Why does the Event Horizon Telescope (EHT) not include telescopes from Africa, Asia or Australia? Why were not they included?
Africa doesn't have a radio telescope in the frequency range necessary (230-450GHz) to participate in the EHT array. For Asia Wikipedia lists the "Yevpatoria RT-70 radio telescope" as capable up to 300 GHz and located in western Crimea. For Australia Wikipedia lists the "Parkes Observatory" whose frequency is also too low, with a maximum of 26 GHz. In Japan there is the "Nobeyama Millimeter Array" but it only goes up to 230 GHz.
The only radio telescopes that go up to the required frequency range (according to the Wikipedia webpage, which may not be a complete list) are:
As you can see the list is certainly incomplete. Another list of the world's radio telescopes is the website TheSkyIsNotTheLimit.org which offers this graphic:
If you center Google Earth on the Atacama Pathfinder Experiment (APEX), the center of the EHT array, you'll notice that those other radio telescopes aren't included in the view:
Would it increase the diameter if they would include some from there? If so, again why did not they?
The west coast of Africa could assist if they had a modern radio telescope on a mountain top, but they don't. Even so, such a radio telescope would be pointing sideways, through the thickness of the atmosphere over the ocean; radio telescopes work best pointing upwards and you want the ability to pan, not to be constricted to a limited range of movement. Such distant points in the array will be simultaneously active only for a short period of time, but they serve as a hand-off as one rotates into position and the other rotates out of view.
If you rotate the globe a bit you lose Hawaii but but gain western Europe (including Crimea, if they upgrade) and the west coast of Africa, including Gamsberg Mountain (in close proximity to the High Energy Stereoscopic System (H.E.S.S.)) in Namibia:
A reason to be interested in Gamsberg is because the mountain is owned by the "Internationale Amateursternwarte" referred to as the IAS. It was established in Germany March 1999 by a group of dedicated amateur astronomers and supported by the Max Planck Institute for Astronomy. This image tells the tale:
There are discussions underway with respect to Gamsberg, see : "The African Millimetre Telescope" (June 2017), by Michael Backes, Cornelia Müller, John E. Conway and Roger Deane, on page 1:
"Although the EHT already constitutes an impressive VLBI network of mm-wave radio telescopes, their spacial distribution is clustered around the Americas, thus limiting its capabilities. Significant improvement can be achieved by adding a single mm-wave radio telescope in Africa, nominally called the Africa Millimetre Telescope.
3. The Africa Millimetre Telescope
Adding the Africa Millimetre Telescope (AMT)$^{12}$, a single mm-wave radio telescope on the African continent, to the EHT network will significantly increase the coverage in the $u$-$v$-plane (see fig.2). This will greatly improve the imaging capabilities of the EHT and, hence, advance its capabilities to directly image the ‘shadow’ of Sgr A∗. In particular, it will enable the ‘Eastern sub-array’, including the IRAM 30 m telescope, NOEMA, SPT, ALMA, and AMT to perform imaging observations, thus adding a significant amount of daily observing time of Sgr A∗ to the EHT (see fig. 1 on the right). As shown in fig. 1, the AMT will have common baselines for observations of Sgr A∗ with all the high-sensitivity telescopes (the IRAM 30 m telescope on Pico Veleta, NOEMA, ALMA, and the LMT) as well as with all the other EHT telescopes. The $u$−$v$-plane coverage of the current EHT configuration is presented in figure 2; included is the improvement in coverage by adding the AMT.
Figure 1: Baselines of the current EHT VLBI network (in yellow) and additional baselines provided by the AMT (in red). Note: the Combined Array for Research in Millimeter-wave Astronomy (CARMA) ceased observations in 2015, IRAM PV denotes the IRAM 30 m telescope on Pico del Veleta, Plateau de Bure denotes NOEMA, and Gamsberg denotes the AMT.
...
Potential sites on the African continent where Sgr A∗ can be observed at elevation angles ≥ 40° and that are at sufficiently high altitude to ensure an average precipitable water vapour column of less than 6 mm include sites on Mt. Kilimanjaro (≥ 4,300 m a.s.l.) in Tanzania and the Sani Pass in the Drakensberg mountains in Lesotho (≥ 3,050 m a.s.l.). However, primarily because
of temporal overlap in visibility of Sgr A∗ with the mm-wave telescopes in the Americas, the western-most site, Mt. Gamsberg (2,347 m a.s.l.) in Namibia has been chosen as the primary site
for further investigations. Additional supporting arguments for this site are that the land is owned by the Max-Planck Society, and that the government of Namibia is very encouraging of astronomy development.
The addition of Gamsberg will significantly improve the coverage, but construction hasn't been approved yet.
In general, the fidelity of images produced by an interferometric array increases as additional telescopes are added to the array.
Yes, but while adding one anywhere does something adding one or more in perfect locations maximizes the return on the enormous investment.
In "Synthesis Imaging in Radio Astronomy II", A Collection of Lectures from the Sixth NRAO/NMIMT Synthesis Imaging Summer School. Edited by G. B. Taylor, C. L. Carilli, and R. A. Perley. ASP Conference Series, Vol. 180, 1999 (.PDF - Warning: 43 MB):
This is a collection of papers, on page 537:
27. Interferometric Array Design
M.A. Holdaway & Tamara T. Helfer
National Radio Astronomy Observatory, Tucson, AZ 85721, U.S.A.
Abstract. We investigate some of the principles which lead to the design of radio interferometric arrays and array configurations, including both abstract issues such as sensitivity and Fourier plane coverage, and practical issues such as moving antennas and site topographical constraints. We draw on the design and history of existing arrays and also give a glimpse of what ideas and algorithms are helping design new instruments such as the Submillimeter Array
(SMA) and the Millimeter Array (MMA).
1. Introduction
Array design can include a variety of topics: how many antennas should the telescope have, and how big should they be? Are there astronomical requirements which dictate an aspect of the array layout? How many antenna configurations will there be, and how will the different configurations work together? How should we design each individual configuration? But the central topic of array
design deals with how to efficiently sample the Fourier plane. Each interferometer, or pair of antennas, at a given moment in time samples a single point in the Fourier plane, and we need to arrange the antennas in such a way that the set of sampled points permits us to make high quality, high sensitivity images. Since most antennas require a fair amount of infrastructure with not insubstantial capital costs on the ground beneath their bases (called antenna pads), it is important to design a good set of antenna configurations which adequately sample the Fourier plane before the array is built.
The above applies to compact arrays (all the antennae involved are interconnected and local to each other) and VLBI where data is recorded and combined at a later date. When the wavelength being measured is less than a millimeter any unwanted movement (or movement not accounted for) of even a millimeter introduces serious errors, each of which needs to be calculated and removed; leaving multiple sources of very tiny errors (which randomly add and subtract from each other, introducing noise).
Read on to page 547:
4.3. VLA-Y and GMRT-Y
The main benefit of the VLA's "Y" configuration is that it is a convenient 2-D arrangement of antennas which gives reasonable 2-D snapshot Fourier plane coverage. The BIMA and OVRO "T" arrays are similar in concept. The bad things about a "Y" or a "T" are that the regularity in the antenna directions along the arms will lead to a sort of grating response in the point spread function, and that it will take several hours of earth rotation synthesis for the Fourier samples to overcome this deficit.
The "Y" and "T" configurations are compromises: they seek to maintain the convenience of a 1-D array in reconfiguring the antennas, but would also like to get good Fourier plane coverage. As such, they are like arrays of fractal dimension 1.5: better than 1-D, not so good as fully 2-D arrays. Examples for the VLA's coverage and beams for a snapshot and a full track are shown in Figures 27-3 and 27-4.
The Giant Metrewave Radio Telescope (GMRT) in India has 14 inner antennas in a 1 km configuration and 16 in an outer configuration in the shape of an irregular "Y" (see Figure 27-5). The two configurations will often be used separately (the 30 m antennas were not designed for reconfiguration). The irregularity of the "Y" mainly stems from where land could be obtained, but it also produces a snapshot beam with 16 antennas which has much lower sidelobes than the VLA snapshot beam has with 27 antennas.
The wye (Y-shaped) configuration uses the least antenna groups to obtain the best result by filling in enough points that it's convenient to fill in the Fourier plane using an optimized algorithm, it is also convenient to position such a shape on existing continents. A five armed star provides better results, again with fewer antennas, but is obviously more expensive than the wye configuration. The best configurations, such as the randomly filled circle, don't fit multiple continents and their intervening oceans.
Notice the Y-shaped configuration of the current EHT array sites in user rugk's question. More sites in a narrow slice of the Earth's circumference allow increased capabilities but if a particular sector is competently (capability) covered it's a duplication of resources and an increase of processing time compared to a perfect location that fills in a hole. A site that is near the edge is useful to increase the observation window, but one too far away to be able to simultaneously observe with a sufficient number of other site isn't anywhere near as useful. Wherever the site is it must be capable of operating at the necessary frequencies and have enough sensitivity to gather useful data at that distance. It's a tall order.
