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Imaging
Published in C. R. Kitchin, Astrophysical Techniques, 2020
Beyond e-MERLIN, the VLA and similar radio systems, we have VLBI. For VLBI, the elements of the interferometer may be separated by thousands of kilometres, range over several continents and provide milliarcsecond resolutions or better. In VLBI, the signals from each element are separately recorded along with timing pulses from an atomic clock (c.f. Event Horizon Telescope [EHT] – Section 1.2.3). The recordings are then physically brought together and processed by a computer which uses the time signals to ensure the correct registration of one radio signal with respect to another. Real-time analysis of the data from a VLBI system comprising the Arecibo radio dish and telescopes in the UK, Sweden, the Netherlands and Poland became possible a decade ago when they were linked via internet research networks – earning the system the name of e-VLBI.
Co-ordinates of heaven and Earth
Published in Martin Vermeer, Antti Rasila, Map of the World, 2019
The currently used method for continuously monitoring the rotation of the Earth is VLBI or very long baseline interferometry, which uses radio telescopes around the world. The method's precision is based on the stability of the places of the radio sources or quasars used. These are much further away than the stars we see in the night sky — millions of light years as compared to tens or hundreds of light years — making this a sensible assumption.
Single-ion, transportable optical atomic clocks
Published in Journal of Modern Optics, 2018
Marion Delehaye, Clément Lacroûte
Many laboratories and astronomical observatories would benefit from an access to those frequency references. For instance, Very Long Baseline Interferometry(VLBI) relies on the simultaneous observation of various radio signals by distant antennas. These signals are down-sampled using a local oscillator, the stability of which is of paramount importance in order to perform low-noise correlations and access high angular resolution. The local oscillators are usually synchronized to active hydrogen masers; a test using a better reference was conducted using a cryogenic sapphire oscillator [13]. However, the need for even better local oscillator stability has emerged recently [14,15]. The first synchronization to a remote atomic clock located in a National Metrology Institute has been reported [16], and is paving the way for synchronization to an optical clock. This would give access to investigation of compact radio sources or molecular emission from the interstellar medium.