Simulating ALMA observations with simalma

How to use simalma to simulate ALMA observations

Summary

The task simalma simulates an ALMA observation by ALMA 12-m, ACA-7m and total power arrays. It takes an input model image or a list of components, plus configurations of ALMA antennas (locations and sizes), and simulates a particular ALMA observation (specified by mosaic setup and observing cycles and times). The outputs are MeasurementSets. The task optionally generates images from the MeasurementSets.

Technically speaking, simalma internally calls simobserve and simanalyze as many times as necessary to simulate and analyze an ALMA observation. Some of the simulation and imaging parameters are automatically set to values typical of ALMA observations. Thus, it has a simpler task interface compared to simobserve plus simanalyze at the cost of limited flexibility. If the user wants to have more control of simulation setup, it is available by manually running simobserve and simanalyze multiple times or by using the simulator (sm) tool.

WARNING: The task simalma is designed to only be invoked once for a simulation setup. It always sets up a skymodel and pointings. That means that simalma is not supposed to be run multiple times for a project, unlike simobserve and simanalyze. The task simalma may ignore or overwrite the old results when it is run more than once with the same project name.

There are options in simalma to simulate observation of ACA 7-m and total power arrays, to apply thermal noise, and/or to generate images from simulated MeasurementSets. One inputs a vector of configurations, and a corresponding vector of total times to observe each component. Thermal noise is added to visibilities when pwv > 0 . The ATM atmospheric model is constructed from the characteristics of the ALMA site and a user defined Precipitable Water Vapour (pwv) value. Set pwv = 0 to omit the thermal noise. Finally, when image = True, synthesized images are generated from the simulated MeasurementSets.

 

Antenna Configuration

The configurations of the ALMA 12-m and 7-m arrays are defined by the antennalist parameter, which can be a vector. Each element of the vector can be either the name of an antenna configuration file or a desired resolution, e.g., ‘alma;cycle1;5arcsec’. Some examples:

  • antennalist = [’alma.cycle2.5.cfg’,’aca.cycle2.i.cfg’]; totaltime = [’20min’,’2h’]’: Will observe the 12-m array in configuration C32-5 for 20 minutes and the ACA 7-m array for 2 hours.
  • antennalist = [’alma;cycle2;0.5arcsec’,’aca.i.cfg’]; totaltime = [’20min’,’2h’]’: Will observe the 12-m array in whatever Cycle 2 configuration yields a zenith synthesized beam as close as possible to 0.5 arcsec (at the center frequency of your skymodel) for 20 minutes and the ACA 7-m array for 2 hours.   
  • antennalist = [’alma.cycle1.2.cfg’,’aca.cycle2.i.cfg’]; totaltime = ’20min’: Will observe the 12-m array in Cycle 1 configuration 2 for 20 minutes and the ACA 7-m array for the default of 2×(12-m time) = 1h20min. This parameter setting will also generate a warning that the user is combining configurations from different ALMA Cycles (but the simulation will run despite that).

Total power can either be included along with interferometric configurations e.g., antennalist = [’alma.cycle1.2.cfg’,’aca.cycle2.i.cfg’,’aca.tp.cfg’], or by using the tpnant and tptime parameters. The latter is preferred since it allows greater control (in particular the number of total power antennas to use – if more than one is used, multiple total power observations will be generated and combined in imaging).

Field Setup

There are two ways to setup pointings, i.e., Rectangle Setup and Multi-Pointing.

In the Rectangle Setups, pointings are automatically calculated from the pointing center (direction) and the map size. A rectangular map region is covered by a hexagonal grid (maptype = ‘alma’) with Nyquist sampling, i.e., 0.48 primary beam (PB) spacing (where PB ≡ 1.2 λ / D), in both ALMA 12-m and ACA 7-m array simulations. A slightly larger area is mapped in ACA total power simulations for later combination with interferometer visibilities. The map area is extended by 1 PB in each direction and covered by a lattice grid with 0.225 PB spacing.

In Multi-Pointing, a list of pointings is defined in the direction parameter or read from a file (when setpointings = False; note that simobserve can read ALMA OT pointing files in the old and new format but the latter only when they are saved as sexagesimal absolute positions). The ALMA 12-m and ACA 7-m arrays observe the specified directions. The ACA total power simulations map either (1) square regions of 2 PB extent centered at each of the pointings, or (2) a rectangle region that covers all the pointings. Either (1) or (2), whichever can be done with the smaller number of points, is selected. The pointing spacing in total power simulations is, again, 0.225 PB in lattice grids.

It is advisable that for Total Power Simulations, the field is chosen sufficiently large, maybe padding at least 1-2 primary beams on each side.

Integration time

The total observation time of each component or configuration is defined by the totaltime parameter as noted above. A scalar will trigger use of the Cycle 2 default time multipliers, 1:0.5:2:4 for the first 12-m configuration, any additional 12-m configurations, any 7-m configuration, and any total power observation.

In general, the integration time (dump interval) of simulations is defined by the integration parameter with an exception. Since the ACA total power array always observes larger areas compared to the ALMA 12-m and ACA 7-m arrays, it is possible that the ACA total power array cannot cover all pointings in the given observation time. In such a case, the integration time in the total power simulation is scaled so that the all pointings are observed at least once in its observation time, i.e., integration_TP = tptime / (the number of total power pointings).

Imaging and combination of ALMA with ACA

The CLEAN algorithm is used in simalma to generate images from visibilities. The visibilities are weighted to UV-plane using Briggs weighting.

When ACA observations are simulated, visibilities of ACA 7-m are weighted by the relative sensitivities to ALMA 12-m visibilities, and both data sets are concatenated before imaging. The relative weight of ACA 7-m visibilities is defined in proportion to the ratio of beam areas squared, i.e., $(7/12)^{4} = 0.11$. This is because simalma uses a bandwidth and an integration time common to both ALMA 12-m and ACA 7-m simulations.

The interferometer and total power images are combined using feather task when total power observations are included. The total power image is scaled by the interferometer primary beam coverage before combination. The final image product is the combined image corrected for the interferometer primary beam coverage. The output image of the feather task is divided by the interferometer primary beam coverage in the final step.