About the e-MERLIN/VLBI National Facility
MERLIN (the Multi-Element Radio Linked Interferometer Network) is an array of seven radio telescopes distributed over central England and operated from Jodrell Bank Observatory (JBO) as a National Facility by the University of Manchester on behalf of the Science and Technology Facilities Council (STFC). The outlying telescopes are connected via microwave links to a central correlator situated at Jodrell Bank. This combination of radio antennas forms the equivalent of a single, integrated radio telescope and is able to image astronomical objects with very high resolution at frequencies from 151 MHz to 24 GHz. MERLIN is the only worldclass astronomical facility based entirely within the UK. The key to MERLIN's success is its high angular resolution. With radio telescope separations of up to 217 km, it is the only groundbased facility in the world that routinely matches the resolution of the Hubble Space Telescope (HST) and the new generation of 8- m class optical/IR telescopes such as Gemini and the Very Large Telescope (VLT). The capabilities of MERLIN are listed in the table below.
MERLIN was developed in the late 1980s from pioneering experiments in long-baseline interferometry at Jodrell Bank. It was designed to provide sub-arcsecond imaging of astronomical sources at centimetre wavelengths, primarily to study in more detail the radio galaxies and quasars imaged by the Cambridge aperture synthesis arrays. The original design goals were quickly surpassed, both technically and astronomically, and MERLIN was immediately recognised as a world-class instrument, evidenced by the many initial publications in Nature. A major upgrade in 1990, in which the resolution was increased by nearly a factor of 2 and the sensitivity by almost an order of magnitude, ensured that MERLIN was transformed into a general-purpose instrument capable of attacking a wider range of astrophysical problems. It has thus remained a world-class facility and is recognised as one of the leading strengths of UK astronomy.
MERLIN is a National Facility open to all users. As with other UK telescopes, the observing year is divided into two observing semesters (February to July and August to January). All observingproposals are peer-reviewed and, if appropriate, allocated observing time by the Panel for the Allocation of Telescope Time (PATT) of the Science and Technology Facilities Council (STFC). PATT allocates MERLIN observing time based on the perceived scientific merit of proposals and the actual observing time available in a given semester. Typically 30-40 proposals are received per semester, requesting a total of approximately 3000 hours of observing time. In order to maximise the efficiency of operation, flexible scheduling is employed so that observations are not normally scheduled in detail more than a few days in advance. This allows the array to take maximum advantage of prevailing weather/ atmospheric conditions or technical limitations. Observers are not normally expected to be present during observations but are encouraged to visit Jodrell Bank Observatory to perform the data reduction.
MERLIN often observes simultaneously with the European VLBI Network (EVN), an array of 13 telescopes distributed across Europe and Asia. In fact, joint MERLIN/EVN observations are an increasingly popular mode of observation within the EVN at 1.4 and 5 GHz due to the ability of the joint array to provide images of a wide range of radio structures from the arcsecond scale down to the milliarcsecond scale. The EVN Consortium Board of Directors and its associated Programme Committee and Technical & Operations Group coordinates EVN activities.
VLBI achieves the highest angular resolution of any branch of astronomy, enabling imaging at angular scales as small as 100 microarcseconds. In 1993 the EVN Board of Directors set up the Joint Institute for VLBI in Europe (JIVE) based in Dwingeloo, the Netherlands, as the home of the EVN data processor. The EVN has been at the forefront of VLBI developments, transforming its capabilities with new technologies. The MkIV tape recording system and data processor provide the world's first 1 Gbit/second-capable VLBI system. This capability, together with the large radio telescopes at Effelsberg, Jodrell Bank and Westerbork, make the EVN the instrument of choice for high sensitivity VLBI.
Lovell surface upgrade
A £2.2M upgrade of the 76-m diameter Lovell Telescope at Jodrell Bank Observatory, the world's third-largest fully-steerable radio telescope, which forms an integral part of MERLIN and the EVN, is complete. A major part of this upgrade was the replacement of the telescope's 340 surface panels, thus enabling the instrument to be used at higher observing frequencies. The inclusion of the refurbished Lovell telescope in 5-GHz MERLIN and EVN observations (from the autumn of 2003) results in a sensitivity increase of a factor of ~3.
MERLIN currently uses microwave links to send astronomical data back from the remote stations. These links have a limited bandwidth so much of the data is thrown away. In order to increase the sensitivity of the telescope the current links are being replaced by optical fibre links which will carry a bandwidth of 4 GHz, compared to the current limit of 30 MHz, increasing the sensitivity of the array by a factor of around 30. This vast increase in data means that the old correlator will no longer be able to cope, so a new correlator is under construction which will be capable of processing over 200 Gbit/s.
Another major development which is part of the upgrade is frequency flexibility - the ability to alter the observing band of the entire array in a matter of minutes using rotating carousels of receivers. Some telescopes in the array already have this capability, while the rest require the visit of an engineer to change the receiver. When e-MERLIN becomes operational the telescope will be able to switch rapidly between 1.4, 5, 6 and 22 GHz. This is required in order to take advantage of optimum conditions for high frequency observations where atmospheric conditions can severely affect results.