1mm VLBI with ALMA and the EHT

Station Positions

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

1.3mm Proposals


Call for proposals
Proposal preparation
Technical parameters for proposal
Proposal submission
TelescopeXYZ
ALMA phased array 2225061.873 -5440061.953 -2481682.084
APEX 2225039.530 -5441197.629 -2479303.360
JCMT -5464584.676 -2493001.170 2150653.982
SMA phased array -5464588.447 -2492884.038 2150756.452
LMT -768715.632 -5988507.072 2063354.852
IRAM 30m 5088967.900 -301681.600 3825015.800
SMT -1828796.200 -5054406.800 3427865.200
SPT 792.600 -802.600 -6359569.200

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.

UV-Coverage

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.

δ=-40deg:

δ=-20deg:

δ=0deg:

δ=+20deg:

δ=+40deg:

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.

Source Elevations

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.

δ=-40deg:



δ=-20deg:

δ=0deg:

δ=+20deg:

δ=+40deg:

System Equivalent Flux Densities

The baseline sensitivity can be calculated from the following equation:

ΔSrms  =   SEFD eq. (1)
ηc   npoltintΔν

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.

TelescopeSEFD (Jy)
ALMA41 phased array 88
APEX 3600
JCMT 4700
SMA phased array 4000
LMT 1400*
IRAM 30m 1400
SMT 11000
SPT 9000

*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.