Manipulation of Ephemeris Objects

How to work with ephemeris objects in CASA

When an astronomical object has a proper motion, in particular objects in our solar system, a static (RA,Dec) position in the FIELD table of the MeasurementSet will not accurately describe the time-dependent position. Prior to CASA 4.2, there was no support for ephemeris objects other than the built-in reference frames for the Sun, the Moon, and the planets out to PLUTO. With CASA 4.2, several new features were introduced which help the user to attach an arbitrary ephemeris to a given field and work with the object from calibration to imaging. These can be used when no ephemeris table was provided by the observatory, or in cases where the use of an improved ephemeris table is necessary.

Ephemeris tables

The CASA format for ephemeris tables was introduced in the early development stages of CASA in connection with the Measures module. The me tool (see CASA Tools on using the tool kit, as well as the inline help on the me tool inside CASA for specific usage) permits position calculations based on ephemerides in this format. Two examples for such tables can be found in the distribution directory in subdirectory data/ephemerides: VGEO is an ephemeris of Venus in the geocentric reference frame while VTOP is an ephemeris for the same object in the TOPO reference fame for the observatory location of the VLA. With the introduction of solar system source models (Butler) in the setjy task, a nearly complete set of ephemerides for the larger bodies in our solar system had to be made available. These are stored in nearly the same format as the above examples VGEO and VTOP (but with a few enhancements) in directory data/ephemerides/JPL-Horizons. If your object’s ephemeris is among those stored in data/ephemerides/JPL-Horizons, you can simply copy the ephemeris from there. Otherwise, you can request the ephemeris from the JPL-Horizons using the CASA commands (for example):

import recipes.ephemerides.request as jplreq
jplreq.request_from_JPL(objnam='Mars',startdate='2012-01-01',enddate='2013-12-31',date_incr='0.1 d', get_axis_orientation=False,
get_sub_long=True, use_apparent=False, get_sep=False,

where you need to fill in the parameters objnam, startdate, enddate,date_incr (the time interval between individual ephemeris table entries), return_address (your email address where you want to receive the ephemeris), and mailserver (the smtp server through which you want to send the request email). The other parameters should be set as shown. Within a short time, you should receive the requested ephemeris as an email from NASA’s JPL-Horizonssystem. Save the email into a file with the “save as” function of your mail client. See the next section on how to attach it to your dataset.

Using fixplanets to attach ephemerides to a field of a MeasurementSet

As of CASA 4.6.0, importasdm fills the SOURCE coodinates with the correct postions based on the ephemerides table. Alternatively, one can use the task fixplanets to set the ephemeris of a given field in a MeasurementSet. Here is an example:

field='Titan', fixuvw=True, direction='mytitanephemeris')

where you need to set the parameters vis to the name of your MS and the parameter field to the name of the field to which you want to attach the ephemeris. The parameter direction must be set to the name of your ephemeris table. Accepted formats are (a) the CASA format (as in VGEO or the ephemerides in data/ephemerides/JPL-Horizons as described above) and (b) the JPL-Horizons mail format which you obtain by saving an ephemeris email you received from JPL-Horizons. The parameter fixuvw should be set to True in order to trigger a recalculation of the UVW coordinates in your MS based on the new ephemeris. The task fixplanets can also be used for other field direction modifications. Please refer to the help text of the task.

Note that among the ephemerides in the directory data /ephemerides/JPL-Horizons/ you should only use those ending in ’’. They are the ones in J2000 coordinates.

Use of the ephemeris after attachment

Once you have attached the ephemeris to a field of an MS, it will automatically be handled in tasks like split and concat which need to hand on the ephemeris to their output MSs. In particular concat recognizes when fields of the MSs to be concatenated use the same ephemeris and merges these fields if the time coverage of the provided ephemeris in the first MS also covers the observation time of the second MS. The ephemeris of the field in the first MS will then be used for the merged field. In order to inspect the ephemeris attached to a field, you can find it inside the FIELD subdirectory of your MS. The optional column EPHEMERIS_ID in the FIELD table points to the running number of the ephemeris table. A value of −1 indicates that no ephemeris is attached. Note that in case an ephemeris is attached to a field, the direction column entries for that field in the FIELD table will be interpreted as an offset to the ephemeris direction and are therefore set to (0.,0.) by default. This offset feature is used in mosaic observations where several fields share the same ephemeris with different offsets. The time column in the FIELD table should be set to the beginning of the observation for that field and serves as the nominal time for ephemeris queries.

Spectral frame transformation to the rest frame of the ephemeris object in task cvel

The ephemerides contain radial velocity information. The task cvel can be used to transform spectral windows into the rest frame of the ephemeris object by setting the parameter outframe to “SOURCE” as in the following example:

outputvis='', outframe='SOURCE', mode = 'velocity',
width = '0.3km/s', restfreq = '354.50547GHz')

This will make cvel perform a transformation to the GEO reference frame followed by an additional Doppler correction for the radial velocity given by the ephemeris for the each field. (Typically, this should happen after calibration and after splitting out the spectral widows and the target of interest). The result is an MS with a single combined spectral window in reference frame REST. From this frame, further transformations to other reference frames are not possible.

Ephemerides in ALMA datasets

The ALMA Science Data Model (the raw data format for ALMA data) now foresees an Ephemeris table. This feature has been in use since the beginning of ALMA Cycle 3 both for science targets and calibrator objects. With ALMA Cycle 3 (or later) datasets, the task importasdm will automatically translate the contents of the ASDM Ephemeris table into separate ephemeris tables in CASA format and attach them to the respective fields.

In the case of mosaics, all fields belonging to a mosaic on an ephemeris object will share the same ephemeris. The individual mosaic pointings will use the offset mechanism described above to define the position of each field.

Imaging ALMA observations with tclean

As of CASA 5.3, the tclean task can automatically handle the imaging of ALMA observations (both single-execution and multi-execution datasets, and both single-field and mosaics) by using the new phasecenter='TRACKFIELD' option. This option will use the ephemeris tables attached to each measurementSet by the ALMA control system. These tables will have ultimately been provided by the observatory for the case of large bodies (those selectable in the ALMA Observing Tool), or by the PI as attachments in the Observing Tool for the case of smaller bodies.

WARNING:  if you want to use the old method of concatenating calibrated MeasurementSets by using the forcesingleephemfield parameter to create a common joint ephemeris table, then you must still set phasecenter='TRACKFIELD' in tclean order to get a sensible image, even though you are passing it only one (concatenated) MeasurementSet. If not, you may get a corrupt image, even if you select a subset of data only from the first execution block in the concatenated MS.

Imaging observations from other telescopes with tclean

For non-ALMA data, or to use a newer ephemeris than was available at the time of the ALMA observations, the user may set the phasecenter parameter to the name of an ephemeris file, constructed as described in the earlier section above. Alternatively, the user may set the phasecenter to the common name of the following bodies: 'MERCURY', 'VENUS', 'MARS', 'JUPITER', 'SATURN', 'URANUS', 'NEPTUNE', 'PLUTO', 'SUN', 'MOON', in which case the standard DE200 ephemeris table distributed with CASA will be used.