Multi-MS simulations

Simulations that include many frequency sub-bands can become very memory heavy when a single concatenated measurement set is used as input. This makes it appealing to run the simulation on multiple smaller measurement sets, containing only a single (or a few) sub-bands.

Of course, it has to be taken care that the appropriate systematics are applied for each frequency channel. This can be achieved by using the fact that most of the corrupting effects in LoSiTo are are stored in a h5parm - file without a frequency axis. So it is possible to create one h5parm - file that includes all these effects (currently all steps but the BEAM operation, BANDPASS operation and NOISE operation) for one of the measurement sets. Then, for each of the input measurement sets, the corruptions in this h5parm - file are added to the simulation with the USE_H5 operation step, and subsequently, a PREDICT operation step is carried out. In this LoSiTo - run, also the BEAM operation, BANDPASS operation and NOISE operation may be included.

For example, suppose that you want to simulate a LOFAR observation consisting of 244 sub-bands, each stored in a single measurement set. The input measurement sets may be called sim_SB150.MS up to sim_SB393.MS. The following LoSiTo parset generates the one h5parm - file containing the corruptions which can be stored in a frequency - independent way. It is sufficient to run LoSiTo on only one of the input measurement sets, e.g.: losito  multims_run1.parset

#LoSiTo parset multims_run1.parset
msin = sim_SB150.MS
skymodel = example.sky
############## CREATE H5PARM FILE ####################
# Add TEC values from von Karman tecscreen
[tec]
operation = TEC
method = turbulence
h5parmFilename = corruptions.h5

# Compute faraday rotation
[rm]
operation = FARADAY
h5parmFilename = corruptions.h5

# Clock delay
[clock]
operation = CLOCK
h5parmFilename = corruptions.h5

# Polarization misalignment
[polmisalign]
operation = POLMISALIGN
h5parmFilename = corruptions.h5

Subsequently, LoSiTo can be run on every single input measurement set, including the effects stored in corruptions.h5 to the simulation using the LoSiTo parset below. Glob-style expressions are supported for the input ms name.

#LoSiTo parset multims_run2.parset
msin = sim_SB*.MS
skymodel = example.sky
############ APPLY CORRUPTIONS ########################
# Add the corruptions stored in the h5parm file:
[tec]
operation = USE_H5
corruption = tec
h5parmFilename = corruptions.h5

[rotationmeasure]
operation = USE_H5
corruption = rm
h5parmFilename = corruptions.h5

[polmisalign]
operation = USE_H5
corruption = polmisalign
h5parmFilename = corruptions.h5

[clock]
operation = USE_H5
corruption = clock
h5parmFilename = corruptions.h5

# Add the Beam
[beam]
operation = beam
mode = default

# Do the predict
[predict]
operation = PREDICT
outputColumn = DATA
predictType = h5parmpredict

# Add bandpass to predicted visibilities
[bandpass]
operation = BANDPASS
outputColumn = DATA
method = ms

# Add noise to the predicted visibilities
[noise]
operation = NOISE
outputColumn = DATA