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

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<title>Virtual Observatory — Python4Astronomers 2.0 documentation</title>

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

<ul>

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<h3>Navigation</h3>

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</a>

</li>

« previous

</a>

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<li>

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

</li>

<li><a href="vo.html" accesskey="U">Online Astronomy and the virtual observatory</a> »</li>

<li>Virtual Observatory</li>

</ul>

</div>

<h1>Virtual Observatory<a class="headerlink" href="#virtual-observatory" title="Permalink to this headline">¶</a></h1>

The <a class="reference external" href="https://github.com/python4vo/coatpy">COATpy</a> wrapping of <a class="reference external" href="https://code.google.com/p/pyvolib">pyvolib</a> exposes a number of different virtual

observatory tools. We will go over each of these in turn.

<li><a class="reference internal" href="#name-position-resolver" id="id1">Name Position Resolver</a></li>

<li><a class="reference internal" href="#catalog-search" id="id2">Catalog Search</a></li>

<li><a class="reference internal" href="#image-search" id="id3">Image Search</a></li>

</ul>

</div>

<h2><a class="toc-backref" href="#id1">Name Position Resolver</a><a class="headerlink" href="#name-position-resolver" title="Permalink to this headline">¶</a></h2>

Names of things

Presently the “names” of things have to be URL encoded or the tool

will not resolve. Use <code class="docutils literal">IC+348</code> or <code class="docutils literal">IC348</code> instead of <code class="docutils literal">IC 348</code>.

Programmatically this looks like:

<div class="highlight-python"><div class="highlight"><pre>urllib.quote_plus("IC 348")

</pre></div>

</div>

This will be fixed in an upcoming update.

</div>

<code class="docutils literal">Sesame</code> is a basic name resolver that returns the position of objects as a

tuple. You can specify which service you wish to use:

<div class="highlight-python"><div class="highlight"><pre>from coatpy import Sesame

simbad = Sesame(opt='S')

ned = Sesame(opt='N')

ned.resolve('M51')

# the services do not agree; doing this will produce an exception.

assert simbad.resolve('IC348') == ned.resolve('IC348')

</pre></div>

</div>

<h2><a class="toc-backref" href="#id2">Catalog Search</a><a class="headerlink" href="#catalog-search" title="Permalink to this headline">¶</a></h2>

The <code class="docutils literal">ConeSearch</code> tool is a standardized interface to one of hundreds of

catalogs. The necessary parameters are:

<li><code class="docutils literal">RA</code>: decimal degrees</li>

<li><code class="docutils literal">DEC</code>: decimal degrees</li>

<li><code class="docutils literal">SR</code>: cone radius in decimal degrees</li>

</ul>

An optional parameter <code class="docutils literal">VERB</code> varies the verbosity of the query response.

Verbs

Only <code class="docutils literal">VERB=1</code> is standardized across all catalogs. A <code class="docutils literal">VERB=1</code> query

returns only the positions of objects found the queried catalog.

<code class="docutils literal">VERB=2</code> returns the “default” set of columns chosen by the

archive. <code class="docutils literal">VERB=3</code> returns all columns. Not all services respond

to <code class="docutils literal">VERB</code> parameters.

</div>

As with the web queries you can wrap up the parameters in a dictionary:

<div class="highlight-python"><div class="highlight"><pre>from coatpy import ConeSearch, Sesame

params = {}

params['RA'] = simbad.resolve("IC348")[0]

params['DEC'] = simbad.resolve("IC348")[1]

params['SR'] = 1./60.

params['VERB'] = 3

</pre></div>

</div>

Discovering catalogs

At this point the system breaks down a little bit as we are not providing

a programmatic tool for discovering catalogs. You have to use an online

<a class="reference external" href="http://nvo.stsci.edu/vor10/index.aspx">Directory</a> to search the catalogs you want. <a class="reference external" href="http://nvo.stsci.edu/vor10/index.aspx#Table||query_string=IC%20348">Here is an example

Directory search.</a> Once a catalog is found, you want to copy the

“accessURL” from the search table or open the “Full Record” and open the

“Simple Cone Search” option. <a class="reference external" href="http://nvo.stsci.edu/vor10/getRecord.aspx?id=ivo://CDS.VizieR/J/AJ/122/866">Here is the catalog used in this example.</a>

</div>

enter the query URL for the target catalog (see sidebar), and create a catalog

handler <code class="docutils literal">ic348cxo</code> that wraps the query tool. We use the <code class="docutils literal">getRaw</code> function

of the ConeSearch to retrieve the raw string result that we then dump into a

file and extract back the data using <a class="reference external" href="http://astropy.org">astropy</a>:

<div class="highlight-python"><div class="highlight"><pre>from astropy.table import Table

url = "http://vizier.u-strasbg.fr/viz-bin/votable/-A?-source=J/AJ/122/866&"

ic348cxo = ConeSearch(url)

with open('ic348cxo.xml','wb') as f:

f.write(ic348cxo.getRaw(**params))

data = Table.read(f.name, format='votable')

</pre></div>

</div>

<h2><a class="toc-backref" href="#id3">Image Search</a><a class="headerlink" href="#image-search" title="Permalink to this headline">¶</a></h2>

<code class="docutils literal">Siap</code> is the tool for querying image services is:

<div class="highlight-python"><div class="highlight"><pre>from coatpy import Siap

</pre></div>

</div>

The <code class="docutils literal">Siap</code> acronym stands for Simple Image Access Protocol and as with

<code class="docutils literal">ConeSearch</code> the inputs are fairly simple:

<li><code class="docutils literal">RA</code>: Decimal Degrees</li>

<li><code class="docutils literal">DEC</code>: Decimal Degrees</li>

<li><code class="docutils literal">SIZE</code>: An image size in degrees</li>

<li><code class="docutils literal">FORMAT</code>: [Optional] default: image/fits (or image/jpeg)</li>

</ul>

Note

Various image archives have appended other query parameters onto

the SIAP standard to provide additional initial filtering of the images

returned. A good example is the <a class="reference external" href="http://irsa.ipac.caltech.edu/applications/2MASS/IM/docs/siahelp.html">IRSA 2MASS</a>

service. The <code class="docutils literal">Siap</code> tool currently throws Exceptions on extra parameters.

</div>

Exercise: find HST images of a young cluster

In this exercise we will go through the steps necessary for downloading

images with <code class="docutils literal">Siap</code> from the Hubble Legacy Archive.

The first step is to set up the query, which is nearly identical to the

previous examples.

<div class="highlight-python"><div class="highlight"><pre>import urllib2

from coatpy import Siap, Sesame

import vo

url = " http://hla.stsci.edu/cgi-bin/acsSIAP.cgi?strict=1"

hla = Siap(url)

params = {}

position = simbad.resolve('IC348')

params.update(zip(('RA', 'DEC'), position))

params['SIZE'] = 1./60.

params['FORMAT'] = 'image/fits'

with open('hla_ic348_images.xml', 'wb') as f:

f.write(hla.getRaw(**params))

</pre></div>

</div>

The main difference between a catalog and an image query is that an image

query results in a series of pointers to the image files that can be

examined and filtered before the reference files are downloaded. We use

the <a class="reference external" href="http://stsdas.stsci.edu/astrolib/vo/html/">vo</a> module to parse the

results, extracting the table of images that satisfy our query.

<div class="highlight-python"><div class="highlight"><pre>vot = vo.table.parse_single_table('hla_ic348_images.xml')

image_table = vot.array

</pre></div>

</div>

The returned array contains 66 fields!

<div class="highlight-python"><div class="highlight"><pre>print(image_table.dtype.names)

</pre></div>

</div>

If all we want is the actual data then the important column is URL

while the filename column is also useful.

<div class="highlight-python"><div class="highlight"><pre># just grab the first image returned

image = image_table[0]

image_url = image['URL']

filename = image['filename'] + '.fits'

with open(filename, 'wb') as image_file:

image_handler = urllib2.urlopen(image_url)

image_file.write(image_handler.read())

image_handler.close()

</pre></div>

</div>

Continue this exercise by examining the resulting image.

</div>

Click to Show/Hide Solution <div class="panel9">We use <a class="reference external" href="http://docs.astropy.org/en/stable/io/fits/">astropy.io.fits</a> to open the image file and examine its contents.

<div class="highlight-python"><div class="highlight"><pre>from astropy.io import fits

import aplpy

import matplotlib.pyplot as plt

hdulist = fits.open(filename)

# check for multiple FITS extensions and their contents

# in this case the "PRIMARY" header is empty

for hdu in hdulist:

print('name: {0} type: {1}'.format(hdu.name, type(hdu.data)))

fig = plt.figure(figsize=(15, 7))

f1 = aplpy.FITSFigure(filename, hdu=1, subplot=[0.1,0.1,0.3,0.65], figure=fig)

f1.set_tick_labels_font(size='x-small')

f1.set_axis_labels_font(size='small')

f1.show_grayscale()

f2 = aplpy.FITSFigure(filename, hdu=2, subplot=[0.4,0.1,0.3,0.65], figure=fig)

f2.hide_xaxis_label()

f2.hide_xtick_labels()

f2.hide_yaxis_label()

f2.hide_ytick_labels()

f2.show_colorscale()

f3 = aplpy.FITSFigure(filename, hdu=3, subplot=[0.7,0.1,0.3,0.65], figure=fig)

f3.hide_xaxis_label()

f3.hide_xtick_labels()

f3.hide_yaxis_label()

f3.hide_ytick_labels()

f3.show_colorscale(cmap='spring')

</pre></div>

</div>

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

<ul>

<li><a class="reference internal" href="#">Virtual Observatory</a><ul>

<li><a class="reference internal" href="#name-position-resolver">Name Position Resolver</a></li>

<li><a class="reference internal" href="#catalog-search">Catalog Search</a></li>

<li><a class="reference internal" href="#image-search">Image Search</a></li>

</ul>

</li>

</ul>

<h4>Previous topic</h4>

<a href="webmodules.html"

title="previous chapter">Standard Python Tools</a>

<h4>Next topic</h4>

<a href="atpy.html"

title="next chapter">ATpy</a>

</div>

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