The use of interferometry for astronomical imaging has successfully been utilized in the aerospace industry since its introduction in the 1950s. As the growing interest in exploring our universe continues to unfold, particularly since the recent SpaceX launch, astronomers continue to work towards discovering new technological methods. To this end, a recent study published by Australian physics and astronomy researchers developed a suitable radio-frequency reference for a type of interferometry known as very-long-baseline interferometry (VLBI) to improve the resolution of this technique for exploratory missions.
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Understanding Interferometry
When radio astronomers attempt to capture high-resolution images of astronomical objects such as stars, nebulas, and galaxies, they typically utilize an astronomical interferometer. This unique array of separate telescopes overcomes the limitations around the diameter of an ordinary telescope and the range of wavelengths it is capable of receiving.
The working principle of a typical interferometer involves taking a beam of light or another type of electromagnetic radiation, splitting it into two halves by a beam-splitter or half-mirror, which will often be a piece of glass that is thinly coated with a layer of silver. Once these beams are split, one reference beam will shine onto a mirror and then a camera or detector, and the second beam will shine at or through the object of interest to be measured. This second beam will eventually meet the first beam at the screen, camera or detector, therefore creating a pattern of dark and light areas known as interference fringes1.
The exact pattern of interference will depend upon the different way or distance in which the second beam has traveled. Interferometers are capable of examining and measuring these interference fringes to provide precise information on the object of interest.
Very-Long-Baseline Interferometry (VLBI)
VLBI is a dominant type of interferometry that is widely used in radio astronomy, as this method is capable of linking together separated radio telescopes to provide precise and vast detail on the universe. The signals received by VLBI radio telescopes are amplified by their antennas, then digitalized and sent to a correlator that either stores them on tape for future use or sends them across the network. The correlator is capable of transforming the received signals from each pair of antennas to determine the brightness distribution of a particular object in the sky at radio frequencies. As computer technology continues to improve, astronomy researchers are hopeful that faster and cheaper network links will develop to maintain VLBI observations as an indispensable astrophysical tool.
Combining VLBI with Radio Frequency Over Fiber (RFOF) Technology
In a recent study published by Optica, a group of Australian researchers led by Tim Rayner developed a suitable radio-frequency reference for VLBI over a telecom optical-fiber link between radio telescopes that are approximately 310 kilometers (km) apart through an innovative phase-conjugation technique. Their RFOF system was found to yield a relative frequency stability that exceeds that of two independent hydrogen masers. Through the use of split-antenna experiments, the researchers found that their RFOF transfer system eliminated atmospheric perturbations reliably and cost-effectively.
In the Optica study, the researchers claim that their passive RFOF method enables phase-coherent fiber-based transfer of a frequency between two widely separated VLBI antennas, thereby demonstrating the first application of this RFOF method for real-world VLBI measurements2.
References:
- “Inteferometers” – Explain That Stuff
- “Long-distance telecom fiber transfer of a radio-frequency reference for radio astronomy” He, Y., Baldwin, K. G. H., Orr, B. J., Warrington, B., Wouters, M. J., Luiten, A. N., Mirtschin, P., Tzioumis, T., Phillips, C., Stevens, J., Lennon, B., Munting, S., Aben, G., Newlands T., & Rayner, T. Optica. (2018). DOI: 10.1364/OPTICA.5.000138.
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