Annals of physics

The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ~1.3 mm, EHT angular resolution (λ/D) is ~25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit s^(−1), exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

/pdf/first-m87-event-horizon-telescope-results-ii-array-and-56170vksd8.pdf

First M87 Event Horizon Telescope Results. II. Array and Instrumentation

Journal of Applied physics,Vol.33 : 1962年に発表されたレオロジー関連の論文

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Interferometry And Synthesis In Radio Astronomy

Frequency stabilization in a high-finesse optical cavity is limited fundamentally by thermal-noise-induced cavity length fluctuations. Scientists have now developed a single-crystal silicon system that offers a fractional frequency instability of 1 × 10−16 at short timescales and supports a laser linewidth of less than 40 mHz at 1.5 µm.

A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity

Two nominally identical ultra-stable microwave oscillators are compared. Each incorporates a sapphire resonator cooled to near 6 K in an ultra-low vibration cryostat using a pulse-tube cryocooler. The phase noise for a single oscillator is measured at −105 dBc/Hz at 1 Hz offset on the 11.2 GHz carrier. The oscillator fractional frequency stability, after subtracting a linear frequency drift of 3.5×10-14/day, is characterized by 5.3×10-16τ-1/2+9×10-17 for integration times 0.1s<τ<1000s and is limited by a flicker frequency noise floor near 1×10-16.

Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators

The design and implementation of a novel frequency synthesizer based on low phase-noise digital dividers and a direct digital synthesizer is presented. The synthesis produces two low noise accurate tunable signals at 10 and 100 MHz. We report the measured residual phase noise and frequency stability of the syn thesizer and estimate the total frequency stability, which can be expected from the synthesizer seeded with a signal near 11.2 GHz from an ultra-stable cryocooled sapphire oscillator (cryoCSO). The synthesizer residual single-sideband phase noise, at 1-Hz offset, on 10and 100-MHz signals was -135 and -130 dBc/Hz, respectively. The frequency stability contributions of these two sig nals was σy = 9 × 10-15 and σy = 2.2 × 10-15, respectively, at 1-s integration time. The Allan deviation of the total fractional frequency noise on the 10- and 100-MHz signals derived from the synthesizer with the cry oCSO may be estimated, respectively, as σy ≈ 3.6 × 10-15 τ-1/2 + 4 × 10-16 and σy ≈ s 5.2 × 10-2 × 10-16 τ-1/2 + 3 × 10-16, respectively, for 1 ≤ τ <; 104s. We also calculate the coherence function (a figure of merit for very long baseline interferometry in radio astronomy) for observation frequencies of 100, 230, and 345 GHz, when using the cry oCSO and a hydrogen maser. The results show that the cryoCSO offers a significant advantage at frequencies above 100 GHz.

Ultra-Stable Very-Low Phase-Noise Signal Source for Very Long Baseline Interferometry Using a Cryocooled Sapphire Oscillator

Two nominally identical ultra-stable cryogenic microwave oscillators are compared. Each incorporates a dielectric-sapphire resonator cooled to near 6 K in an ultra-low vibration cryostat using a low-vibration pulse-tube cryocooler. The phase noise for a single oscillator is measured at -105 dBc/Hz at 1 Hz offset on the 11.2 GHz carrier. The oscillator fractional frequency stability is characterized in terms of Allan deviation by 5.3 x 10^-16 tau^-1/2 + 9 x 10^-17 for integration times 0.1 s < tau < 1000 s and is limited by a flicker frequency noise floor below 1 x 10^-16. This result is better than any other microwave source even those generated from an optical comb phase-locked to a room temperature ultra-stable optical cavity.

Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators with 1 x 10^-16 frequency instability

Extension of very long baseline interferometry (VLBI) to observing wavelengths shorter than 1.3 mm provides exceptional angular resolution (~20 μas) and access to new spectral regimes for the study of astrophysical phenomena. To maintain phase coherence across a global VLBI array at these wavelengths requires that ultrastable frequency references be used for the heterodyne receivers at all participating telescopes. Hydrogen masers have traditionally been used as VLBI references, but atmospheric turbulence typically limits (sub)millimeter VLBI coherence times to ~1-30 s. Cryogenic sapphire oscillators (CSOs) have better stability than hydrogen masers on these timescales and are potential alternatives to masers as VLBI references. Here, we describe the design, implementation, and tests of a system to produce a 10 MHz VLBI frequency standard from the microwave (11.2 GHz) output of a CSO. To improve long-term stability of the new reference, the CSO was locked to the timing signal from the Global Positioning System satellites and corrected for the oscillator aging. The long-term performance of the CSO was measured by comparison against a hydrogen maser in the same laboratory. The superb short-term performance, along with the improved long-term performance achieved by conditioning, makes the CSO a suitable reference for VLBI at wavelengths less than 1.3 mm.

https://iopscience.iop.org/article/10.1086/660156/pdf

Adapting a Cryogenic Sapphire Oscillator for Very Long Baseline Interferometry

A low maintenance long-term operational cryogenic sapphire oscillator has been implemented at 11.2 GHz using an ultra-low-vibration cryostat and pulse-tube cryocooler. It is currently the world's most stable microwave oscillator employing a cryocooler. Its performance is explained in terms of temperature and frequency stability. The phase noise and the Allan deviation of frequency fluctuations have been evaluated by comparing it to an ultra-stable liquid-helium cooled cryogenic sapphire oscillator in the same laboratory. Assuming both contribute equally, the Allan deviation evaluated for the cryocooled oscillator is σy ≈ 1 × 10-15τ-1/2 for integration times 1 <; τ <; 10 s with a minimum σy = 3.9 × 10-16 at τ = 20 s. The long term frequency drift is less than 5×10-14/day. From the measured power spectral density of phase fluctuations, the single-sideband phase noise can be represented by Lφ(f) = 10-14.0/f4+10-11.6/f3+10-10.0/f2+10-10.2/f+ 10-11.0 rad2/Hz for Fourier frequencies 10-3 <; f <; 103 Hz in the single oscillator. As a result, Lφ ≈ -97.5 dBc/Hz at 1-Hz offset from the carrier.

http://inspirehep.net/record/852153/files/arXiv:1004.2886.pdf

Nitin R. Nand

Papers

Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators

Ultra-Stable Very-Low Phase-Noise Signal Source for Very Long Baseline Interferometry Using a Cryocooled Sapphire Oscillator

Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators with 1 x 10^-16 frequency instability

Adapting a Cryogenic Sapphire Oscillator for Very Long Baseline Interferometry

Ultra-Low Vibration Pulse-Tube Cryocooler Stabilized Cryogenic Sapphire Oscillator With ${\hbox{10}}^{-16}$ Fractional Frequency Stability