1mm VLBI with ALMA and the EHT
The EHT + phased ALMA array is expected to comprise the following stations:
|ALMA phased array||2225061.873||-5440061.953||-2481682.084|
|SMA phased array||-5464588.447||-2492884.038||2150756.452|
where 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 stations.dat file for NRAO SCHED containing these coordinates is provided.
The above array results in the following uv-coverage for targets at declinations of -40, -20, 0, +20, and +40 degrees. 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.
The following plots show source elevations for the 1mm VLBI array as a function of declination for a target at RA=0hr. The elevation as a function of time for other sources can be obtained adding the source RA to the UT time shown. In these plots, the yellow curve denotes a NOEMA single antenna at Plateau de Bure. Neither NOEMA nor the IRAM 30m antenna at Pico Veleta can usefully observe sources at -40 degrees declination. Likewise, the South Pole Telescope cannot see northern-hemisphere sources.
System Equivalent Flux Densities
The baseline sensitivity can be calculated from the following equation:
|ΔSrms =||SEFD|| eq. (1)
where "SEFD" is the geometric mean of the System Equivalent Flux Densities (SEFDs) of antennas included, ηc is the correlator efficiency (0.88), npol is the number of polarizations being considered (this should be 1 for single baseline fringe detection), tint 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 1mm VLBI array.
|ALMA41 phased array||88|
|SMA phased array||4000|
*The LMT value assumes installation of a new receiver illuminating 32 m of the surface. It may be lower if the full 50 m can be illuminated.
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.