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# 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)]>