Array Broad-banding : to link all telescopes with optical fibres and install a
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.
- 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.
This first phase is designed to:
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.
- 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.
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
- 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
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
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.
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 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 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
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.