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Thread: The positioning of radio telescopes.

  1. #1
    Join Date
    Jan 2020

    The positioning of radio telescopes.

    I have asked this question on many forums before but I have never had a satisfactory answer. Why are the radio telescope dishes at ALMA spaced in such an apparently random way? They are all over the place! Why is this? Thanks.

  2. #2
    Join Date
    Sep 2003
    The beautiful north coast (Ohio)
    Hi Chris Martin, welcome to CQ

    The short answer

    ALMA consists of a giant array of 12-m antennas (the 12-m array), with baselines up to 16 km, and an additional compact array of 7-m and 12-m antennas to greatly enhance ALMA's ability to image extended targets, located on the Chajnantor plateau at 5000m altitude. The antennas can be placed in different locations on the plateau in order to form arrays with different distributions of baseline lengths. More extended arrays will give higher spatial resolution, more compact arrays give better sensitivity for extended sources. In addition to the array of 12-m antennas, there is the Atacama Compact Array (ACA), consisting of twelve 7-m antennas and four 12-m antennas. This array will mostly stay in a fixed configuration and is used to image large scale structures that are not well sampled by the ALMA 12-m array.
    The ALMA 12-m array will cycle from its most compact configuration, with maximum baselines of ~150 m, to its most extended configuration, with maximum baselines of ~16 km (when completed), and back. The Atacama Compact Array (ACA) has two configurations, one of which is a north-south extension to provide a better beam shape for far-north/far-south targets. See Appendix A of the Proposer's Guide for the configurations available in the current Cycle.
    Link to the Proposer's Guide

    4.2 Summary of capabilities offered in Cycle 7
    The Cycle 7 capabilities are described in Appendix A. In summary they are:

    Number of antennas

    At least forty-three antennas in the 12-m Array.

    At least ten 7-m antennas (for short baselines) and three 12-m antennas (for making single-dish maps) in the ACA.

    Receiver bands

    Receiver Bands 3, 4, 5, 6, 7, 8, 9, and 10 (wavelengths of about 3.0, 2.0, 1.6, 1.3, 0.85, 0.65, 0.45, and 0.35 mm, respectively).

    12-m Array Configurations

    Maximum baselines between 0.16 km and 16.2 km depending on array configuration and subject to the following restrictions:

    The maximum possible baseline for Bands 8, 9 and 10 is 3.6 km.

    The maximum possible baseline for Bands 3, 4, 5, 6 and 7 is 16.2 km.
    My take on all that is that it is not random. Different sets of dishes are arranged in different configurations, so that ALMA can be used for multiple projects at the same time, using different sets of dishes.

    I believe similar things are done at the Very Large Array, where a particular project may use one or several of the dishes at a time.
    At night the stars put on a show for free (Carole King)

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  3. #3
    Join Date
    Oct 2001
    The Space Coast
    Dang it, Swift beat me to it. I had a more concise version of his post ready to go, but the browser tanked!

    "Off went his rocket at the speed of light
    Flying so fast there was no day or night
    Messing around with the fabric of time
    He knows who's guilty 'fore there's even a crime

    Davy, Davy Crockett
    The buckskin astronaut
    Davy, Davy Crockett
    There's more than we were taught"

    -They Might Be Giants, "The Ballad Of Davy Crockett (In Outer Space)"

  4. #4
    Join Date
    Mar 2004
    In more detail - each pair of antennae (elements) in a radio interferometer measures, at each time, how much the field of view looks like
    a sine-wave pattern of brightness, with a finer cycle spacing the farther apart the two elements project in that direction (compared to the wavelength being used) and direction of the spacing parallel to the spacing between elements. One pair of antennas provides very limited information. But for N antennas, the number of different baselines is N(N-1), rising very rapidly. For the VLA and ALMA, for example, the array layout incorporates both the desired baseline lengths and useful behavior as the Earth rotates, to fill in as many different directions and spacing of the sine-wave pattern as possible, allowing the highest-fidelity reconstruction of the actual distribution of radio sources on the sky. (In the jargon, "filling the (u,v)-plane.) So the VLA is not symmetric E-W because that would lead to unnecessarily redundant baselines for sources near the celestial equator.

    Sometimes local topography rears its head (or rocks). The VLA's northern arm is a bit shorter than the SW and SE ones because there are hills there. Which is a shame, because having a longer north arm would give more consistent angular resolution both NS and EW for objects seen looking south.

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