7.1 Sending queries to web services (Vizier, SkyBoT)
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Retrieving data from Vizier
-----------------------------------

AsPyLib allows to send a simple "cone search" request to the Vizier service, and retrieve star positions and information from any catalog.
To do it you only need to specify:

* a catalog 
* a position (alpha, delta)
* a cone radius in arcmin, or a rectangular box size in arcmin x arcmin

To see how it works, the simplest way is to show an example. The script below retrieves all stars with magnitude smaller than 11, in an area of 7deg*10deg centered on the Pleiades, in Taurus.
::

	from aspylib import astrometry, astro
	import matplotlib.pyplot as plt

	#------ retrieving information from Vizier ------
	catalog = 'USNO-B1'
	position = [56., 24.]		#[alpha, delta] in degrees
	conesize = [600., 420.]		#[boxsize_alpha, boxsize_delta] in arcmin
	constraint = 'B1mag<=11'

	astrometry.send_query_vizier(catalog, position, conesize, constraint)
	stars = astrometry.read_votable("vizier.xml")

	#------ printing information --------------------
	print
	print len(stars)," stars downloaded"
	print
	print "catalog fields:"
	print stars.dtype.names

	#------ generating sky map ----------------------
	plt.figure()
	plt.scatter(stars['RAJ2000'],stars['DEJ2000'],s=4*(12-stars['B1mag']))
	plt.grid()
	plt.xlim([52,60])
	plt.ylim([22,27])
	ax = plt.gca() 
	ax.set_xlim(ax.get_xlim()[::-1])
	plt.show()

Then we get the following messages in the Python console:

.. image:: vizier1.png

And the following sky map is shown:

.. image:: vizier2.png


The first instruction sends a request to Vizier, and saves the returned VOtable file under the name "vizier.xml"
::

	astrometry.send_query_vizier(catalog, position, conesize, constraint)

The filename may be changed, as follows:
::

	astrometry.send_query_vizier(catalog, position, conesize, constraint, filename = "data.xml")
	
Then the second instruction opens the file, and convert it to a Numpy record array. Numpy record arrays are special arrays that allow to store columns with different data type, with a name and format for each column.
They are discussed `here <http://docs.scipy.org/doc/numpy/user/basics.rec.html>`_ for instance.
::

	stars = astrometry.read_votable("vizier.xml")

The name of the columns can be obtained with:
::

	print stars.dtype.names

And the data in a given column:
::
	
	print stars['column name']


Retrieving data from SkyBoT
----------------------------------------

Sending queries to SkyBoT is pretty much the same. An example of script is given below:
::

	from aspylib import astrometry

	#------ retrieving information from SkyBoT ------
	date = 2455923.0
	position = [56., 24.]		#[alpha, delta] in degrees
	conesize = [30., 30.]		#[boxsize_alpha, boxsize_delta] in arcmin

	astrometry.send_query_skybot(date,position,conesize)
	cat = astrometry.read_votable("skybot.xml")

	#------ printing information --------------------
	print
	print len(cat)," objects downloaded"
	print
	print "catalog fields:"
	print cat.dtype.names
	print
	print cat
	print
	raw_input("done")

Then the following messages are printed in the Python console:

.. image:: skybot.png

More information
----------------------------------------

Getting more information about the AsPyLib functions is quite easy thanks to the console help. Just by typing this:
::

	help(astrometry.send_query_vizier)
	
we get the following:

.. image:: help_query.png