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Converts a direction from ICRS to J2000 using the CASA measures tool and computes the angular separation. See also J2000ToICRS. As discussed in CAS-7885, the optically-defined FK5 frame differs from ICRS by a maximum of 31.57 mas, while the CASA J2000 frame differs from ICRS by 23.15 mas. Note that J2000 is also used as a name to label the epoch that corresponds to 2000-Jan-01 at 12h UT. In ALMA Cycles 0-3, the antennas point in the FK5 frame, while the delays are pointed in the ICRS frame. In Cycle 4, the antennas will also point in the ICRS frame.


au.ICRSToJ2000(radec, date='', verbose=True)
  • radec: sexagesimal string format
  • date: optional -- CASA shows no date dependence in this calculation! Any of these formats is valid (The time portion is optional):
    • 2011/10/15 05:00:00
    • 2011/10/15-05:00:00
    • 2011-10-15 05:00:00
    • 2011-10-15T05:00:00
    • 2011-Oct-15T05:00:00


CASA <10>: au.ICRSToJ2000('18:37:24.5088 -33:19:08.760')
Separation: radian = 2.33282e-08, degrees = 0.000001, arcsec = 0.004812
  Out[10]: '18:37:24.50901, -033:19:08.755972'
-- ToddHunter - 2015-09-24
Topic revision: r3 - 2016-03-08, ToddHunter
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