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.
For more information see the MERLIN/VLBI National Facility pages
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