ARlogo Annu. Rev. Astron. Astrophys. 2014. 54:
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There are four major VLBI arrays that are regularly doing radio astrometry: the VLBA (Very Long Baseline Array) in US, the EVN (European VLBI Network), the VERA (VLBI Exploration of Radio Astrometry) array in Japan, and the LBA (Long Baseline Array) in Australia. Basic parameters of these arrays are summarized in Table 1.

Table 1. Major Radio Astrometric Interferometers

Array Country/ Antenna diameters & Maximum Operating beam size (mas)
  region number in array baseline (km) frequencies  

VLBA USA 25m × 10 8600 km 0.3–86 GHz 0.17 at 43 GHz
  homogeneous and best imaging capability

VERA Japan 20m × 4 2300 km 6.7–43 GHz 0.63 at 43 GHz
  dedicated to astrometry with dual-beam system

EVN Europe 14m–100m, × ~ 10 3000-10000 km 1.6–22 GHz 0.30 at 22 GHz
  high sensitivity with large dishes

LBA Australia 22m–70m, × ~ 10 1700 km 1.4–22 GHz 1.7 at 22 GHz
  only VLBI array in the southern hemisphere

JVLA USA 25m × 27 36 km 0.07–50 GHz 40 at 43 GHz

ALMA Chile 12m × 54 + 7m × 12 16 km 43–900 GHz 4.5 at 850 GHz
  large connected array at mm and sub-mm wavelengths

SKA Au/SA ~ 1 km2<> aperture ~ 3000 km 0.1–22 GHz 0.7 at 22 GHz
  future large cm-wavelength array

The VLBA consists of ten 25-m diameter telescopes spread over US territory from Hawaii in the west to New Hampshire and Saint Croix in the east. With a maximum baseline length of ≈8000 km and frequency coverage from 300 MHz to 86 GHz, the VLBA is a flexible and sensitive array. The homogeneity of the array, with all the antennas and instruments identical, is advantageous as it makes calibration straightforward. While the VLBA is a general-purpose imaging array, recently phase-referencing astrometry has occupied a major portion of its observing time. Currently there are several large astrometric projects on the VLBA. The Bar and Spiral Structure Legacy (BeSSeL) Survey is measuring parallaxes and proper motions of hundreds of Galactic masers associated with high-mass star forming regions. The Gould's Belt Distances Survey aims to measure distances to and provide a detailed view of star-formation in the Solar neighborhood. The Radio Interferometric Planet Search (RIPL) is an astrometric search for planets around nearby low-mass stars. The Pleiades Distance Project seeks absolute parallaxes for up to 10 cluster stars in order to resolve the current cluster distance controversy and provide a solid foundation for many aspects of stellar astrophysics. Finally, the Megamaser Cosmology Project seeks to measure H0 directly for 10 H2O megamaser galaxies well into the Hubble flow in order to independently constrain H0 with ± 3% accuracy.

Figure 5

Figure 5. Schematic portrayal of the locations of the Very Long Baseline Array (VLBA) antennas. The VLBA has ten 25-m diameter antennas spanning the globe from Hawaii to New Hampshire and St. Croix.

The VERA array consists of four 20-m diameter telescopes located across Japan with a maximum baseline length of 2300 km (see Figure 6). Each antenna is equipped with two receiver systems that are independently steerable in the focal plane. As such VERA can observe target and reference sources simultaneously to effectively cancel tropospheric fluctuations. VERA is also the only array dedicated full-time to phase-referencing astrometry. Most of the observing time is spent on parallax measurements of maser sources tracing spiral structure in the Milky Way and of red giant stars.

Figure 6

Figure 6. Schematic portrayal of the locations of the VERA antennas. The VERA array has four 20-m diameter antennas spanning the Japan and is dedicated to astrometric observations.

The EVN array has antennas distributed across Europe as well as in other countries, including China, South Africa and USA. The EVN is most sensitive at frequencies < 10 GHz, owing to some large antennas: the Effelsberg 100-m, the Jodrell Bank 76-m and soon the Sardinia 64-m telescope. The array has been used for astrometric measurements of OH masers at 1.6 GHz, methanol masers at 6.7 GHz, and active stars.

The LBA in Australia is the only VLBI array regularly operating in the southern hemisphere. It has high sensitivity when the Parkes 64-m and Tidbinbilla 70-m telescopes are included. The LBA has provided astrometric measurements for southern pulsars, and hopefully it will soon be used for maser parallaxes. This is necessary in order to trace the 3-dimensional structure of the roughly one-third of the Milky Way that cannot be observed from the north.

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