Although Semester 97A was primarily intended to be a MERLIN L-Band session, some time was allocated at the PATT meeting for C-Band observations during February. This was to minimise the frequency changes and thus loss in observing time, as a MERLIN plus EVN observing period at 5 GHz was scheduled to start in late February. During this 'MERLIN only' C-Band observing period, observations progressed very well, with remarkably few equipment failures and loss of time due to weather.
The MERLIN + EVN observations at 5GHz, which were carried out during the last two weeks of February and the first week of March, started somewhat earlier than had originally been expected when the MERLIN PATT time was allocated. Fortunately, because of the very little time loss, it had been possible to complete all the C-Band Semester 97A high priority observations. Unfortunately, the weather was extremely bad throughout Europe during the MERLIN + EVN period and there was a considerable time loss. Since the programme for VLBI observations cannot be changed once it has been arranged prior to the start of the observing period, several of the 17 separate programmes allocated time may not have a sufficient amount of useable data.
Once started with the full system at the end of March, the L-Band observations progressed very well and all of the A and B priority programmes were completed. Some 'short track' runs were also obtained for 15 sources, allocated C-priority or fill-in' time, as well as long 'full-track' runs on two sources, observed under the heading of Director's time. As indicated in the last PATT newsletter, it was hoped to extend the lower frequency of the lower band at L-Band to 1330 MHz to enable the observation of red-shifted hydrogen from more extra-galactic objects. This was achieved and all the sources requiring this, for which observing time was given conditional upon its availability, were appropriately observed. At the lowest frequencies, the presence of radar interference will have had some effect on the data, though as yet, this must be quantified.
During May, there was a much more successful MERLIN + EVN observing period at L-Band, during which period the very first observations were made in conjunction with the VSOP VLBI satellite antenna ( now renamed HALCA, meaning far-away ), which was successfully launched by the Japanese early in February. The first map from these observations was produced within a few weeks, quite an achievement when one considers the movement of the satellite and the consequently rapidly changing baselines.
A pulsar proper motion experiment, which only required the Lovell and Cambridge telescopes and was scheduled to be carried out during the 10 days of high frequency VLBI observations during May/June, was also successfully completed.
The continuation of the modifications to the E-Systems telescopes, as part of the 'frequency agility programme', and other essential engineering maintenance work started in early July and is expected to continue until the end of October.
Observations of the HST 'Deep Field' at L-Band for a two week period have been completed and analysis of the data so far shows that the expected r.m.s. noise level in a map of 7.8 mJy per beam has been achieved - the first time that it has been demonstrated that it is possible to integrate for such a long period of time and expect the r.m.s. noise level to decrease in proportion to the square root of the observing time. Once combined with data from the VLA, the noise level should be reduced to approximately 4 mJy per beam. Not surprisingly, the extremely large data set which has resulted from these observations has meant the implementation of special file-handling techniques as the files are too large for 'standard' Unix systems.
The MERLIN pages on the world-wide web, which it is intended to regularly update, have been redesigned and now contain much more information than previously. It is now possible to easily and interactively access the MERLIN archive through these pages.
One of the MERLIN highlights presented at N.A.M. was a remarkable 5 GHz MERLIN ( + VLA ) continuum map of M82, which had recently been produced. Even though the physical size of the map created for display at N.A.M. was over 4 feet in length, it was not large enough for the detail of the supernovae shells in the galaxy to be seen. Further enlarged versions of a few of these had to be displayed alongside the main map. This image is visible on the MERLIN pages of the world-wide web, sections of which can be enalrged at the request of the viewer.
A list of the L-Band observing programmes allocated time for Semester 97B can be found on the MERLIN World Wide Web page (http://www.jb.man.ac.uk/merlin) under the appropriate heading.
As this year, a change in the MERLIN observing frequencies will not be made at the end of January 1998, i.e. at the end of Semester 97B, because of MERLIN and VLBI observations at 5 GHz and 22 Ghz in mid-February / early March. The operational frequency for the remainder of the Semester 98A and the timing of the change of frequency will then be determined by the demand of the users for particular frequencies and the views of the MERLIN TAC on the importance of the science to be undertaken. It is to be noted, however, that unless considerable loss of observing time is to occur, there can be only one frequency change to either 408 MHz or L-Band. Should it be that the change is to 408 MHz, then Semester 98B observations will be at L-Band. If the change is to L-Band, then Semester 98B observations will be at 5 Ghz / 22 Ghz.
It is to be noted that as solar activity is now increasing, 408 MHz observations without considerable degradation due to ionospheric variations are unlikely to be possible beyond 1998 for some considerable time. This is, therefore, the last time for some time that operations at 408 MHz will be offered.
The maximum angular resolutions at C-Band, L-Band and 408 MHz are approximately 40, 130 and 750 milliarcseconds and the minimum r.m.s. noise levels in maps of continuum sources will be about 50, 60 (35microJy per beam using the Lovell telescope ) and 700 microJy per beam for 12 hr. on source observations respectively. The maximum bandwidth per polarisation at the two higher frequencies remains at approximately 15 MHz. At 408 MHz, it will be only 4 MHz.
For spectral line work throughout the Semester, users are referred to Section 3.4 of the MERLIN User Guide Version 1.1. However, 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 125 kHz. 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.
Proposal forms and further information can be obtained via either
ftp ftp.jb.man.ac.uk
directory: pub/merlin/semester_98A
www URL http://www.jb.man.ac.uk/merlin
email merlin@jb.man.ac.uk
Observers wishing to apply for time for the MERLIN + VLBI observing session in February / March 1998 should apply directly to the EVN programme committee. The deadline for such proposals is 1 October 1997. Further information about applying for such time can be obtained from the Secretary, Dr. Rolf Schwartz at the Max Planck Institut fur Radioastronomie in Bonn. (email: p589evn@mpifr-bonn.mpg.de)
The provisional schedule for future MERLIN observations is as follows:
Semester Dates Frequency Band Proposal Deadline 98A Feb98-Mar98 5GHz/22GHz 30th Sept. 97 98A Mar98-Jul98 408MHz or L-Band 30th Sept. 97 98B Oct98-Jan99 L-Band or 5GHz/22GHz 31st March 97