The Future Development of MERLIN

MERLIN - Future Developments

MERLIN (The Multi Element Radio Linked Interferometer Network) is a cornerstone of the UK's ground-based astronomical facilities, providing sub-0.1 arcsecond radio imaging, polarimetry, spectroscopy and astrometry. As we enter a new era of high resolution astronomy, with infrared adaptive optics already demonstrated on GEMINI and with NGST and ALMA on the horizon, MERLIN is uniquely placed to provide complementary radio observations. The development plan for MERLIN capitalizes on this position. After completion of the current programme of frequency flexibility, replacement of the Defford telescope, installation of new optical fibre links and the inclusion of the resurfaced Lovell Telescope, the instrument will be transformed. Continuum imaging will be possible with more than an order of magnitude improvement over the current sensitivity and at three times the current resolution. As well as providing a vital radio complement to key programmes on other major facilities, such as the study of starformation at high redshift and within our own galaxy, new and unique science will be possible with the enhanced MERLIN:

the imaging, at resolutions of 15--50 mas, of warm (100s K) gas and dust in stars, stellar envelopes and star--forming regions and in Seyfert and starburst galaxies.
the imaging of non--thermal emission, at resolutions from 15--50 mas, from normal galaxies in the local group to blue (starburst) irregulars at cosmological distances.
the polarimetric imaging, at resolutions of 15--120 mas, of non--thermal emission from stars and stellar winds.
the imaging of maser emission, at resolutions of 10--120 mas, from relatively un-explored species such as methanol, ammonia and excited state OH in protostellar disks and star formation regions.
the polarimetric imaging, with astrometric precision, of OH and H2O masers in stellar envelopes.
the multi--wavelength polarimetric imaging of rapidly variable sources such as turbulent stellar winds, X--ray transients, novae, extragalactic SNR, GRBs and gravitational lenses.

Road map to MERLIN3

The development of MERLIN3 has been envisaged in three stages

Array Upgrade : new 12-15 GHz receivers, full frequency flexibility, replacement of Defford Telescope

Prototype Fibre Link between Jodrell Bank and Cambridge

Array Broad-banding : to link all telescopes with optical fibres and install a new correlator Along with the concurrent resurfacing of the Lovell telescope, these upgrades will provide, without significant increase in running costs, a 25-fold sensitivity increase at the prime operating frequency of 5 GHz with 50 mas resolution and the ability to carry out reliable imaging at 21-24 GHz with 8 mas resolution.

MERLIN3 Capabilities

Maximum baseline: 217 km
Observing bands: 1.3 - 1.8, 4 - 6, 12 - 15 and 21 - 24 GHz
Resolution: 150 mas at 1.4 GHz to 8 mas at 24 GHz
Astrometry: < 1 mas at 4 - 15 GHz
Field-of-View: 10 arcmin diameter at 1.4 GHz
Sensitivity: 1.0 uJy rms in 100 hr at 4 - 15 GHz
Spectroscopy: Up to 4096 channels per IF < 10 m/s velocity resolution

The exact sequencing of the components of the development is not critical, nor is the division into three stages. However, for clarity we discuss the three stages separately below.

Array Upgrade

This first phase is designed to:

extend the short wavelength limit where good imaging is possible by a factor of at least three; at a stroke this will increase the potential angular resolution of the array by the same factor.
and provide for very flexible deployment of a common suite of receivers permanently mounted on each telescope.

Specifically, this involves the installation of new 12-15 GHz receivers. This frequency band is chosen for the optimum balance of resolution and sensitivity as well as covering the 12 GHz methanol line. The current Defford telescope cannot operate above 5 GHz, nor can it be made frequency flexible, therefore a replacement telescope must be built. The most cost-efficient approach is to copy the existing 25m E-systems design: all the required design data have been made available and the contruction and outfitting can be done at modest cost. Control, receiver and communication equipment would be identical to that used in the three other MERLIN E-systems telescopes. In order to make the larger 18-21cm receiver permanently available on these telescopes, a deployable microwave lens will be added. Scaled optical models have already been built and tested and the final design is in progress.

Achieving these technological goals will immediately give a major boost to the scientific capabilities of MERLIN and provide markedly improved operational effiency without increasing the running costs of the array. The immediate astronomical goals are to provide new and unique data on:

the early stages of the formation of stars and pre--planetary disks via observations of the methanol line. This line appears to be the most luminous tracer of active starformation and pioneering observations have revealed rotating discs around high mass protostars. Continuum observations at 15 GHz will have a resolution of 15 AU at 1 kpc, while the positions of bright masers can be fitted to < 1 AU at this distance.
the mass outflows from evolved stars which replenish the interstellar medium with dust and elements lighter than iron via simultaneous observations of OH (1.6 GHz) and H2O (22 GHz) masers.
a variety of variable radio sources, including X--ray transients (``micro--quasars''), galactic novae and gravitational lenses: a rapid response can only be made with frequency flexible receivers. Unlike the VLA, MERLIN is always ready at maximum resolution.
the interaction of relativistic jets with the interstellar medium in the nuclei of ``active'' galaxies.

Prototyping a broad-band link

Phase 2, which could proceed in parallel with Phase 1, is aimed at completing the development of the fibre optic technology required for phase 3 and prototyping the operation of the Jodrell Bank--Cambridge baseline in particular for remote wide--band VLBI.

The development work will follow on from The University of Manchester's PIPSS award in collaboration with British Telecom which has already shown that bandwidths of several GHz can be achieved over 200 km links on a single fibre using analogue techniques.

The broad-band Jodrell_Cambridge link will enable optimum use to be made of the 32m telescope with the new JIVE 1 Gbit/sec VLBI correlator, which is now in operation. Currently the data rate from the 32m is limited to 128 Mbit/sec by the microwave links used to send the radioastronomy signals back to Jodrell Bank. The Jodrell--Cambridge link will also allow experience to be gained in the operation of broad--band fibre links preparatory to phase 3. The very high sensitivity, high availability single baseline between Jodrell Bank and the 32m will also allow new science to be done independent of the VLBI Network. An example is the study of the rapidly variable radio emission associated with the expanding fireball of gamma ray bursters.

Broad-banding

The third major phase of the transformation is an order--of--magnitude increase in sensitivity of the entire array whose effect is essentially to produce a new scientific instrument. The main features will be:

The current narrow--band microwave links will be replaced with optical fibres, increasing the bandwidth from 16 MHz per polarization to 2 GHz.
A new correlator will be built to process the wide-band data and act as a powerful and flexible spectrometer with built-in interference rejection.

The order--of--magnitude improvement in sensitivity will enable MERLIN3 to image thermal emission from warm gas and dust with a brightness temperature of a few hundred~K with resolutions of 15--50 mas. If, as is expected, the Lovell telescope is upgraded to provide good performance at < 6cm wavelength, the sensitivity of MERLIN3 will be 25 times that of the current system.

The unique combination of resolution, sensitivity and frequency coverage of MERLIN3 opens up a spectacular range of new possibilities in both galactic and extra-galactic astronomy. In particular it will bring within reach the thermal and ionized universe of stars, nebulae and active galaxies in a wavelength regime unobscured by dust. Stellar astronomy in particular will be revolutionised by the ability to detect thermal emission from warm gas and dust with linear resolution of a few AU or better. MERLIN3 will therefore provide complementary science to ALMA (which studies cold/cool material at somewhat lower resolution) and NGST. Other MERLIN3 opportunities include fundamental astrometry with a positional accuracy < 0.1 mas; planetary searches; studies of star forming regions and active stars, extragalactic SNR and starburst galaxies, AGN and a range of cosmological studies.