The ALMA Phasing Project (APP)

APP is an ongoing ALMA Development Project (approved in 2012)to provide the hardware and software necessary to coherently sumthe signals of up to 61 ALMA antennas and record VLBI format data.

PI: Sheperd Doeleman (MIT Haystack/Harvard-Smithsonian CfA)

APP Participating Organizations:

- MIT Haystack Observatory (lead)- NRAO- Max Planck Institut für Radioastronomie- University of Concepción- ASIAA- NAOJ

- Harvard-Smithsonian Center for Astrophysics- Onsala Observatory

Sponsors:- ALMA North America Development Fund- NSF Major Research Instrumentation Program- Cost sharing partners

Quick Summary of Phased ALMA Capabilities

Equivalent collecting area: 84-m parabolic dish (assuming 50 phased 12-m antennas)

Frequencies of operation: Band 3 (84-116 GHz = λ2.6 - 3.6 mm) Band 6 (211-275 GHz = λ1.1 - 1.4 mm) (In the future, Band 7 and beyond) Band 7 (275-373 GHz = 0.8-1.1 mm; planned) Bandwidth: 7.5 GHz

Polarization: Full Stokes

VLBI Recording: 64 Gbps aggregate (4 Mark-6 units, each 16 Gbps)

Angular resolution of ALMA-Mauna Kea baseline: ~30 μas @ 1.3 mm ~70 μas @ 3 mm

What Does Phasing ALMA Entail?

Hardware: • Installation of a new frequency standard (H maser) to replace ALMA’s Rb clock

• Design and install Phasing Interface Cards (PICs) in ALMA correlator (2 per quadrant, 8 total) to serve as VLBI backend

• Install set of Mark 6 high-speed VLBI recorders at ALMA OSF

• Build and install optical fiber link system to carry data from ALMA high site to VLBI recorders

Software:• Implement new VLBI Observing mode (VOM) into existing ALMA software to phase the array and operate the VLBI backend

CSV:• Perform commissioning and science verification of the entire system

All APP hardware is now installed and accepted.

(Half of)Optical FiberLink System

Mark 6VLBIRecorders

PhasingInterface Cards (PICs)

HydrogenMaser

APP Software Status

Nearly all software tasks for the APP have been implemented asdesigned for Cycle 3.

• On-line capabilities of the APP software are now complete

• 76 out of 96 ICT software development tasks are now closed; the remainder are in progress for Cycle 4 and beyond

• VEX-driven VLBI observations have been demonstrated

Remaining work:

• Integrate VLBI observing capability into a “proposal-driven” process (i.e., use OT to produce a VLBI scheduling block that can execute VEX specifications)

Slow Loop

Fast Loop

Schematic of ALMA Phasing Software

• Commissioning and Science Verification (CSV) for the APS officially kicked off in January 2015.

• 3 full-fledged CSV missions so far (plus sundry isolated tests)

• For gory details, see ALMA Technical Notes 16, 17, & 18 (Matthews & Crew)

In a nutshell:

• The phasing system is working- Fast and slow phasing loops successfully demonstrated with up

to 51 antennas- Phasing system meets efficiency requirements in Band 3 & 6,

all 4 correlator quadrants, both polarizations

• ALMA can now do VLBI- Successful VLBI fringe detections in Bands 3 and 6 on baselines >5000 km

Commissioning and Science Verification of the ALMA Phasing System

APP Commissioning and Science Verification (APP-CSV)

The formal APP-CSV Plan has three Phases:

Phase 0 (“Pre-commissioning”):- Test VLBI-mode scan execution- Insure APS operation does not negatively impact standard ALMA operations- Verification of data products- Insure use of APS does not corrupt polarization

Phase 1 (“Commissioning”):- Rigorous stability/efficiency testing- “Local” VLBI (interquadrant fringe tests; fringe tests between AOS-OSF, AOS-APEX)- End-to-end checkout of integrated APS hardware and software

Phase 2 (“Science Verification”):- Global VLBI observing campaigns in Bands 3 and 6- Correlation and post-correlation analysis

Demonstrations of the Phasing System

5 minutes

mean ϕrms ~ 9 degrees

- Band 3 - 25 phased antennas- Baselines <1.4 km- dual polarization (XX, YY)- 4 ✕ 1.875 GHz bands

ALMA Phasing System observation from August 2015:

2π

ALMA Phasing System observation from August 2015:

5 minutes

- Band 3 - 25 phased antennas- 4 ✕ 1.875 GHz basebands- XX pol

0.007

0.001

jump in correlated amplitude by ~sqrt(25)upon phase-up

10 minutes

Δϕ=180o

Green, Orange: no phasing correctionsBrown, Black: fast loop operational

Evaluation of Phasing Efficiency

Demonstrations of VLBI

ALMA VLBI “First Light”: January 13, 2015

• ALMA – APEX baseline • Band 6 (1.3 mm); Source: 0522-364• True VLBI with independent frequency standards at both sites• ALMA is unphased• SNR~68 (5 seconds of data)

multi-band delay

delay rate

Phased ALMA − IRAM 30m: March 30, 2015

• Band 6 (1.3 mm); Source 3C273• 9770 km baseline

Phased ALMA – VLBA (MK) (plus 5 other VLBA stations): August 1, 2015

• Band 3 (3 mm); Source 3C454.3• 6600 km baseline

First Intercontinental VLBI Fringes with ALMA

multi-band delay

multi-band delay

delay rate

delay rate

single-band delay

single-band delay

averaged cross-power spectrum

averaged cross-power spectrum

VLBI tests with the APS to date demonstrate:

All components of the APS hardware are working to spec

There is a viable path to correlation of VLBI data on ALMA baselines in spite of the different sampling rate used by ALMA and most VLBI sites

• Incremental Acceptance Review (October 26)

• Final full-scale CSV mission (March/April 2016?)

• On track to offer VLBI capabilities on ALMA to the general community in ALMA Cycle 4 (“go/no-go” decision will be made in December)

Remaining issues for Cycle 4:

- Demonstration of VEX2VOM (i.e., demonstration of VLBI mode observations using a scheduling block rather than a “manual mode”)

- Complete study of optimal limiting radius for inclusion of antennas in the phased array

Upcoming APP Activities

• Additional testing of PolConvert capability

• Correlation of full VLBI data sets

• Imaging and closure analysis of VLBI observations

• Preparation of documentation on use of APS for science observations

Remaining APP Tasks