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.
Total time
on source
  Split the time into this number of snapshots
(hrs)   1 2 3 4 8
20 seconds Dynamic range 120 - - - -
1 minute Dynamic range 178 - - - -
0.25 Dynamic range 410 - - - -
0.5 Dynamic range 500 930 1130 1150 -
Elevation coverage -
UV coverage -
Image -
1 Dynamic range 820 - 1350 - 2000
Elevation coverage - -
UV coverage - -
Image - -
3 Dynamic range 2670 - 2800 -
Elevation coverage - -
UV coverage - -
Image - -
8 Dynamic range 5060 - - -
Elevation coverage - - -
UV coverage - - -
Image - - -

  • 480 minutes
    • Full 8-hour track (480min) cleaned down to 1mJy
      • amp vs. time (ant#0 baselines only), image
      • rms of 0.164 mJy/beam: Max = 0.83, dynamic range = 5061, rms is 5 times worse than without the 3 bright sources
      • with 10% pwv phase noise: rms = 1.44 mJy/beam, but Max dropped to 0.097
      • with 5 um rms phase noise: rms = 0.268 mJy/beam, Max is 0.83Jy
  • 180 minutes (rms would be 0.27 scaling the 8-hour track)
    • One x 180 minutes cleaned down to 1.3mJy
      • rms of 0.31mJy/beam, Max = 0.823, dynamic range = 2670
    • Three x 60 minutes spread over 6.5 hours
      • rms of 0.30mJy/beam, Max = 0.830, dynamic range = 2800
  • 64 minutes (rms would be 0.45mJy scaling the 8-hour track)
    • Eight x 8-minute snapshots (64min) spread over 8 hours
      • amp vs. time, image
      • image rms of 0.41 mJy/beam, Max = 0.83, dynamic range = 2024, rms is 4.4 times worse than without the 3 bright sources
      • 5um phase noise gives 0.77 mJy/beam
    • Three x 21-minute snapshots spread over 7 hours
      • image rms of 0.62 mJy/beam, Max = 0.83, dynamic range = 1351
    • One 64min observation at transit
      • amp vs. time
      • image rms = 1.0mJy/beam, Max = 0.82Jy, dynamic range = 820, rms is 1.39x worse than splitting into 8 chunks
  • 32 minutes (rms would be 0.64 scaling the 8-hour track)
    • Four x 8-minute snapshots (32min) spread over 6 hours
      • amp vs. time
      • rms = 0.72 mJy/beam, Max = 0.83 Jy, dynamic range = 1153, rms is 5.5 times worse than without the 3 bright sources
      • with 5um rms phase, new rms = 1.18mJy/beam
    • Three x 10.67-minute snapshots spread over 4 hours
      • amp vs. time
      • rms = 0.73 mJy/beam, Max = 0.83 Jy, dynamic range = 1137
    • Two x 16-minute snapshots spread over 4 hours (with one at transit)
      • amp vs. time
      • rms = 0.89mJy/beam, Max = 0.83, dynamic range = 932
    • One 32 minute observation near transit
      • amp vs. time
      • rms = 1.59 mJy/beam, Max = 0.82 Jy, dynamic range = 516
      • rms is 2.21x worse than splitting into 4 chunks, 2.18x worse than splitting into 3 chunks, 1.94x worse than splitting into 2 chunks
  • 15 minutes (rms would be 0.93 scaling the 8-hour track)
    • One 15 minute observation near transit
      • rms = 2.0 mJy/Beam, Max = 0.82 Jy, dynamic range = 410
  • 1 minute: rms = 4.6 (would be 3.6 scaling the 8-hour track)
  • 20 seconds: rms = 6.6 (would be 6.2 scaling the 8-hour track)

Results for thermal-noise-limited case (5-source field)

Time (hours) Sensitivity calculator
noise (mJy/beam)
Achieved noise
8 0.038 0.032
1.067 0.104 0.094
0.533 0.147 0.141

  • 480 minutes
    • Full 8-hour track cleaned to 0.1mJy (image)
      • image rms = 0.032mJy
      • Max = 9.3mJy, dynamic range = 291
  • 64 minutes
    • Eight x 8-minute snapshots spread over 8 hours
      • rms = 0.094 mJy
      • Max = 8.98 mJy, dynamic range = 96
  • 32 minutes
    • Four x 8-minute snapshots spread over 8 hours
      • rms = 0.127 mJy
      • Max = 9.02mJy, dynamic range = 69
    • Three 10.67-minute snapshots over 4 hours
      • rms = 0.135 mJy
      • max = 9.29 mJy, dynamic range = 69
    • Two 16-minute snapshots over 4 hours (with one at transit)
      • rms = 0.142
      • Max = 9.24 mJy, dynamic range = 65
    • One 32-minute observation near transit
      • rms = 0.141 mJy, which is 1.11x worse than splitting into 4 chunks, 1.044x worse than 3 chunks
      • max = 9.23 mJy, dynamic range = 65

-- ToddHunter - 2011-07-06
Topic attachments
I Attachment Action Size Date WhoSorted ascending Comment
ngc6334i_345.elev.pngpng ngc6334i_345.elev.png manage 17 K 2011-07-14 - 16:55 ToddHunter  
ngc6334i_345.noisy.manualclean.pngpng ngc6334i_345.noisy.manualclean.png manage 22 K 2011-07-14 - 17:28 ToddHunter  
ngc6334i_345.uvcov.pngpng ngc6334i_345.uvcov.png manage 56 K 2011-07-15 - 15:14 ToddHunter  
ngc6334i_345_2017cycle5.3.noisy.manualclean.image.imview.pngpng ngc6334i_345_2017cycle5.3.noisy.manualclean.image.imview.png manage 35 K 2017-05-04 - 10:44 ToddHunter  
ngc6334i_345_8hours.pngpng ngc6334i_345_8hours.png manage 20 K 2011-07-06 - 16:30 ToddHunter  
ngc6334i_345_amp_vs_time.pngpng ngc6334i_345_amp_vs_time.png manage 56 K 2011-07-06 - 16:30 ToddHunter  
ngc6334i_345_clean_2017version.py.txttxt ngc6334i_345_clean_2017version.py.txt manage 22 K 2017-05-03 - 16:52 ToddHunter partially revised script to generate Cycle 5 config 3 result
ngc6334i_345_elev_vs_time.pngpng ngc6334i_345_elev_vs_time.png manage 17 K 2011-07-06 - 18:52 ToddHunter  
ngc6334i_345_faint5.pngpng ngc6334i_345_faint5.png manage 21 K 2011-07-06 - 19:39 ToddHunter  
ngc6334i_345_flag.elev.pngpng ngc6334i_345_flag.elev.png manage 15 K 2011-07-14 - 16:42 ToddHunter  
ngc6334i_345_flag.noisy.manualclean.pngpng ngc6334i_345_flag.noisy.manualclean.png manage 25 K 2011-07-14 - 17:28 ToddHunter  
ngc6334i_345_flag.uvcov.pngpng ngc6334i_345_flag.uvcov.png manage 37 K 2011-07-15 - 14:50 ToddHunter  
ngc6334i_345_flag2.elev.pngpng ngc6334i_345_flag2.elev.png manage 15 K 2011-07-14 - 16:42 ToddHunter  
ngc6334i_345_flag2.noisy.manualclean.pngpng ngc6334i_345_flag2.noisy.manualclean.png manage 27 K 2011-07-14 - 17:30 ToddHunter  
ngc6334i_345_flag2.uvcov.pngpng ngc6334i_345_flag2.uvcov.png manage 28 K 2011-07-15 - 14:50 ToddHunter  
ngc6334i_345_flag2_amp_vs_time.pngpng ngc6334i_345_flag2_amp_vs_time.png manage 22 K 2011-07-06 - 18:27 ToddHunter  
ngc6334i_345_flag3.elev.pngpng ngc6334i_345_flag3.elev.png manage 15 K 2011-07-14 - 16:42 ToddHunter  
ngc6334i_345_flag3.noisy.manualclean.pngpng ngc6334i_345_flag3.noisy.manualclean.png manage 28 K 2011-07-14 - 17:30 ToddHunter  
ngc6334i_345_flag3.uvcov.pngpng ngc6334i_345_flag3.uvcov.png manage 22 K 2011-07-15 - 14:50 ToddHunter  
ngc6334i_345_flag3_amp_vs_time.pngpng ngc6334i_345_flag3_amp_vs_time.png manage 25 K 2011-07-06 - 18:20 ToddHunter  
ngc6334i_345_flag4.elev.pngpng ngc6334i_345_flag4.elev.png manage 15 K 2011-07-14 - 16:43 ToddHunter  
ngc6334i_345_flag4.noisy.manualclean.pngpng ngc6334i_345_flag4.noisy.manualclean.png manage 29 K 2011-07-14 - 17:30 ToddHunter  
ngc6334i_345_flag4.uvcov.pngpng ngc6334i_345_flag4.uvcov.png manage 22 K 2011-07-15 - 14:50 ToddHunter  
ngc6334i_345_flag4_amp_vs_time.pngpng ngc6334i_345_flag4_amp_vs_time.png manage 21 K 2011-07-06 - 18:25 ToddHunter  
ngc6334i_345_flag5.elev.pngpng ngc6334i_345_flag5.elev.png manage 15 K 2011-07-14 - 16:43 ToddHunter  
ngc6334i_345_flag5.noisy.manualclean.pngpng ngc6334i_345_flag5.noisy.manualclean.png manage 27 K 2011-07-14 - 17:30 ToddHunter  
ngc6334i_345_flag5.uvcov.pngpng ngc6334i_345_flag5.uvcov.png manage 25 K 2011-07-15 - 14:50 ToddHunter  
ngc6334i_345_flag5_amp_vs_time.pngpng ngc6334i_345_flag5_amp_vs_time.png manage 21 K 2011-07-06 - 18:40 ToddHunter  
ngc6334i_345_flag6.elev.pngpng ngc6334i_345_flag6.elev.png manage 15 K 2011-07-14 - 16:43 ToddHunter  
ngc6334i_345_flag6.noisy.manualclean.pngpng ngc6334i_345_flag6.noisy.manualclean.png manage 27 K 2011-07-14 - 17:29 ToddHunter  
ngc6334i_345_flag6.uvcov.pngpng ngc6334i_345_flag6.uvcov.png manage 26 K 2011-07-15 - 14:49 ToddHunter  
ngc6334i_345_flag6_amp_vs_time.pngpng ngc6334i_345_flag6_amp_vs_time.png manage 21 K 2011-07-06 - 18:55 ToddHunter  
ngc6334i_345_flag7.elev.pngpng ngc6334i_345_flag7.elev.png manage 15 K 2011-07-14 - 16:44 ToddHunter  
ngc6334i_345_flag7.noisy.manualclean.pngpng ngc6334i_345_flag7.noisy.manualclean.png manage 27 K 2011-07-14 - 17:29 ToddHunter  
ngc6334i_345_flag7.uvcov.pngpng ngc6334i_345_flag7.uvcov.png manage 31 K 2011-07-15 - 14:49 ToddHunter  
ngc6334i_345_flag8.elev.pngpng ngc6334i_345_flag8.elev.png manage 15 K 2011-07-14 - 16:44 ToddHunter  
ngc6334i_345_flag8.noisy.manualclean.pngpng ngc6334i_345_flag8.noisy.manualclean.png manage 24 K 2011-07-14 - 17:29 ToddHunter  
ngc6334i_345_flag8.uvcov.pngpng ngc6334i_345_flag8.uvcov.png manage 39 K 2011-07-15 - 14:49 ToddHunter  
ngc6334i_345_flag9.elev.pngpng ngc6334i_345_flag9.elev.png manage 15 K 2011-07-14 - 16:44 ToddHunter  
ngc6334i_345_flag9.noisy.manualclean.pngpng ngc6334i_345_flag9.noisy.manualclean.png manage 24 K 2011-07-14 - 17:29 ToddHunter  
ngc6334i_345_flag9.uvcov.pngpng ngc6334i_345_flag9.uvcov.png manage 41 K 2011-07-15 - 14:49 ToddHunter  
ngc6334i_345_flag_amp_vs_time.pngpng ngc6334i_345_flag_amp_vs_time.png manage 27 K 2011-07-06 - 16:30 ToddHunter  
Topic revision: r16 - 2017-05-04, ToddHunter
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