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FP6 Workpackage 5: Advanced Radiometric Phase Correction (ARPC)
JAO ­ ALMA Enhancement Team Meeting

Bojan Nikolic
(Project Scientist for Advanced WVR Techniques)

John Richer
(Project Leader for Advanced WVR Techniques)
Cambridge

24 July 2009

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 1)


The need for accurate phase correction

ALMA is attempting to improve the angular resolution of (sub)mm-wave images by a factor of 50 over current facilities
Aiming for 5 mas on longest baselines/highest frequencies

Phase errors due to atmospheric turbulence are the major obstacle
This is because the effect of atmospheric phase error is greatest on long baselines required for high-resolution imaging

Other consequences of phase errors (e.g. Memo # 582):
Loss of sensitivity Astrometry errors (snapshot/mosaic only) Flux calibration errors (snapshot/mosaic only)

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 2)


High-z quasar observation with the SMA

Krips et al 2007 (2007ApJ...671L...5K):

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 3)


Simulated proto-planetary disk with ALMA
This is the aim [Wolf & A'Angelo, 2005. (50+100pc, 1+5 Mjupiter )]

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 4)


ALMA phase correction strategy
Water Vapour Radiometry
Measure atmospheric proper ties along the line of sight of each telescope

Fast-switching
Obser ve nearby quasars Calculate antenna phase errors Calibration cycle down to 10­15 s (fast antennas!) Expect calibrators about two degrees from science target Can calibrate at 90 GHz and transfer up to 950 GHz

+

Use dedicated 183 GHz radiometers on each telescope Measurements at about 1 Hz Infer excess path Correct either in correlator or in post-processing

+ Self-Calibration in a limited number of cases

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 5)


Fast switching phase calibration
(Simulation)
50

40

1

antenna #

30 (rad) 20 -1 10 0 0 50 1 40 0.5 20 40 60 80 time (integration #) 100 120 140 0 1 30 (rad) 20 -0.5 10 -1 0 0 20 40 60 80 time (integration #) 100 120 140 0 1 0 0

antenna #

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 6)


The 183 GHz Water Vapour Line
Blue rectangles are the production WVR filters
250

200

150 Tb (K) 100 50 0 175 177.5 180 182.5 (GHz) 185 187.5 190
Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 7)


Goal for ALMA Phase Calibration

Formally: Lcorrected 1 + w 1 mm 10 µm + 0.02 â Lraw (1)

For most projects the above means atmospheric fluctuations are eliminated as a scheduling constraint ALMA will rely on accurate phase correction If it doesn't work as expected for a project, the scientific objectives are likely to be compromised

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 8)


WP5 Description from Contract

Description of work
Develop more sophisticated atmospheric models both ab initio but also through the empirical analysis of the radiometric data already taken at the Sub-Millimeter Array (Hawaii) and later from the ALMA site in Chile, from Early Science onwards. Implement and test these methods as par t of the ALMA software system, with the deliverable being a turnkey system in the ALMA pipeline by the end of the project.

Deliverable D9
Complete software package for correcting phase and amplitude errors applied to the ALMA astronomical data integrated into the ALMA system. The software will be released in stages in accordance with the standard ALMA Computing release cycle, with increasing functionality in each 6-monthly release.

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 9)


WP5 within the ALMA Project
Radiometric phase correction is a part of baseline ALMA. It was however recognised that phase correction by water vapour radiometr y is a complex technique that needs an extended research and development process to become a simple and usable solution for astronomers and the obser vator y; ALMA had limited resources in the Science and Computing IPT budgets for this work; Leadership and exper tise in this within European institutions made it attractive to continue the technical development in here. WP5 grew out of this: we recognised that we could significantly enhance ALMA's scientific capabilities using European exper tise. Enhancement comes in two forms: better phase correction (hence better imaging and sensitivity) for a given obser vation; and capabilities to exploit wider range of weather conditions (efficiency).
Nikolic & Richer, U. Cambridge JAO ­ ALMA Enhancement Meeting: WP5 ESO 24 July 2009 (slide 10)


Summary of current status of WP5
All internal milestones as agreed at the star t of the project have been achieved so far (and associated payments made by ESO to UC) Project is fully staffed (Richer/Nikolic/Cur tis) 4 ALMA memos covering aspects of WVR phase correction First field trip to the site commencing tonight!
Tests at the OSF Discussion/collaboration with ALMA staff

We have requested a zero-cost extension to the work package to allow it to overlap with Early Science ­ end date Dec 2011 No realistic test data yet ­ all work so far either theoretical or with shor t sections of SMA data `Baseline' radiometric phase correction only now being designed and implemented

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 11)


Internal Milestones
Assuming Dec 2011 completion

1 Oct 2007 1 Apr 2008 1 Oct 2008 1 Apr 2009 1 March 2010

1 Dec 2010 1 June 2011 31 Dec 2011

Repor t on the testing at SMA. Repor t on effect of beammismatch. Initial version of on-line stub code. Repor ts on implications of observing strategies and antenna vs baseline correction. Initial version of physics-based algorithms and repor t on this. Refined version of physics algorithm. Initial version of machine-learning based algorithm Repor t on initial testing at the AOS. Refinement of physics algorithms. Measurement of empirical correlation phase and WVR output. Machine-learning algorithms based on real data. Repor t on this approach. Near-final algorithms based on extensive AOS testing Final revised versions of algorithms and documentation, and repor t on their performance after Early Science feedback

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 12)


Primary outputs of WP5 so far I
Full analysis of test data from the SMA
Written up as a note distributed to the project
Date 20060217 -- 20060224 -- 20060503 20060524 -- 20060601 -- 20060718 20060920 -- 20061030 Time (UT) 16.9­17.9 19.8­20.2 18.4­19.4 20.0­21.75 15.3­16.8 5.1­5.7 5.9­8.8 5.3­6.7 7.3­8.3 4.3­6.5 4.2-5.5 6-6.7 19.3-20.3 Elev (deg) 16­30 38­44 25­40 47­72 44-65 50­55 55­64 57­64 55 ­63 40­62 27­41 46­61 67­72 Baseline ( m) 212 212 212? 212? 64 212 212 212 212 212 64 64 415 Raw (µm) 207 238 81 258 54 103 90 62 154 271 83 72 332


5-min ( µ m) 153 239 79 241 37 87 73 40 72 163 71 62 282



Res. ( µ m) 62 73 47 72 28 34 31 31 56 56 60 50 139

c ( mm) 3.6 2.0 2.5 2.4 1.4 2.6 1.8 2.4 2.4 2.3 3.1 2.3 7.0

Sp ec ( µ m) 68 47 51 52 35 53 41 48 50 50 60 48 119

Sampling (s) 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 1.3 1.3 1.3

Comment

11 s offset, timing issues. High intf. noise. Timing issues. -- Very wet conditions. Quality of fit limited by time drift.

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 13)


Primary outputs of WP5 so far II

Simulation of errors due to par tial overlap of WVR and astronomical beams
Documented as ALMA Memo # 573 Implemented in C++/Python and publicly available under LPGL

Turbulent layer at 250 m
0.2 0.2 0.1 0.1

Turbulent layer at 750 m



0.02

0 5 10 15 (arcmin) 20 25 30

0.05

0.05

0.02

0.01

0.01

0

5

10

15 (arcmin)

20

25

30

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 14)


Primary outputs of WP5 so far III

Simulation of ALMA phase correction strategy as a system
Documented as ALMA Memo # 582 Implemented in C++/Python, publicly available under LPGL, and compatible with CASA

Positional error
1
1

Fractional flux error

(arcsecs)

P

0.01

S

2

-S /S

0.1

0.1

2

2

0.01

0.001 0.01

0.02

0.05

0.1

0.2 rms (rad)

0.5

1

2

5

0.001 0.01

0.02

0.05

0.1

0.2 rms (rad)

0.5

1

2

5

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 15)


Primary outputs of WP5 so far IV
Calculation of phase correction coefficients from ab-initio and empirical methods
Documented in detail as ALMA memos #587 and #588 Fully implemented in standalone C++ and made publicly available under LPGL

Estimated
200 200 p (µ m) 0 p (µ m) 0

Optimum

-200

-200

750

750

500

500

250 p (µ m) p (µ m)

250

0

0

-250

-250

-500

-500

-750 16.8

17

17.2 t (hours UT)

17.4

17.6

17.8

-750 16.8

17

17.2 t (hours UT)

17.4

17.6

17.8

Residual RMS 74 µm
Nikolic & Richer, U. Cambridge JAO ­ ALMA Enhancement Meeting: WP5

Residual RMS 71 µm
ESO 24 July 2009 (slide 16)


Primary outputs of WP5 so far V

Technical input for reviews of the production WVR hardware design

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 17)


Highlight of recent progress

Basic versions of both ab-initio and empirical algorithms implemented and shown to produce encouraging results with the SMA test data Emily Cur tis joined us in April (working par t-time on phase correction) and she's now up to speed on the project First data from the OSF with production WVRs this month Mid-term review successfully passed

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 18)


Near-term plans

Trip to OSF/AOS to collect WVR sky brightness data, talk to ALMA staff, and familiarise with the operation of the telescopes (next two weeks) Analysis of sensitivity of the dispersive scaling factor to atmospheric conditions (e.g., ground level temperature, water vapour scale height) (some results already available) Analysis of the WVR sky brightness data from OSF/AOS (next three months) Initial design of the CASA interface (probably commence in September)

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 19)


Test Data
Realistic test data is a critical input to our work, in order to Understand in which directions to fur ther develop the algorithms we already have Test how well the algorithms work and tune them from use in production Currently we have: About five hours of useful test data from the SMA with the prototype WVRs Sky-brightness data from production WVRs in the lab at OSF (collected in the beginning of July) Near term test plan exists and we fully suppor t it: Distributed by Richard Hills 18 items of which about 5 completed already

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 20)


Test Data Shopping list

Ideally we would have simultaneous interferometric and WVR sky brightness data: With good instrumental phase stability Taken at the AOS (but at OSF would be useful too) Over a range of atmospheric conditions (daytime/nighttime and winter/summer) In a number of different configurations (compact to most extended, where the challenge is greatest) Over a range of obser ving frequencies (to understand dispersive effects) Extension of work to December 2011 is crucial to getting the majority of these data

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 21)


Integration plans
It will be possible to do phase correction both on-line (based in the TELCAL package) and off-line (CASA) All code already developed is standard C++/Python and compatible with both systems The plan is to first integrate into CASA
Offline correction is better except in cases where limited by high data rates Initial testing of algorithms will always be done off-line This integration is straightforward and can largely be done independently of main CASA development

One proven, some algorithms will be integrated into TELCAL
Only a subset of algorithms will be suitable for use in TELCAL Code already produced can be called from TELCAL Integration will be more straightforward once baseline algorithms are implemented

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 22)


Risks/issues to keep in mind

Availability of suitable quality/quantity test data
Much reduced with the extension to Dec 2011

Loss of key personnel Will phase transfer from 90GHz work well enough? If not alternative strategies will need to be developed User experience of (offline) phase correction
Integration into pipeline processing? Added complexity to off-line data reduction?

Role of WVRs in absolute flux calibration? Input from phase correction algorithms into scheduling software
How well can we predict residual phase fluctuation? How do we decide on fast-switching cycle time?

Nikolic & Richer, U. Cambridge

JAO ­ ALMA Enhancement Meeting: WP5

ESO 24 July 2009 (slide 23)