1mm VLBI Call for Proposals: Proposal Preparation
The EHT + phased ALMA 1mm VLBI array is expected to comprise 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 30m telescope, and phased ALMA.
For Cycle 5 VLBI observations at 1mm (Band 6), ALMA will be operated as a phased array comprising approximately 41 phased 12-m antennas. The effective collecting area will therefore be approximately equivalent to a single 76-m diameter antenna with a gain of 1.14 K/Jy and an SEFD of ~88 Jy. To maximize phasing efficiency, baselines between phased antennas will be restricted to ~1km and correlated 1-mm flux densities on 1-km baselines must be ≥500mJy.
Antenna positions and SEFDs are given at the The EHT without phased ALMA is not being offered as part of this Call., 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 available telescopes (which may be more or fewer than those listed) will be used during the 1mm VLBI observing session. Note that only VLBI observing including phased ALMA may be requested at this time.
Sky Coverage and Scheduling Considerations
The performance of the phased arrays (SMA and ALMA) will degrade at low elevation, and other elements of the 1mm 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 EHT web page for 1mm 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 this Call, proposers should be aware that all these scheduling constraints will be taken into account when developing the approved VLBI program, and would likely affect any sources outside the RA range of 10h-20h.
The 1mm VLBI session dates are not yet fixed, but are likely to be in March or April, 2018.
It is intended that a single tuning covering 212.1 to 216.1 GHz and 226.1 to 230.1 GHz will be available at all 1mm VLBI stations, supporting continuum science only. At ALMA, this will be provided by four 1.875GHz bands centered at 213.1, 215.1, 227.1 and 229.1 GHz. The other stations will be tuned to fully overlap these four 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 the ALMA Cycle 5 Technical Handbook.
Recording Rate and Polarization Products
In Cycle 5, all partner telescopes will record 2-bit samples of both polarizations in each band for a total of 64 Gbps. The recorded VLBI data from all stations will be shipped to the EHT facilities (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
The phasing mode offered for Cycle 5 requires the phase-up of the array to be performed directly on the science target. Science targets are therefore limited to sources with correlated 1-mm flux densities of ≥500mJy on 1-km baselines and having sufficient flux density on intercontinental baselines to allow self-calibration on timescales of 10 seconds (~50mJy). Note that these sensitivity requirements correspond to source brightness temperatures of a few x108K on 5000km 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 at the EHT web page for .
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 hours to sample a range of parallactic angles. The overall observing efficiency on the science target within this 3 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.
The technical justification for a 1mm 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:
Proposers are encouraged to use the following cues to structure their technical justification: