EHT Proposals

The ALMA Phasing Project

The ALMA Phasing Project (APP) has provided the hardware and software necessary to phase up the ALMA dishes to form a single station for VLBI. It is an international collaboration led by MIT Haystack Observatory, including contributions from NRAO, MPIfR, ASIAA, NAOJ, University of Concepcion, Harvard-Smithsonian CfA, and Onsala Space Observatory. It has been funded by the ALMA North America Development Fund, the NSF Major Research Instrumentation Program, and associated cost-sharing partners. From ALMA Cycle 7 the phasing system is offered using Band 3 (wavelength 3 mm) with the Global 3-mm VLBI Array, Band 6 (wavelength 1.4 mm) and, from Cycle 9,  Band 7 (wavelength 0.88mm) with the EHT. The maximum bandwidth that can be output from the phasing system is 7.5 GHz. The fringe spacing on a baseline from ALMA to Mauna Kea (JCMT, SMA) is ~30 microarcseconds at wavelength 1.3 mm. Further technical information about phased ALMA may be found in each Cycle's ALMA Technical Handbook (e.g. Cycle 7).

The Event Horizon Telescope

The Event Horizon Telescope is a 1 mm VLBI array whose primary goal is to observe and image nearby supermassive black holes with sufficient angular resolution to resolve the hot material just outside the black hole event horizon. The EHT network currently consists of telescopes in Chile, France, Greenland, Mexico, the South Pole, and the USA (Arizona and Hawaii) that together provide a resolution of better than 30 microarcseconds. To provide the sensitivity required to observe sources at such fine detail, EHT partners have developed ultrawide-bandwidth instrumentation and correlation facilities capable of handling very high data rates. For each cycle, the EHT aims to offer a limited amount of array time to enable new science programs that can make use of the EHT in conjunction with the APP.

Proposal Review Process

All proposals requesting both phased ALMA and EHT time will undergo review through the normal ALMA proposal review and time allocation process, as well as being reviewed by the EHT Consortium. A merged list of acceptable proposals will be made by the ALMA VLBI Coordinating Committee. The final 1 mm VLBI observing schedule will comprise proposals of the highest scientific and technical merit that are also able to fit into the VLBI session given its scheduling constraints.

Station Positions

The EHT with phased ALMA array is expected to comprise the following stations:

In the table above antenna positions are given in meters in an Earth-centered, right-handed coordinate system. X is in the direction of the Greenwich meridian, Y is oriented to give a right-handed coordinate system with X and Z, and Z is in the direction of the north pole. A text file for NRAO SCHED containing these coordinates is provided here: stations.txt.

UV-Coverage (plots shown for 1.4mm)

The above array results in UV-coverage for targets at declinations of -40, -20, 0, +20, and +40 degrees as shown for 1.4mm in the plots below. The UV-coverage plots show the complete coverage possible above 20 degrees elevation. Baselines that include ALMA are shown in red. Baselines between other EHT stations are shown in blue. Baselines to JCMT and to SMA provide redundant (U,V) coverage. The same is true of APEX and ALMA. This redundancy, along with the very short spacings provided by the JCMT-SMA and APEX-ALMA baselines, is helpful for calibrating the array.

In the figures below, PDBURE = NOEMA, PICO = IRAM 30-meter, THULE = GLT, and KP, APEX, and JCMT are not listed explicitly because of their proximity to SMT, ALMA, and SMA, rerspectively.
δ = -40 deg.:
δ = -20 deg.:
δ = 0 deg.:
δ = +20 deg.:
δ = +40 deg.:

Source Elevations The following plots show source elevations for the 1-mm VLBI array as a function of declination for a target at RA = 0 h. The elevation as a function of time for other sources can be obtained adding the source RA to the UT time shown. Neither NOEMA nor the IRAM 30-meter antenna at Pico Veleta can usefully observe sources at -40 deg. declination. The Greenland Telescope (GLT) cannot observe sources on the southern hemisphere. Likewise, the South Pole Telescope cannot observe northern-hemisphere sources. In the figures below, PDBURE = NOEMA, PICO = IRAM 30-meter, THULE = GLT, and KP, APEX, and JCMT are not listed explicitly because of their proximity to SMT, ALMA, and SMA, rerspectively.
δ = -40 deg.:
δ = -20 deg.:
δ = 0 deg.:
δ = +20 deg.:
δ = +40 deg.:

System Equivalent Flux Densities

The baseline sensitivity can be calculated from the following equation: ΔS_rms  =  SEFD η_c^-1 (n_polt_intΔν)^-1/2

In the equation above "SEFD" is the geometric mean of the System Equivalent Flux Densities (SEFDs) of antennas included, η_c is the correlator efficiency (0.88), n_pol is the number of polarizations being considered (this should be 1 for single baseline fringe detection), t_int is the on-source integration time in seconds, and Δν is the bandwidth in Hz. The table below gives the SEFDs for the various elements of the 1-mm VLBI array.

(*) This assumes 37 antennas in phased ALMA and 6 antennas in phased SMA, which are medians of the number of antennas used in 2017 observations. Full array (12 dishes) assumed for phased NOEMA. With very good observing conditions in 2017, ALMA's SEFD was 74.

(**) The LMT value is a conservative estimate based on scaling the SEFD measured for the receiver illuminating 32-meter surface to the recently completed 50-meter surface. With recent upgrades, SEFD is expected to be in the 700-800 Jy range in 2020. The GLT and SPT values account for the fact that typically sources are observed at a low observation.

The above SEFD values assume typical weather conditions and a source at moderately high elevation. Actual SEFD values can be much higher in poor weather or for low-elevation sources. As a guideline, a source with 50 mJy of correlated flux density on intercontinental baselines should be easily detectable on all baselines to ALMA unless the weather is exceptionally poor.

Telescopes

The EHT with phased ALMA 1-mm VLBI array comprises of the following millimeter-wave telescopes: the Submillimeter Array (SMA), the James Clerk Maxwell Telescope (JCMT), the Submillimeter Telescope (SMT), the Atacama Pathfinder Experiment (APEX), the South Pole Telescope (SPT), the Large Millimeter Telescope Alfonso Serrano (LMT), the IRAM 30-meter telescope, the Greenland Telescope (GLT), Kitt Peak 12-meter telescope (KP), Northern Extended Millimeter Array (NOEMA), and ALMA.

ALMA is operated as a phased array comprising approximately 39 phased 12-m antennas. The effective collecting area will therefore be approximately equivalent to a single 75-m diameter antenna with a gain of ~1.06 K/Jy and an SEFD of ~94 Jy at 1.4mm. To maximize phasing efficiency, baselines between phased antennas will be restricted to ~1 km. Correlated 1-mm flux densities on 1-km baselines must exceed 500 mJy, unless passive phasing is used on a nearby calibrator can be used and is offered in the Call for Proposals.

Antenna positions and SEFDs are given in the "Technical Information" section above, for proposers wishing to perform UV-coverage or other simulations. Proposers should assume this array in considering UV-coverage, sensitivity, and other technical aspects, although all telescopes that are available (which may be fewer than those listed) will be used during the 1-mm VLBI observing session. Note that only VLBI observing including phased ALMA may be requested at this time. The EHT without phased ALMA is not being offered as part of current Calls.

Sky Coverage and Scheduling Considerations

The performance of the phased arrays (SMA and ALMA) will degrade at low elevation, and other elements of the 1-mm VLBI array have elevation limits determined by the local terrain. Soft limits on the declination range that can be observed with the array are -45 deg. < δ < +45 deg. Proposers should consult the "Technical Information" section above for 1-mm VLBI with ALMA and the EHT for plots of target visibility as a function of declination and VLBI station.

Solar avoidance for the EHT is 45 degrees, and no source will be observed if it comes with that angular distance from the Sun during the EHT observing period. There may also be additional scheduling constraints at some of the VLBI stations. For example, the performance of some of the telescopes degrades during the daytime, and they will not observe after sunrise. In addition, some of the telescopes may have other programmatic constraints. While there are no formal restrictions on the target RAs that can be requested in a Call, proposers should be aware that all these scheduling constraints will be taken into account when developing the approved VLBI program, and will likely affect any sources outside the RA range of 10-20 h.

The 1-mm VLBI session dates are not yet definitive, but are likely to be in March or April.

Frequencies Supported

Band 6 and 7 VLBI is offered for continuum and spectral line science. The standard frequency tunings used by the EHT are shown below. Flexible tuning is available on a limited basis only and projects that seek to observe which non-standard tunings should contact the EHT Operations Manager prior to submission of a proposal.

• Band 6: a single tuning simultaneously covering 212.1 to 216.1 GHz (lower sideband) and 226.1 to 230.1 GHz (upper sideband), corresponding to an LO of 221.1 and IF of 5-9 GHz. At ALMA, this will be provided by four 1.875 GHz bands centered at 213.1, 215.1, 227.1 and 229.1 GHz,
• Band 7: a single tuning simultaneously covering 334.6 to 338.6 GHz (lower sideband) and 346.6 to 350.6 GHz (upper sideband), corresponding to an LO of 342.6 and IF of 4-8 GHz. At ALMA, this will be provided by four 1.875 GHz bands centered at 335.6, 337.6, 347.6 and 349.6 GHz.

The other EHT stations will be tuned to overlap the four ALMA sub-bands. When ALMA is operated as a VLBI station, all ALMA standard interferometry data products are also created by the ALMA correlator, providing the equivalent of a standard single-field interferometry observation of the target field for all four of the above bands. Further information about the phased ALMA system is available in each Cycle's Technical Handbook (e.g. Cycle 7).

Recording Rate and Polarization Products

All partner telescopes will record 2-bit samples of both polarizations in each of the four frequency bands for a total of 64 Gbps. The recorded VLBI data from all stations will be shipped to the EHT facilities (MIT Haystack Observatory and MPIfR Bonn) for correlation. At ALMA, the polarizations are linear (X and Y), while circular polarizations (L and R) are used at the other sites. The ALMA observations will therefore include additional calibration to allow conversion of the ALMA X/Y data to equivalent L/R in post-correlation processing (see Marti-Vidal et al. 2015). Note that correlation of full polarization is required even for Stokes I continuum science goals.

While the EHT has carried out polarimetry in the past, VLBI polarimetry with phased ALMA is not fully commissioned and is offered with the caveat that it is on a best-efforts basis.

Sensitivity and Target Flux Densities

Science targets require sufficient flux density on intercontinental baselines to allow self-calibration on timescales of 10 seconds (~50 mJy). Note that these sensitivity requirements correspond to source brightness temperatures of a few times 10⁸ K on 5000-km baselines. Proposers should be aware that this array is therefore sensitive only to non-thermal emission. Further information on the SEFDs of individual telescopes can be found in the "Technical Information" section above. The phasing mode offered requires the phase-up of the array to be performed directly on the science target or via passive phasing on a nearby calibrator. Phasing targets are limited to sources with correlated 1-mm flux densities exceeding 500 mJy on 1-km baselines. 

Calibration Overhead

Phased ALMA data need full-polarization correlation even for Stokes I continuum science goals, for the reasons described above; it is therefore necessary to calibrate the ALMA polarization leakage terms. To do this, the polarization leakage calibrator for any individual science target must be observed over an hour angle range of at least 3 h to sample a range of parallactic angles. The overall observing efficiency on the science target within this three-hour block is expected to be ~25%, with the remaining 75% of the time taken up with ALMA-specific, single dish, and VLBI calibrations. Proposers should request the full amount of time needed to perform their science observations, including calibration overhead.

Proposal Technical Justification

The technical justification for a 1-mm VLBI proposal should be used to specify how the technical set-up enables the proposal’s scientific goals to be met. For VLBI proposals it is entered as free-format text in the ALMA OT (maximum 4000 characters). It should include the following:

• Explain the reason for using the 1.3 mm and/or 0.87 mm bands (ALMA Bands 6 and/or 7).
• Specify whether the science goal is Stokes I continuum or continuum polarimetry. Note that all polarization products must be requested whether or not the science goal includes polarimetry.
• Give the expected flux densities of the targets on (a) 1-km baselines and (b) 5000-km baselines.
• Specify the VLBI calibrators to be observed, and their expected flux densities. These should include calibrators for determining delays, bandpass, and instrumental polarization leakage and polarization angle.
• Justify the sensitivity required to achieve the science goal. For non-imaging experiments, justify the required baseline sensitivity.
• Specify the required on-source integration time to achieve the required sensitivity, and the total observing time including overhead. Include considerations such as uv-coverage needed for precision imaging. Please also verify that the time request on the proposal cover page is consistent with that specified here.
• Note whether the imaging is expected to be limited by sensitivity, dynamic range, or both. Describe any potential imaging issues expected (e.g., due to nearby strong sources, complex source structure, etc.).
• Note any other special technical considerations with either the set-up or the data processing.
• If the source is outside the RA range of 10-20 h, include a statement describing the requested time for each telescope after 8 am and before 7 pm local time.

Proposers are encouraged to use the following cues to structure their technical justification:

• Frequency/band:
• Polarization science goal:
• Target flux densities:
• Calibrators:
• Sensitivity:
• Observing time:
• Imaging:
• Other:

Proposal Submission

Proposals for 1-mm VLBI using phased ALMA and the EHT must be submitted to ALMA by the proposal submission deadline, typically a Wednesday in the middle of April, at 15:00 UT.

The scientific justification for an ALMA proposal is limited to 4 pages (A4 or US letter), at no smaller than 12-pt font. The technical justification is entered through a free-format text field in the OT, and is in addition to the pages allowed for the scientific justification.

For a complete description on the ALMA proposal submission process and the relevant documentation for the current Call for Proposals visit the ALMA Science Portal (North America, Europe, East Asia). Note that starting from Cycle 8, ALMA's review process is dual-anonymous and it is the responsibility of the proposers to ensure anonymity is preserved when writing their proposals. A FAQ on the proposal process is available here. Further questions about submitting proposals as well as any other technical or scientific questions about ALMA can be submitted by logging into the ALMA Helpdesk at help.almascience.org.