COOSYS element#

The COOSYS element is allowed in the VOTABLE and RESOURCE elements. It describes a coordinate system and has the following attributes:

  • ID: a label,

  • system: the reference frame of the coordinates. Since VOTable version 1.5, it follows the IVOA reference frame vocabulary.

  • equinox: fixes the equatorial or ecliptic systems

  • epoch: the epoch of the positions, as Julian or Besselian years

  • refposition: it exists since VOTable version 1.5 and follows the IVOA reference positions vocabulary.

They are all optional. The CooSys class represents a COOSYS element. It can be created from scratch:

>>> from astropy.io.votable.tree import CooSys
>>> coosys = CooSys(system="FK4", equinox="B1950", epoch="B1980")

and converted into an astropy built-in frame:

>>> coosys.to_astropy_frame()
<FK4 Frame (equinox=B1950.000, obstime=B1950.000)>

Using the COOSYS element#

The COOSYS element is often used to describe the coordinates of a FIELD element. In that case, the FIELD will have a ref attribute corresponding to the COOSYS’s ‘ID field. See for example this VOTable:

>>> from astropy.io.votable import parse as parse_votable
>>> from io import BytesIO
>>> votable = parse_votable(BytesIO(str.encode("""<?xml version="1.0" encoding="utf-8"?>
...    <VOTABLE version="1.3" xmlns="http://www.ivoa.net/xml/VOTable/v1.3" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.ivoa.net/xml/VOTable/v1.3 http://www.ivoa.net/xml/VOTable/VOTable-1.3.xsd">
...    <RESOURCE type="results">
...      <COOSYS ID="coosys_c1356galcand" epoch="2016.0" system="ICRS"/>
...      <TABLE ID="result_S1744377631073" name="result_S1744377631073" nrows="3">
...      <FIELD ID="Source" datatype="long" name="Source" ucd="meta.id;meta.main">
...        <DESCRIPTION>Unique source identifier (unique within a particular Data Release) (source_id)</DESCRIPTION></FIELD>
...      <FIELD ID="RA_ICRS" datatype="double" name="RA_ICRS" ref="coosys_c1356galcand" ucd="pos.eq.ra;meta.main" unit="deg">
...        <DESCRIPTION>Right ascension (ICRS) at Ep=2016.0 (ra)</DESCRIPTION></FIELD>
...      <FIELD ID="DE_ICRS" datatype="double" name="DE_ICRS" ref="coosys_c1356galcand" ucd="pos.eq.dec;meta.main" unit="deg">
...        <DESCRIPTION>Declination (ICRS) at Ep=2016.0 (dec)</DESCRIPTION></FIELD>
...        <DATA><TABLEDATA>
...        <TR><TD>3680787975936</TD><TD>45.01912373733</TD><TD>0.16323692972</TD></TR>
...        <TR><TD>5570573422080</TD><TD>44.85334508786</TD><TD>0.12810861107</TD></TR>
...        <TR><TD>7941395337344</TD><TD>44.97411533912</TD><TD>0.22751845955</TD></TR>
...        </TABLEDATA></DATA>
...      </TABLE></RESOURCE></VOTABLE>""")))
>>> table = votable.get_first_table()
>>> for field in votable.iter_fields_and_params():
...     print(field)
<FIELD ID="Source" datatype="long" name="Source" ucd="meta.id;meta.main"/>
<FIELD ID="RA_ICRS" datatype="double" name="RA_ICRS" ref="coosys_c1356galcand" ucd="pos.eq.ra;meta.main" unit="deg"/>
<FIELD ID="DE_ICRS" datatype="double" name="DE_ICRS" ref="coosys_c1356galcand" ucd="pos.eq.dec;meta.main" unit="deg"/>

We can now generate a SkyCoord object from the VOTable information:

>>> from astropy.coordinates import SkyCoord
>>> SkyCoord(table.array.data["RA_ICRS"],
...          table.array.data["DE_ICRS"],
...          frame=votable.get_coosys_by_id(
...                         table.get_field_by_id("RA_ICRS").ref
...                         ).to_astropy_frame(),
...          unit="deg")
<SkyCoord (ICRS): (ra, dec) in deg
   [(45.01912374, 0.16323693), (44.85334509, 0.12810861),
    (44.97411534, 0.22751846)]>