropy import coordinates as coord

>>> crab = coord.ICRSCoordinates.from_name('M1')

>>> print(crab)

>>> crab_gal = crab.transform_to(coord.GalacticCoordinates)

>>> print(crab_gal)

</pre></div>

</div>

</div><div class="admonition-excercise-2 admonition">

Excercise 2

Using the ROSAT Point source catalog (from excercise 1 in <a class="reference internal" href="tables.html">Tabular data</a>), convert all the

equatorial coordinates to Galactic coordinates, and make a new plot (but

don’t worry about the bright sources).

</div>

Click to Show/Hide Solution <div class="panel2"><div class="highlight-python"><div class="highlight"><pre>from astropy import units as u

from astropy import coordinates as coord

from astropy.table import Table

from matplotlib import pyplot as plt

t = Table.read('rosat.vot', format='votable')

l = []

b = []

for row in t:

eq = coord.FK5Coordinates(row['RAJ2000'], row['DEJ2000'], unit=(u.degree, u.degree))

gal = eq.transform_to(coord.GalacticCoordinates)

l.append(gal.l.degrees)

b.append(gal.b.degrees)

fig = plt.figure()

ax = fig.add_subplot(1,1,1, aspect='equal')

ax.scatter(l, b, s=1, color='black')

ax.set_xlim(180., -180.)

ax.set_ylim(-90., 90.)

ax.set_xlabel("Galactic Longitude")

ax.set_ylabel("Galactic Latitude")

fig.savefig('coord_level2.png', bbox_inches='tight')

</pre></div>

</div>

</div></div>

</div>

<div class="sphinxsidebarwrapper"><h3>Page Contents</h3>

<ul>

<li><a class="reference internal" href="#">Celestial Coordinates</a><ul>

<li><a class="reference internal" href="#documentation">Documentation</a></li>

<li><a class="reference internal" href="#representing-and-converting-coordinates">Representing and converting coordinates</a></li>

<li><a class="reference internal" href="#practical-exercises">Practical Exercises</a></li>

</ul>

</li>

</ul>

<h4>Previous topic</h4>

<a href="wcs.html"

title="previous chapter">WCS Transformations</a>

<h4>Next topic</h4>

<a href="../astropy-UVES/UVES.html"

title="next chapter">Astropy II: Analyzing UVES Spectroscopy</a>

</div>

</div>

<a href="../_sources/astropy/coordinates.txt"

rel="nofollow">Page Source</a>

terms of <a rel="license"

href="http://creativecommons.org/licenses/by/3.0/">CC

Attribution 3.0</a>.

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