The ALMA Phasing Project (APP) APP is an ongoing ALMA Development Project (approved in 2012) to provide the hardware and software necessary to coherently sum the 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 ( GHz = λ mm) Band 6 ( GHz = λ mm) (In the future, Band 7 and beyond) Band 7 ( GHz = 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: ~ mm ~70 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 Fiber Link System Mark 6 VLBI Recorders Phasing Interface Cards (PICs) Hydrogen Maser

APP Software Status Nearly all software tasks for the APP have been implemented as designed 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 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 ϕ rms ~ 9 degrees - Band phased antennas - Baselines <1.4 km - dual polarization (XX, YY) - 4 ✕ GHz bands ALMA Phasing System observation from August 2015: 2π

ALMA Phasing System observation from August 2015: 5 minutes - Band phased antennas - 4 ✕ GHz basebands - XX pol jump in correlated amplitude by ~sqrt(25) upon phase-up

10 minutes Δ ϕ =180 o Green, Orange: no phasing corrections Brown, 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: 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 3C km baseline Phased ALMA – VLBA (MK) (plus 5 other VLBA stations): August 1, 2015 Band 3 (3 mm); Source 3C km baseline First Intercontinental VLBI Fringes with ALMA multi-band delay 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

Project Timeline and Milestones Fall 2011: Project inception May 2013: Critical Design Review January 2014: Initial phasing system (R10.6) and first VLBI recordings June 2014: Installation of Hydrogen Maser July 2014: Switchover of ALMA to Maser October 2014: Completion of Hardware installation; Software verification of all major features completed (R2014.4) November 2014: Hydrogen Maser Acceptance Review December 2014: Hardware Acceptance Review (Conditional Pass) January 2015: Start of Commissioning and Science Verification

Maser Installed in Seismic Rack at AOS in June 2014

APP System Block Diagram Functional area ExistsNew