China
Africa
Explore chapters and articles related to this topic
Imaging
Published in C. R. Kitchin, Astrophysical Techniques, 2020
The Very Long Baseline Array (VLBA) VLBI system uses ten 25-m telescopes spread out over the United States from Hawaii to the US Virgin Islands. Its maximum baseline is 8,600 km and around 50 GHz, its resolution can be 150 microarcseconds. It has, for example, recently been able to measure the parallax for the Orion nebula to an accuracy of 100 microarcseconds, reducing the previously accepted distance for the nebula from 475 to 385 pc. It has also recently had new receivers installed and with upgraded computers it is now improved by a factor of around 5,000 compared with its performance when it first started operating in 1993. Along with the Arecibo, Effelsberg and Green Bank Telescope (GBT) dishes and the VLA it also forms the High Sensitivity Array (HSA). This can improve upon the sensitivity of the VLBA by a factor of ten and has a maximum baseline of 10,300 km.
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.
Radio Galaxies and Quasars
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
Many of the unanswered questions about radio galaxies relate to the initial stages of the jets. What causes the jets? How are they collimated? How are the particles accelerated to such high energies? Because of their great distance, extremely high angular resolution is needed to study the structure of these regions. The highest resolution observations are made using Very Long Baseline Interferometry (VLBI, discussed in Volume I, Chapter 6). Conventional VLBI observations include wavelengths as short as 1 cm, at which VLBI can obtain a resolution of 8×10−10 radians, or a few tenths of a milliarcsecond. Very long baseline interferometry (VLBI) This is analogous to being able to see the date on a penny (width about 1 mm) at a distance of 1250 km. A concerted effort to use a global VLBI array at mm and sub-mm wavelengths has recently been undertaken with the goal of resolving the event horizons of two nearby supermassive black holes Black hole!supermassive — SgrA*, the 4×106M⊙ black hole in the center of the Milky Way galaxy, Galactic center!SgrA* Galactic center!black hole and the 5×109M⊙ black hole in M87 (i.e., Virgo A). Virgo A, M87!black holeActive galactic nuclei (AGN)!Virgo A, M87 The resolution of this instrument, called the Event Horizon Telescope, Event horizon telescope (EHT) or EHT for short, will be of order tens of micro-arcseconds. At the time of the writing of this volume, the results of the first set of EHT observations have not been reported. At a distance of approximately 16 Mpc, the event horizon of the 5×109M⊙ black hole in M87 has an angular size of 22 micro-arcseconds. The event horizon of SgrA* is expected to be approximately 53 micro-arcseconds. Unfortunately, though, for all other radio galaxies and quasars, at much larger distances, the resolution with VLBI at even the shortest wavelengths will be insufficient to image the AGN engine itself.
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.