Ephemeris Objects

It is possible to obtain the ephemeris used during the observation integrated in an ALMA raw dataset or Science Data Model (SDM). Upon import to MS format, the ephemeris is automatically attached to the corresponding field(s) and used whenever the direction of the object is needed. For the spectral frame transformation, however, the user still needs to run cvel or, as a faster alternative, the new task mstransform.

Since ALMA Cycle 3, the ALMA observatory will include in each SDM all necessary ephemerides in the so-called Ephemeris table, an XML file inside the SDM. Each ephemeris in the table has a separate ID. The task importasdm will translate each of these ephemerides into a separate CASA ephemeris table which has the same format as those used by the task setjy. Examples can be found in the subdirectory "data/ephemerides/JPL-Horizons" in each CASA distribution. The task importasdm will then automatically attach these CASA ephemeris tables to the fields they belong to such that they are ready for use when importasdm is completed.

The ephemerides used by ALMA are originally for the observer location of ALMA. They use the ICRS coordinate reference frame and typically have a time spacing of a few tens of minutes. For the later transformation of the spectral reference frame, however, a geocentric ephemeris is preferable and presently required. The importasdm task will therefore by default also perform a conversion from the ALMA observer location to the geocenter. This is controlled by the importasdm parameter convert_ephem2geo which is True by default.

The spectral reference frame of the visibilities in the SDM is always topocentric (TOPO), the observer reference frame. For the observation of lines in the rest frame of the ephemeris object, it is necessary to transform the visibilities to that frame. This software Doppler tracking can be achieved with either the traditional task cvel or its faster implementation cvel2 which uses internally the same code as the task mstransform. All three tasks should produce the same result.

The user must set the outframe parameter of cvel, cvel2, or mstransform to "SOURCE". This will lead to a transformation from the TOPO to the GEO reference frame followed by an additional time-dependent Doppler correction according to the radial velocity of the object as given by its ephemeris.

When an ephemeris is attached to a field of the MS FIELD table, the object position is no longer taken from the direction columns of the FIELD table but linearly interpolated from the ephemeris table for the given time. The nominal entry of the direction column then changes its meaning to an angular offset with respect to the ephemeris. Thus, if the object is exactly at the position described by the ephemeris, the nominal entries of the direction column are zero for right ascension and declination. If, e.g. in the case of a mosaic, there are a number of fields with nearby positions, the fields can share an ephemeris; they all reference the same ephemeris table via the optional column EPHEMERIS_ID, but have in their direction column entries different offsets.

Because the user can not expect that the nominal field direction entries are the actual object position, it is necessary to obtain the field direction with special tool method

or the more general

(see the built-in CASA help for details). The default time of the position is taken from the TIME column of the FIELD table.

In summary, with ephemerides included in the ALMA raw data and the added support for this in the importasdm task, the user does not have to worry about how to obtain the right ephemeris in the right format and how to attach it properly to the MS. This process is transparent and only a few logger messages indicate that it is happening. The correct time-dependent positions, radial velocities, and object distances are used in all relevant tasks such as listobs, plotms, and, as described above, cvel, cvel2, and mstransform. For Solar-System object flux calibrators, the task setjy will, however, only extract the nominal position from the SDM ephemeris and otherwise use its internal set of ephemerides since these contain additionally needed object parameters. Care has to be taken when trying to extract the field positions from the FIELD table as the nominal direction column entries will only be _offsets_ (w.r.t. the ephemeris position) when an ephemeris is attached.

As opposed to ALMA data which uses a tabulated representation of the ephemerides, VLA data use a polynomial representation of the positions and radial velocities. Also this representation is supported. The polynomial ephemeris is internally tabulated with a default time step of 0.001 days and then processed as in the ALMA case. The parameter polyephem_tabtimestep can be used to control the step size of the tabulation.