algorithm development · algorithm development saspo meeting, charlottesville, nov. 9th 2010 s....

Algorithm Development

SASPO Meeting, Charlottesville, Nov. 9th 2010

S. BhatnagarNRAO

2/21S. Bhatnagar: SASPO Meeting, CV, Nov. 9th 2010

Overview

• Issues requiring new algorithms– Wide-band imaging + off-axis effects

– Wide-band polarization calibration

– HPC, Pipeline processing, Automatic RFI removal(EVLA Memo #139 & #122)

• Progress so far• Various algorithmic dependencies

• Staged deployment recognizing the available resources

– Connection between algorithms R&D and HPC

– Issues and challenges: Research, required software development, etc.


Algorithms R&D Plan

• Guiding principles for algorithms R&D plan 1. Enable new scientific capabilities as soon as possible

(with the given resources)

2. Recognize algorithmic dependencies

3. Develop an integrated plan for staged algorithm R&D, HPC

4. Develop algorithms that can utilize HPC

● Work closely with the HPC development group and plan


Imaging issues

• Requirements– Full-beam, wide-band, full-Stokes imaging

– Mosaicking (including heterogeneous arrays)

• Algorithms R&D required:– Wide-band issues (2:1 Bandwidth ratio)

– Account for frequency dependence of the sky-emission (MFS)

– Polarization dependence of the sky-emission (RM-Synthesis)

– Wide-field issues– Time varying Primary Beam effects

– Frequency and polarization dependence of the PB

– Full-field, wide-band issues– Integration of the various new algorithms

– Mosaicking


Calibration issues

• On-axis (Direction independent)– Wide-band bandpass calibration

– Time varying component

– Frequency dependent polarization – Time varying component

• Off-axis (Direction dependent)– Frequency dependence of the PB

– Time dependence due PB rotation/shape

– Ionosphere (L/S band), atmosphere (ALMA)

– Beam polarization

• RFI– Auto-/semi-automatic flagging algorithms

– High dynamic range imaging: Removal of weak RFI


Time varying DD gains due to PB

1.0 GHz

1.5 GHz

2.0 GHz

+

Continuum wide-bandPower pattern


Wide-band implies Wide-field imaging

● EVLA @L-Band● BW=600 MHz (1.2 – 1.8 GHz)

● Algorithmic Challenge: - Time-varying direction-dependent gains

- Wide-band effects

- Extended emission with superimposed compact emission

- Full Stokes + Mosaicking


Wide-band Imaging

● 3C286, BW=1.0-2.1 GHz● No wide-band modeling of the sky emission

● DR: 1600

● 3C286, BW=1.0-2.1 GHz● With MS-MFS (freq. Dependent model for the sky emission)

● DR: >110,000


Wide-band Imaging

● EVLA @ C-Band● Total BW: 2x2GHz

Stokes-I Image Spectral Index Image


Status: Imaging

• Wide-field imaging– W-Projection: In use (IEEE, 2010)

– PB Corrections» Basic algorithm: AW-Projection (Testing/A&A,2008)» Frequency dependence: Work in progress» Extending to Heterogeneous arrays (ALMA): In progress

» Inclusion of arbitrary antenna PBs: Some work required » Integration with mosaicking: Work required» Polarization dependence: Work needed

– Pointing SelfCal» R&D stage: In progress


Status: Imaging

• Wide-band imaging– MS-MFS (Rau, PhD Thesis (2010))

» No correction for the PB effects

– Scientific testing in progress

– Integration with PB-correction» Re-factoring of the code: Work in progress

● Integration of MS-MFS and A-Projection● Numerical testing● Extension to full-Stokes: R&D required

• Full-Stokes, wide-band imaging– R&D required

• Automatic RFI removal - programming work needed• Pipeline processing – work needed


● 3C147 field at L-Band

● Only 12 antennas used● Bandwidth: 128 MHz● ~7 hr. integration

● Dynamic range: ~700,000:1

● A single baseline based correction was applied

EVLA L-Band Stokes-I: Before correction


● 3C147 field at L-Band

● Only 12 antennas used● Bandwidth: 128 MHz● ~7 hr. integration

● Dynamic range: ~700,000:1

EVLA L-Band Stokes-I: After correction


EVLA L-Band Stokes-V: Before correction

Is it M s , Poln ?Or is it I s , Poln?

V ijObs

= Mij∫M ijS s I s e2 bij . sd s


EVLA L-Band Stokes-I: After correction

Use physical model for the Stokes-V pattern:

Contours: Stokes-I power patternColour: Stokes-V power pattern


3C147: Residual errors in full field

Smearing + W-Term errors!

Errors due toPointing errors?

Errors due PBside-lobes?


Pointing SelfCal: Time dependent solutions

R-beam

L-beam

arcs

ec

Time


● EVLA polarization squint solved as pointing error (optical pointing error).● Squint would be symmetric about the origin in El-Az plane in the absence of antenna servo pointing errors.

● Pointing errors for various antennas detected in the range 1-7 arcmin.● Pointing errors confirmed independently via the EVLA online system.

[paper in preparation]

DD SelfCal algorithm: EVLA Data

R-beam

L-beam

Pointing errorR-beam

L-beam

Pointing error

● El-Az mount antennas● Polarization squint due to off-axis feeds - The R- and L-beam patterns have a pointing error of +/- ~0.06

● DoF used: 2 per antenna● SNR available for more DoF to model the PB shape

D


I/O load

• Near future data volume (0-1 years)• Recent data with the EVLA: 100-500 GB

• Next 5 years• 100X increase (in volume and effective I/O)

• Non-streaming data processing• Expect 20-50 passes through the data (flagging + calibration

+ imaging)– Effective data i/o: few TB

• Exploit data parallelism

• Distribute normal equations (SPMD paradigm looks promising)

• Deploy computationally efficient algorithms (‘P’ of SPMD) on a cluster


Computing challenges

• Significant increase in computing for wide-field wide-band imaging

• Correction of direction dependent terms• As expensive as imaging

• 1 TB effective I/O for calibrating and imaging a 100 GB data-set

• Run-time algorithm efficiency is an important parameter

• Run-time memory requirement


Near term future activities

• Re-factor existing code– Integration of MS-MFS and AW-Projection algorithms

– Use A-Projection for mosaicking (ALMA and EVLA)

– Allow use of arbitrary antenna PB (CASA/ARDG)

• Test wide-band wide-field imaging with EVLA

• Deploy on HPC platforms

• Test mosaicking with PB corrections

• Explore GPU processing

• Full-Stokes PB Corrections• Extend it for wide-band imaging

• Full Stokes Imaging

• Implement and test RFI flagging algorithms

• Pipeline processing using all the above for scientific projects

In progress/Programing

In progress+R&D

In progress/Programing

Programing/R&D?

R&D required <--

R&D required

Prog. + possibly R&D

Programming and R&D <--

R&D required

algorithm development · algorithm development saspo meeting, charlottesville, nov. 9th 2010 s....

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