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The 32-metre Telescope at Cambridge used both in MERLIN and the European VLBI Network

Changing a VLBI tape.

The EVN correlator at JIVE. This is located in the Netherlands to which the tapes, recorded at telescopes all over the world, are brought for the signals recorded on them to combined.

The Hubble Deep Field was observed for 10 consecutive days by the Hubble Space Telescope. Over a thousand galaxies were detected at various stages of evolution. The majority were so faint that they had never been seen before. The most distant are seen as they existed not long after the origin of the Universe.

MERLIN/VLA contour line maps superimposed onto the corresponding galaxy images from the Hubble Deep Field. That shown above has an active galactic nucleus ejecting beams of relativistic particles out to either side, giving rise to twin lobes of radio emission. That shown below is a typical starburst galaxy, the radio emission coming from the central region of the galaxy where there is a very high rate of star formation.

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The Merlin and VLBI National Facility

The development of MERLIN

The array of telescopes now known as MERLIN, the Multi-Element Radio-Linked Interferometer Network, first came into operation in 1980. The signals from five remote 25-metre telescopes, having a maximum separation of 134 km, were brought back over microwave radio links to Jodrell Bank. Here they were combined with signals from the Mark II or Lovell telescopes in a "correlator" which provided the raw data from which detailed images of radio sources could be produced in a computer. In a major upgrade to MERLIN, completed in 1991, a new 32-metre telescope was built at Cambridge to increase the maximum telescope separation to 217 km. This now gives MERLIN a typical resolution of 1/20 of a second of arc, equivalent to resolving a one-pound coin from a distance of 100 km. To make the system more sensitive, the bandwidths of the microwave links were doubled and a new, multi-bit, correlator built to combine the data more efficiently.

Using the Lovell Telescope in MERLIN

At wavelengths of 18cm and longer, the Lovell Telescope can be incorporated into the array to more than double the sensitivity. When the Lovell Telescope upgrade is completed in 2002, its inclusion into the array will boost the sensitivity of MERLIN in the key wavelength range around 6cm.

Very Long Baseline Interferometry (VLBI)

Even higher resolution than that achievable with MERLIN can be obtained by combining the signals from widely spaced telescopes in VLBI arrays. Jodrell Bank Observatory is a founder member of the European VLBI Network (EVN) whose telescopes span Europe from Spain to Finland and from the United Kingdom to Poland. The EVN now has a dedicated facility, JIVE (the Joint Institute for VLBI in Europe), at Dwingeloo in the Netherlands, where its new wideband correlator is located. Unlike MERLIN, where the data are combined together (correlated) in real time, the data received at the EVN telescopes are recorded on tape and played back weeks or months later at the correlator. Jodrell Bank computer and engineering staff made significant contributions to the development of the EVN correlator which combines the signals to allow radio source images to be made with angular resolutions as small as 100 microarcseconds. The 32-metre telescope at Cambridge and either the Lovell or Mark II telescopes at Jodrell Bank spend around 12 weeks each year as part of the EVN array. At wavelengths of 18 cm and longer the Lovell Telescope is used, whilst the Mark II is used for wavelengths of 6cm and below. The upgraded Lovell Telescope will significantly enhance the performance of the EVN at 5 and 6cm wavelengths. Due to the number of large telescopes participating, including the 76-metre Lovell Telescope, the Westerbork array in Holland and the 100-metre Effelsberg Telescope in Germany, the EVN provides the highest sensitivity VLBI array in the world.

The National Facility

The MERLIN and VLBI systems at the Jodrell Bank Observatory are now operated as a National Facility by the University of Manchester on behalf of the Particle Physics and Astronomy Research Council (PPARC). Its remit is to operate MERLIN for the benefit of the whole astronomical community and to
provide the support necessary to participate in the European and Global VLBI operations.

MERLIN's strengths

MERLIN is a unique instrument for high resolution radio imaging. It plays an invaluable role in being the only general purpose ground-based facility that routinely matches the angular resolution of the Hubble Space Telescope and the new generation of ground-based instruments, allowing much collaborative research to be carried out. MERLIN baseline lengths range from 11 to 217 km, overlapping the longest baselines of the VLA, (the Very Large Array) at 36km, and the shortest of the EVN at ~100 km. Thus MERLIN is capable of imaging classes of radio sources that are generally too small for the VLA to map or too extended for the EVN. An increasing practice is to combine MERLIN data with those from the VLA or EVN to produce combined-array images with high sensitivity and good image quality over a wide range of angular scales. Some of the many observations made using MERLIN and these other arrays will be described in the following pages.

The Future: e-MERLIN

There now exists an ambitious plan for the future development of MERLIN into a successor, e-MERLIN. The result will be an instrument with greatly enhanced capabilities able to address many new and exciting areas of science.

Optical Fibres

The most significant part of the development would be the replacement of the current microwave links with optical fibre links. These could carry a bandwidth of 4 GHz compared to the 28 MHz of the current links. With a new correlator capable of processing these wide-band signals the result would be an order of magnitude increase in MERLIN's sensitivity. If this is allied to improvements in the low-noise receivers, and the incorporation of the upgraded Lovell Telescope into the array at wavelengths below 18cm, the sensitivity of e-MERLIN will up to 30 times that of the current system!

Receivers and the Defford Telescope

It is hoped that the 1960-vintage Defford Telescope, unable to be used at the shortest MERLIN wavelengths, will be replaced with a low-cost copy of the existing 25-metre VLA-type telescopes. New receiving systems covering the 6-10 and 12-15 GHz frequency ranges would be installed on all MERLIN telescopes to enable exciting new science do be carried out, particularly the study of stars and cosmology. In addition, further receiver developments will allow full frequency flexibility on 1 minute timescales between all the main MERLIN operating frequency bands.

MERLIN/VLA observations of the Hubble Deep Field

MERLIN was used to observe the Hubble Deep Field for 18 days and its data were combined with that from 42 hours of observations with the VLA. A total of 87 ultra-faint radio sources were observed in the HDF and surrounding area and the very precise positions obtained by MERLIN were used to calibrate the positions of related galaxies seen in other wavebands. So far, 85% of the radio sources have been optically identified, of which 30% have active galactic nuclei and 70% are starburst galaxies.

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