|Number 5: September 2001|
|MERLIN||VLBI AT JBO||PROPOSALS||ARCHIVE||CONTACT|
Call for Proposals
The deadline for proposals for Semester 02A (February - July 2002) on MERLIN is October 8, 2001. All details are in the MERLIN web area, specifically;
Observing frequencies available:
L-Band and 6 GHz C-Band will be available from the start of the Semester**. The timing of the change of frequency from L-Band to C and K-Band will be determined by the user demand, and the MERLIN TAG's view of the proposed science. There can only be one such change. Unfortunately, because of the continuing upgrade to the Lovell telescope, it will not be available throughout the Semester apart from the February MERLIN+EVN observations. The system parameters for observation of a continuum source in good weather cond itions are;
|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||~15|
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;
* There will only be a single baseline between Cambridge and Mk2 available in th
e 6-7 GHz methanol band.
** There will be MERLIN+EVN observations at L-Band in February 2002. Application s to the EVN PC.
The period since the last Newsletter has been one of intense activity on the e-MERLIN front. The situation regarding the funding of the construction is very positive, with commitments having been made from several different sources. In late June PPARC organized a review, chaired by Prof. Richard Wade, of the future operating costs of e-MERLIN. The outcome of the review was positive and the report of the committee has been passed to PPARC's Science Committee. On September 12 PPARC is conducting another review, this time of the project implementation plan and the risk analysis. Hopefully, we will have another good outcome. The results of all this careful scrutiny are due to be discussed by PPARC's Science Committee in October in the context of the future of the UK's ground-based astronomy programme. Hopefully, decisions will be announced soon afterwards.
Meanwhile, normal MERLIN operations continue. Observing in Semester 01A occurred at 5 and 22 GHz, we are returning after the summer engineering break at the same frequencies and will be switching to L-band (1.3 and 1.8 GHz) during November. The exact date of the changeover is yet to be determined and will be influenced by the availability of the Lovell Telescope (see article below). The summer shutdown for engineering was occupied with routine engineering tasks at several of the telescopes, including investigation of some non-fatal problems that were noticed during the previous observing session e.g. periodic stalling of the sub-reflector at Cambridge. The major scheduled task was to be the replacement of the drive system at Pickmere, the third and final telescope to undergo such surgery. However, the farm adjacent to Pickmere was the subject of a Foot and Mouth scare and, although it was eventually declared clear of the disease, the quarantine period that was enforced resulted in us having to postpone the work on Pickmere until next summer.
The final article in this newsletter advertises the public release of the interactive MERLIN archive. We hope that all astronomers will make full use of this archive, especially as it grows and all publicly available data become available. As the article discusses, Cheung et al. have recently published a paper using MERLIN archive data; I look forward to seeing more such papers.
Related to the release of the archive is the news that Jodrell Bank Observatory are members of the Astrogrid consortium, which has been successful in obtaining UK e-science funding, and the Astrophysical Virtual Observatory (AVO) consortium, which has been awarded EU funds to develop Phase A of a virtual observatory. Our participation in both of these projects means that radio astronomical applications will be fully discussed and developed as the ideas for VOs proceed.
Finally, we are about to publish the 1999-2000 biennial report. This will soon be visible on our web page and looks very impressive as a glossy hard copy. Please e-mail Janet Eaton (firstname.lastname@example.org) with your mailing address if you wish to be included on the hard-copy circulation list.
Gravitational lens systems are fascinating objects and have proved exceptionally useful in a wide range of astrophysical problems. Most famously they enable us to measure the Hubble parameter in a single step, but they also enable us to constrain lens galaxy mass profiles and also to explore the ISM of the lensing galaxy due to its influence on the radiation of the background lensed quasar. A recent discovery was the phenomenon of radio microlensing in at least one of the images of CLASS B1600+434, a 1.4-arcsec separation double lens system (Figure 1 (a)). Whilst optical microlensing is a relatively common phenomenon, this was the first detection of microlensing in the radio, and was made possible due to the two 'copies' of the background source, which allowed a separation of the intrinsic variability and that superimposed due to some external propagation effect. Through an analysis of the microlensing variability it is possible to infer several properties of both the lensed source and also the microlenses. In the case of CLASS B1600+434 the microlensing is believed to be due to a population of > ~0.5 solar mass located in the halo of the lensing galaxy. This has prompted a search for microlensing in a larger sample of lens systems.
A MERLIN key project to look for microlensing in other systems has been running since February 21 of this year and the sample of eight JVAS/CLASS lens systems has been observed on 32 occasions. The calibration strategy is a vital part of the project and builds on the previous experience of monitoring lens systems both with MERLIN and the VLA. Over a 24-hour period each lens system is observed over five different hour angles and additional nearby phase calibrators and 'control' sources are observed. The latter are GPS sources which shouldn't vary in flux density from epoch to epoch and which therefore allow correction of the lensed light curves for calibration errors.
Although no spectacular events on the scale seen previously in CLASS B1600+434 have been seen we believe that we have found evidence for microlensing in at least one other lens system, whilst CLASS B1600+434 continues to show intrinsic variability and has allowed us to measure a time delay. Using a technique similar to a cross-correlation gives a delay of approximately 45-46 days and a flux ratio of 1.23 from the light curve in Figure 1 (b) (J1545+4751 is the control source). This compares with values of 48 and 51 days found from VLA 8.4-GHz and optical NOT monitoring respectively. This is the first time that a time delay has been unambiguously measured in a lens system using MERLIN.
|Figure 1: (a) MERLIN image of CLASS B1600+434. (b) MERLIN light curves of CLASS B1600+434. CLICK ON IMAGES FOR FULL SIZE VERSIONS.|
The Lovell Telescope Upgrade
This summer has seen major progress towards the upgrading of the Lovell Telescope to make it fully efficient at 5 GHz and so able to greatly improve the sensitivity of MERLIN in this key operational frequency band. SHAL Engineering staff have been stripping off the old surface panels, now badly corroded around their edges, and replacing them with new ones made of galvanized steel. Whereas the old panels had been spot welded into place, the new are being screwed into position so avoiding the local deformations caused by the welding process. In addition, some errors in the shape of the backing structure of one of the tiers of panels are being corrected, and new adjusting screws are being installed behind each panel. As a result, the new panels will be individually smoother and conform to the correct parabolic shape to higher precision than the originals and, when all have been installed, it will be possible, using both laser and holographic techniques, to set them into the correct parabolic surface with great accuracy.
The accompanying photograph taken on August 21st shows that work to replace alternate panel segments was almost complete. On the upper left engineers working on a 'BEV' (Bowl Excursion Vehicle) are replacing the final panels of one segment. Having cleaned the backing structure ribs they are covered with mastic and the new panels screwed into place. Work is continuing into September so that over 60% of the panels will have been replaced this year. The resurfacing will be completed next summer to then allow the whole surface to be set with high precision.
Following on from this year's resurfacing work, a complete replacement of the telescope's control and drive system is scheduled for completion in November. When operating at the higher frequencies made possible by the new surface, the telescope beam will be correspondingly smaller. It is thus necessary for the telescope to track radio source positions with greater accuracy. By providing individual drive control to each of the four azimuth motors and six elevation motors the new control system will be much more responsive than that which is being replaced so giving the required positional accuracy.
When work is finally completed in the autumn of 2002, not only will the Lovell Telescope extend the capabilities of MERLIN, but it will also be able to take part in additional VLBI observations - so further enhancing the role of our National Facility. More details and pictures can be found on the Jodrell Bank Observatory web-site at:
|Figure 2: (a) Work continuing on the Lovell Telescope upgrade. (b) Grinding the backing structure ribs. CLICK ON IMAGES FOR FULL SIZE VERSIONS.|
The MERLIN Archive
All MERLIN observations are archived and placed in the public domain one year after the end of the session in which they were made. A complete list of all observations made can be searched via the MERLIN archive web pages at:
In order to make access to archive data much easier and faster, we have started a major programme to process all standard continuum observations made since 1992 and make the processed data and preview images available via the web. The first tranche of 579 observations at C-band and L-band is now available. The pipeline process used to generate these images does all the calibration and phase referencing, including polarization calibration, and works on the data in spectral line mode, so that archive users can make wide-field images if they wish. The original proposers are sent preview pages of their data before release. These reference images are not designed for publication, (although in many cases the image quality is perfectly adequate) but primarily for archive users to make a quick decision on whether to download or request the calibrated visibility data and produce an image which meets their requirements.
The archive web pages allow the database of processed observations to be searched by position, source name, date and frequency. Having selected a particular observation, a preview page shows details of the observations, preview images and plots of the data in different formats. Registered users can download FITS files of the images. FITS files of the calibrated visibility data will shortly be made available via the web; at present these must be requested from the MERLIN archivist. As part of the pipeline process, each phase calibrator source, which may well be interesting in its own right, is also imaged and can be retrieved via the same web page.
The MERLIN Archivist also supports individual requests for data. Cheung et al. obtained MERLIN archive data (Figure 5(a)) to support analysis of an X-ray jet in the blazar 3C371 newly discovered by Chandra (Pesce et al. 2001, ApJL, in press). This continuum image, showing a radio jet extending for more than 5 kpc to the west, goes much deeper than any published radio map of 3C371. These data were originally taken in order to map HI absorption in the central regions.
Jodrell Bank is now a partner in AVO and AstroGrid. Work done as part of these projects will concentrate on interoperability issues of using radio aperture synthesis data and on-the-fly imaging to allow archive users to produce small images from large visibility data sets held at Jodrell Bank.
|Figure 3: (a) 3C371 image recently made with MERLIN archive data by Cheung et al. (b) Example of the web-based access to the new MERLIN data archive. CLICK ON IMAGES FOR FULL SIZE VERSIONS.|