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Alexander Imaging ppt Slide Position
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- 12/23/2010
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Document Sample
Overview of Calibration and
Dynamic Range Challenges
Paul Alexander
AAVP 2010 Dynamic Range Paul Alexander
Required Dynamic Range
• Sensitivity: 2000 m2/K at 150 MHz; 300 MHz BW; station beam ~ 1 degree
• In 24 hrs integration s = 0.2 mJy
• ~ 1 500 mJy source per sq degree at 150 MHz
2.5 × 106
• EoR signal ~ 10 mK in presence of ~1000K foreground. Image at > 10s
1 × 106
Take this estimate with a “pinch of salt” – limited by
foreground subtraction
Suggests 107 : 1
AAVP 2010 Dynamic Range Paul Alexander
Required Dynamic Range
• Note do need to think about source confusion
• In 24 hrs integration s = 0.2 mJy
• Source density ~ 1.5 x 105 sources per sq degree
Required baseline ~ 100km at 450 MHz
AAVP 2010 Dynamic Range Paul Alexander
Achieving high dynamic range now
What do we know we have to include in an analysis:
Include Discussion Maturity
Antenna-based complex Standard calibration and self
gains calibration – iterative
Removing sources in Removing bright sources from UV
global sky model data even with local phase solution is
relatively robust
RFI and “bad data” Can be critically important:
excision • still largely done by hand for GMRT,
eVLA and LOFAR
• Expert algorithms not well
developed
Bandpass calibration Well defined, but often more ?
problematic than it should be –
software limitation
AAVP 2010 Dynamic Range Paul Alexander
Achieving high dynamic range now
Include Discussion Maturity
Debugging the system We learn a great deal about our
instruments over time and correct
often 2nd order errors
Position-dependent effects Hugely Important relatively recent
advance
• Time dependent pointing errors –
antenna models may be limit
• Position-dependent phase screen –
critically important for the
ionosphere – modelling?
• Many algorithms (peeling. A-
projection ...)
Full stokes imaging A position-dependent effect –
polarization response changes
across FoV
AAVP 2010 Dynamic Range Paul Alexander
Achieving high dynamic range now
Other known issues
• Algorithm approximations mean analysis has known problems and errors
which are not necessarily well dealt with
Wide-field imaging approximations (faceting, w-projection)
Deconvolution errors and artefacts – still an art using human judgement to
drive non-linear algorithms
Time-averaging and bandwidth smearing poorly dealt with (but also useful
in very wide fields).
AAVP 2010 Dynamic Range Paul Alexander
AA Pros and Cons
• AA is operating at low frequency Ionosphere!
• Physical stability (wind etc.) Good, study details
• Unblocked aperture Inherent
• Smaller beams are better >60m collectors
• Narrow band is important AA is Wide Band but
many channels
• Calibration capability Excellent, by channel
• Trade DR for sensitivity AA v. flexible
AAVP 2010 Dynamic Range Paul Alexander
Designing for dynamic range
Stable, known antenna patterns are key
• AA advantages
• AA’s mechanically stable
• Unblocked aperture
• Direct measurement of field
• But
• Need to calibrate 105 elements per station
• What accuracy of element calibration is needed
• Model dependent calibration – how many parameters can we solve for?
• Station beam is time dependent – transit experiment for individual elements
• Multiple independent elements for AA-low
AAVP 2010 Dynamic Range Paul Alexander
Designing for dynamic range
Element-level calibration options and issues
• Importance of phase versus amplitude – how accurate? How often?
• Where are the main errors introduced in the RF chain -
• If copper is used for signal transport – active measurement of cable lengths?
• Deployment issues – position, orientation, misalignment
• If digitisation at the element – accuracy of clock distribution
• Temperature variations – large ambient fluctuations – fibre better than copper?
• Expert health monitoring system at element level – flag failed or failing
elements
• Noise injection?
AAVP 2010 Dynamic Range Paul Alexander
Pathfinders
• The design decision must be informed by the pathfinders and precursors
• SKA community mu go beyond – “waiting to see what we will learn”
SKA team must pose the questions that we want to be answered
• Get answers either from experience of the pathfinders or doing explicit
experiments and measurements
Obvious area of immediate cooperation between all the experiments
and the AAVP team
AAVP 2010 Dynamic Range Paul Alexander
Other design issues
• Sufficiently good ionospheric model
• Station size and UV coverage – competing issues
• Larger stations – smaller station beam easier
ionospheric model? Lower cost & less processing
• Smaller station size better – more stations better
UV coverage, better imaging capability
• Hierarchical beam former
• Hierarchical beamformer in which data decimated
reduced accuracy of station beam
AAVP 2010 Dynamic Range Paul Alexander
Algorithm issues (AA emphasis)
• Maturity of approaches is not there yet
• Much use of human intervention still required
• Transitioning to totally automated pipelines will be a major challenge
• Expert system for RFI excision?
• Are wide-field imaging approaches sufficiently accurate?
• Is our underpinning understanding of interferometry based too much on
“experience” rather than a formal understanding of the underpinning
formalism?
• Relying very much on a tiny group of real experts who have both the
“experience” and the formal analysis
AAVP 2010 Dynamic Range Paul Alexander
Magnitude of the task
Grid science reduction and
visualisation
Science Observation
proposal definition M&C
database
Global
and local
sky model
Monitor and Control system
Cloud store
Data product
Collectors
distribution
Data excision
Data excision
Correlator
Calibration
loop
Imaging Science
processor product
archive
Data Visibility
routing processors
Local
science
reduction
AAVP 2010 Dynamic Range Paul Alexander
SKA1 Data Rates and Configuration
• AA Line experiment 50 AA-low stations
• 100 sq degrees, 10000 channels over 380 MHz bandwidth
3.3 GS/s
• Issues
• What data rate can we process?
• Trade UV coverage (Ns) for FoV and hence survey speed (W)
• Line vs continuum requirements
• What is the longest baseline
• Single or multi-pass algorithms increase data rate and
buffering
AAVP 2010 Dynamic Range Paul Alexander
Reducing the data rate
• Relax criteria for dump rates and frequency resolution
– My criteria based on uniqueness in UV plane
– Can the criteria be relaxed and still achieve high dynamic range?
• Dump times and frequency resolution baseline dependent
Design correlator for worse case upgrade path
AAVP 2010 Dynamic Range Paul Alexander
AAVP 2010 Dynamic Range Paul Alexander
