MERLIN/VLBI National Facility

Newsletter

Number 6: March 2002
MERLIN VLBI AT JBO PROPOSALS ARCHIVE CONTACT

Contents
  1. Call for Proposals
  2. Director's Report
  3. Recent Science: MERLIN Measures Magnetic Fields In PNe
  4. High Temperature Superconducting Filters For MERLIN



Call for Proposals

The deadline for proposals for Semester 02B (October 2002 - January 2003) on MERLIN is April 9th, 2002. All details are in the MERLIN web area, specifically;

http://www.merlin.ac.uk/propsub/call.

Observing frequencies available:

L-Band: 1330MHz to 1430MHz & 1550MHz to 1730MHz
C-Band: 4.5GHz to 5.2GHz & (6GHz to 7GHz)*
K-Band: 21GHz to 24GHz

*Only a single baseline between Cambridge and Mk2 is available in the 6GHz to 7GHz methanol band.

The availability of the various frequency bands will be determined by the user demand, and the MERLIN TAG's view of the proposed science. The Mk2 telescope will be the MERLIN homestation for L-Band observations in Semester 02B. The Lovell telescope will be available for MERLIN L-Band operations for a period within Semester 03A.

Please note that as a result of recent cuts in the MERLIN operations budget, it is no longer possible to guarantee that all requested observing bands will be available in any given Semester. Within the limits of practicality, the choice of observing band will be driven by user demand. This situation is likely to persist until full frequency flexibility is achieved around 2005 as part of the e-MERLIN upgrade.

The system parameters for observation of a continuum source in good weather conditions are;

L-BandC-BandK-Band
Maximum angular resolution (mas)~150~40~8
R.M.S. noise level for 12 hr. on source (microJy/beam)~60 ~50 ~400
Maximum bandwidth per polarization (MHz)~15~15~1 5

The use of the Lovell telescope at L-Band reduces the 12 hour RMS noise level to ~35 microJy/beam. The maximum rate at which the observing frequency can be switched within an observing band will be approximately once every five minutes for multi-frequency synthesis observations. For spectral line work throughout the Semester, users are referred to Section 3.4 of the MERLIN User Guide Version 1.1. The maximum number of frequency channels per baseline to be divided between the 4 polarizations for bandwidths of 16 MHz, 8 MHz and 4 MHz are 64, 128 and 256, respectively. The number of frequency channels per baseline to be divided between the 4 polarizations will be 512 for bandwidths of 2 MHz or less. The minimum total bandwidth is 250 kHz.

Proposal forms, information on MERLIN Key Programmes, and further general information can be obtained via;

www: http://www.merlin.ac.uk
ftp: ftp.jb.man.ac.uk, directory: /pub/merlin/proposals
email: merlin@jb.man.ac.uk



Director's Report

As you are probably aware, e-MERLIN is now a funded and active project. On December 5 2001, at the same time as PPARC announced the decision that the UK is to join the European Southern Observatory (ESO), it also announced that it would continue to fund the operation of MERLIN, and later e-MERLIN. This decision enabled the release of funds from the Universities of Manchester, UMIST and Cambridge and from the UK's North West Development Agency. This fantastic news was greeted with champagne and mince pies at Jodrell Bank Observatory (JBO).

The total cost of e-MERLIN is estimated at 7.8M, with an additional contingency of 0.8M. This will fund the connection of the telescopes to JBO with fibre optic cables, enabling the transmission of bandwidths of up to 2 GHz per polarization; the construction of a new broadband correlator; the construction of broadband receivers; the replacement of much of the signal-processing electronics at the telescopes and the rewriting of much of the array control software. We aim to deliver the first broadband data to users in 2007.

This does not mean that MERLIN will stagnate in the intervening period. There will be two visible signs of progress along the way to the full e-MERLIN. First, in October 2003, timed to coincide with the new high-frequency capability of the JIF-funded upgrade of the 76-m Lovell Telescope, we hope to bring on-line new broadband receivers covering the range 4-8 GHz. This will produce an immediate increase in MERLIN's sensitivity of a factor of ~3 in this band as well as providing the ability to observe with the whole array at frequencies of CH3OH, H2CO and several excited-state OH lines. Then, in the autumn of 2005 we hope to have full frequency-flexibility, i.e. all of MERLIN's principal observing bands (1.4-1.6, 4-8 and 22 GHz) will be available on the array at all times. This will remove a major operational headache, will enable us to take full advantage of good weather to observe at 22 GHz and will enable rapid switching between observing bands for those projects that require them.

However, all is not good news. In order to be able to afford entry into ESO, PPARC has had to impose severe cuts on the operational budgets of most of the UK's observatories. For MERLIN this amounts to a cut of ~18%. This has two effects: first, it has a knock-on effect on e-MERLIN. Part of the original funding plan was to divert manpower resources from MERLIN operations into e-MERLIN construction, accepting a temporary decrease in operational efficiency. This plan has been badly affected by the cuts. The University of Manchester has mitigated the situation by increasing its funding but we are left with some hard choices. We are still examining the situation but it appears that some parts of the e-MERLIN project will have to be deferred until more funding is found. These will not affect the science that e-MERLIN will deliver but will affect its future operational efficiency.

Secondly, these operational cuts will have a major impact on our current ability to deliver science during the next few years at the high level of efficiency our users have come to expect. Austerity measures across the board are being introduced. Effects visible to MERLIN users will be: less Lovell Telescope time available due to a reduction in the access charges paid to the University of Manchester; the probability of observations in fixed frequency bands (L-band or C/K band) for a year at a time until frequency flexibility is in place; limitations on overtime, resulting in reduced operational efficiency; longer summer engineering periods (currently ~3 months); a reduction in the support available for visitors and remote users (see below). These restrictions are regrettable but are forced on us by the cuts mentioned above.

The MERLIN staff have always had an excellent reputation for user support but, for the foreseeable future the time that they have available for processing MERLIN data for users in absentia will be limited. However, this will not be zero and users who wish to avail themselves of this service should make a request to merlin@jb.man.ac.uk. The request will be assessed and, if it is deemed that it can be fulfilled in a reasonable time, it will join the 'data processing queue' and the user will be informed when they might expect to receive their processed data. We still strongly encourage users to come to Jodrell Bank to process their data themselves. However, the support available locally will also be more limited. If you wish to visit JBO please e-mail to the above address well in advance to ensure that help can be provided. We are unable to handle such requests with notice of less than two weeks.

P.J. Diamond (pdiamond@jb.man.ac.uk)



Recent Science: MERLIN Measures The Magnetic Field In Planetary Nebulae For The First Time

Many current theories regarding the shaping mechanisms that produce the observed morphologies of planetary nebulae (PN) detail the potential role played by magnetic fields during their evolution. With the aim of detecting these fields and testing the models, a group from Hertfordshire, JBO, and UCL led by Dr Indra Bains have obtained MERLIN full polarization spectral line observations of the 1612MHz and 1667MHz OH maser emission from a sample of proto-planetary nebula (PPN) candidates with an angular resolution of 0.2 arcsec. Here they report the first results of the study, concerning the PPN OH17.7-2.0 (also known as IRAS18276--1431).

(Click on image for full-size version)
Figure 1: Distribution of linearly polarized maser components at 1612 MHz plotted relative to the phase centre of the observations. The size of each symbol is proportional to the log of the component Stokes I flux density; vector magnitude (1 arcec~0.92 Jy/beam) is proportional to the component linearly polarized flux density. The velocity range of the components is 47.74--74.98 km/s and is indicated by the colour scale (redder components are more redshifted). The cross indicates the centre of expansion estimated from the data. The direction of the linear polarization vectors is equivalent to that of the electric field vectors. Assuming it is the sigma-transitions of the Zeeman pattern that we observe, the orientation of the B-field in the plane of the sky is perpendicular to the linear polarization vectors.

They have detected one fully-resolved Zeeman sigma-component pair at each observing frequency, from which they measure magnetic field strengths of +4.6 mG (1612 MHz) and +2.5 mG (1667 MHz). They believe this is the first time that a magnetic field strength has been measured in a PPN. The magnetic energy density given by the field strength measurements is comparable to the estimated bulk mechanical energy of the outflow, showing that there is enough energy stored in the magnetic field to shape the mass loss from the star. The observations (See Figure 1) suggest a large-scale, regular magnetic field structure, which they show to be consistent with that of a stretched dipole field. The data show that the stellar magnetic field can produce the necessary latitude-dependent mass loss that is required in addition to the Generalised Interacting Stellar Winds models to eventually produce ellipsoidal and bipolar PN.

I. Bains (ib@ast.man.ac.uk)



High Temperature Superconducting Filters for MERLIN.

The utilisation of the radio spectrum is becoming increasingly crowded and is set to increase at an extraordinary rate. Radio astronomy is particularly sensitive to interference, which is effectively converted to noise in the receiver. Figure 2 shows the noise figure of one of the MERLIN E-system telescopes at Knockin, a rural location near Oswestry, as compared to an identical system near Darnhall, which is located close to the town of Winsford. It can be seen that the noise temperature and hence the sensitivity of the telescope in the area less prone to interference is substantially better. The difference is primarily caused by mobile communications and air traffic control whose bands are adjacent to the astronomy bands.

Figure 2: L-Band system noise temperature. Knockin and Darnhall telescopes November 2000.

A High Temperature Superconducting (HTS) filter at the front end of the receiver has the potential to effectively eliminate the interference from adjacent bands. We have a development project under way that will deploy L-band HTS filters on the telescopes at JBO and then across the MERLIN array to demonstrate their efficiency. In addition, there are other potential applications of HTS microwave components that have application in astronomy and other radio applications. These are being investigated and developed in an interdisciplinary collaboration between the University of Birmingham and the University of Manchester.

The role of the Emerging Device Technology (EDT) research group at the University of Birmingham is the design of the receiver filters. Figure 3 shows the layout of an 8-pole quasi-elliptical response filter, together with the measured frequency response at 77 K. It is evident from Figure 3 that the filter has an exceptional frequency response.

Figure 3: (a) Filter layout, (b) insertion loss.

Almost all of the MERLIN receivers are cooled to approximately 15 K, in order to minimise the amount of thermally generated noise in each system. This environment is ideal for HTS filters and although space is limited, suitably designed HTS filters will fit within the current cooling system cryostats. An HTS bandpass filter fitted between the antenna feed and the first low-noise amplifier (LNA) could improve the receiver noise performance dramatically, as it would make it virtually lossless in-band and give a sharp cut-off at the band edges, thus considerably reducing the level of unwanted signals reaching the first LNA.

Work has begun between the two collaborating institutes on an L-Band filter. The collaboration between JBO, with our expertise in cryogenically cooled low noise receivers and the Birmingham EDT group, with its expertise in HTS development, provides a great opportunity to develop valuable products which could have application throughout the radio industry.

Danielle Kettle (dkettle@jb.man.ac.uk)




MERLIN/VLBI National Facility, Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11 9DL, United Kingdom
Telephone: +44 (0)1477 571321, Facsimile: +44 (0)1477 571618