Cooperate元素#

的 COOSYS 元素允许在 VOTABLE 和 RESOURCE 元素它描述了一个坐标系并具有以下属性：

ID ：一个标签，
system ：坐标的参考系。自VOTable 1.5版本以来，它遵循 IVOA reference frame vocabulary .
equinox ：修复赤道或黄道系统
epoch ：位置的时代，如朱利安或贝塞尔时代
refposition ：它自VOTable 1.5版本以来就存在，遵循 IVOA reference positions vocabulary .

它们都是可选的。的 CooSys 类表示一个 COOSYS 元素它可以从头开始创建：：

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

并转换成占星内置框架：：

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

使用Cooperate元素#

的 COOSYS 元素通常用于描述 FIELD 元素在这种情况下， FIELD 将有一个 ref 与 COOSYS '的' ID '字段。例如，请参阅此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"/>

我们现在可以生成一个 SkyCoord 来自VOTable信息的对象：：

>>> 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)]>