Friday, 18 February 2011

How long are LOFAR baselines

There has been a bit of misinformation floating about the Internet recently about just how long the LOFAR baselines are. A 'baseline', in this sense, is a short-hand term for the inter-receiving-element distance. In other words, how far apart are the antennas. In the case of LOFAR, the elements are the stations, so the baselines are the distances between the LOFAR stations.

The reason why this is significant is that, ignoring calibration issues, the resolution of the radio telescope is governed by the baselines and the observing frequency. This means that for any given frequency, the longer the baselines, the better the resolution (which is the fineness of detail that the radio telescope can make out). Of course, you can't just have one long baseline... you need the shorter ones too, so that you can resolve all levels of structure in the astronomical object you are observing. (This is what the so-called uv-diagrams show: just how good the coverage of a sparse radio telescope is over all the baselines.)

But it is these 'long baselines' that get quoted in press releases, as they represent the limit of the radio telescope's resolving power.

For LOFAR, although the collecting area and intermediate baselines are dominated by the Dutch stations, it is the so-called International Stations that dominate the long baselines. These stations are:
  • LOFAR DE601 — Effelsberg, Germany
  • LOFAR DE602 — Unterweilenbach, Germany
  • LOFAR DE603 — Tautenburg, Germany
  • LOFAR DE604 — Potsdam, Germany
  • LOFAR DE605 — Jülich, Germany
  • LOFAR FR606 — Nançay, France
  • LOFAR SE607 — Onsala, Sweden
  • LOFAR UK608 — Chilbolton, United Kingdom
Additional stations, such as the proposed station at Birr Castle, Ireland, are not listed here. At the time of writing this, DE605 and SE607 are still under construction or being commissioned. And, to date, DE602 and DE604 has not yet been used in long baseline observations to the western stations.

This means that the longest baselines used so far by LOFAR is the DE603 to UK608 distance, which is 920 km. This was the longest baseline used in the image that recently made the mainstream news.

DE603 (Tautenburg, Germany)... one end of
the longest LOFAR baseline so far. (Photo:
ASTRON & Landessternwarte Tautenburg)

And at the other end, UK608 (Chilbolton, UK). Together these
two stations have a baseline of 920 km. (Photo: STFC Images)

Once all the stations under construction have been completed, the longest baseline will be FR606 to SE607, which would be 1301 km.

If KAIRA is connected to the LOFAR network, the it will add several long baselines, exceeding all of these. The shortest would be the KAIRA to SE607 baselines, at 1350 km, while the longest would be KAIRA to FR606 at 2594 km. As can be seen, ignoring position angles, these KAIRA baselines would extend the LOFAR International Telescope very well.

However, even with KAIRA, LOFAR will still fall well short of the 10,000 km that is currently being claimed by some web sites. If only the funding agencies would make those sorts of errors, then, yes, we'd definitely have 10,000 km baselines! ;-)

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