Simulations of multiple snapshot performance for ALMA
This page was originally created for ALMA Early Science Cycle 0 (16 antennas), and all of the results below the Goals section are for that configuration.
However, please note that the CASA simulator does not account for any sources of dynamic range limitations, except for uv coverage and integration time.
Real results will be worse than these theoretical performance limits. To get an idea of the dynamic range limit for an ALMA snapshot with 31 antennas, see CSV-553 and the 05-Mar-2014 report which achieves ~5600 in Band 3 on a 5.62 Jy quasar (J0522-364) with 2.5 minutes on source.
A recent paper in ApJL demonstrates a dynamic range of order 23000 in Band 6 on a 0.5 Jy grid-source quasar (J0635-7516).
To compare this to the theoretical limit, I re-ran the simulation of an 8-hour track using Cycle 5 configuration 3 (with 43 antennas) with the same simulated field
as below. The resulting image reaches a dynamic range of 45000.
Goals
Explore the performance of imaging quality of an ALMA Cycle 0 configuration that can be obtained by combining several short snapshots at different hour angles.
Run a case where the image is dynamic range limited, and one where it is thermal noise limited.
Simulated field
Start with Band 7 (345 GHz), Cycle 0 ES-extended configuration, Dec = -35 deg
Create a field with 3 bright sources (1, 0.5, 0.25 Jy), and five fainter ones: 10, 5, 2.5, 1 mJy, and a second 0.5 mJy adjacent to the brightest one
Create a second field without the 3 bright sources
Simulate both with simdata for an 8 hour track, and flag portions of the data corresponding to different numbers of snapshots
Include atmospheric noise but not phase noise (since with bright sources, self-cal should remove most of the phase noise)
Clean interactively like an observer would do, with appropriate small boxes
Compute the dynamic range as the ratio between max peak flux density of brightest source divided by image rms (computed over a source-free region that represents a significant fraction of the area within the half-power beam diameter)
Results for dynamic-range limited case (8-source field)
Note: These values are computed without phase noise. Running the same simulations with a realistic level of residual phase noise (about 5um rms) results in a factor of 2 decrease in all the dynamic ranges.