Durin's Day

One evening, as the leaden winter grey skies were gathering to hang heavy over the chill Lappish air, there was a small gap in the clouds. And the light of the setting sun shone through to light up Saana... the lonely mountain.

The lonely mountain, as seen from the KAIRA site.

It reminded me of this passage from an old children's book...

"Stand by the grey stone when the thrush knocks," read Elrond, "and the setting sun with the last light of Durin's Day will shine upon the key-hole."
"Durin, Durin!" said Thorin. "He was the father of the fathers of the eldest race of Dwarves, the Longbeards, and my first ancestor: I am his heir."
"Then what is Durin's Day?" asked Elrond.
"The first day of the dwarves' New Year," said Thorin, "is as all should know the first, day of the last moon of Autumn on the threshold of Winter. We still call it Durin's Day when the last moon of Autumn and the sun are in the sky together. But this will not help us much, I fear, for it passes our skill in these days to guess when such a time will come again."

-- J.R.R.Tolkien, "The Hobbit", 1937.

We didn't see any dwarves that day nor, thankfully, any dragons. Yet still this is a magical place; one of the few remaining quiet places in the world. And winter is at hand.

Last days of autumn

Autumn is a lovely time of year in Lapland (actually... every time of year is lovely in Lapland). But in autumn, the landscape turns to bright yellow, then flame red, then rust... before it finally succumbs to the winter snows. We had a particularly lovely day the other day, so here are some photographs...

Looking across the KAIRA LBA. Saana is visible in the background. (Photo. D. McKay-Bukowski)

From the base of the mound, looking north. The HBA array catches the glorious afternoon sunshine. (Photo: D. McKay-Bukowski)

Using LOFAR LBA and HBA arrays simultaneously

Most LOFAR stations have two antenna fields. These are the Low-Band Antenna (LBA) array and the High-Band Antenna (HBA) array. The LBA and HBA antennas are capable of receiving a range of 10-90 and 110-270 MHz respectively. However, the signal processing is arranged slightly differently. Each "channel" in the signal processing system has three inputs into which the antennas are connected. These receiver units (RCUs), then have several signal paths that can be used, each of which switch in different filters. Thus, the different antennas and frequency bands can be selected and sampled.

Because each individual receiver unit has both an LBA and an HBA polarisation connected to it, it is not possible to observe with both of these simultaneously. Furthermore, because of the filters used in the RCUs, it is not possible to observe across the entire frequency range of the HBA simultaneously either. You must select one of these "RCU modes" for each given channel.

•  RCU mode 3 = LBA input, 10-90 MHz filters
•  RCU mode 5 = HBA input, 110-190 MHz filters
•  RCU mode 7 = HBA input, 210-270 MHz filters

There are other modes as well, such as RCU mode 6, which uses not just different filters, but also a different clock rate (160, rather than 200 MHz) in order to sample the frequencies around 200 MHz (which would normally be aliased in the other modes). The point is that each RCU is limited to a single mode. This has led some to believe that LOFAR stations cannot observe with the LBA and HBA simultaneously.

Actually... they can.

Here at KAIRA, we have been using combinations of modes. In what we refer to as "RCU mode 357", we have been observing with RCU modes 3, 5 and 7 simultaneously. The modes are interleaved, so that there is still a distribution of antennas for each; this allows beam-forming to take place.

Of course, this is not for the faint-hearted, and one must be careful in mapping the channels and powering-up the HBA tiles in such a way so as not to cause power supply failures. However, when done correctly, multiple beams across all bands can be formed giving frequency coverage over nearly the entire VHF band. Here is one of our first results:

A small sample of data from KAIRA, ranging from ~15 to ~275 MHz. Each frequency channel has been divided by the median to bring out the radio features. The bright arcs are ionospheric scintillation during observations of Cas A. (D. McKay-Bukowski, J. Vierinen, and R. Fallows.)

Already we have seen some interesting things and we will be reporting these over the next few days.

First full set of subbands on the LBA

This evening, we managed to get good spectra from all LBAs at KAIRA. This has been a challenging task with blood, sweat and tears... right to the end, due to the problems with cable-ground shielding. But we've managed to do it and therefore proudly present our first full set of LBA bandpasses.

LBA spectra from all 96 RCUs. (Click to enlarge)

We still have more work to do on this and, in fact, we will need to do further subband testing to ensure that we have flushed out all the cabling problems. However this is another step for the project and we can now look forward to facing the next task!

First international experiment

This week, KAIRA is participating for the first time in a large international experiment with baselines of several thousands of kilometres. Apart from hoping to discover some new science, this is a perfect opportunity to link EISCAT, LOFAR and, of course, KAIRA itself. As the week goes on, we will be incorporating more and more stations into this experiment network. However, on this first night of observing the following stations are involved (from North to South): 

• ESR (EISCAT Svalbard Radar, Norway)
• KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array, Finland)
• Kiruna (EISCAT receiver station, Sweden)
• SE607 (LOFAR station at Onsala, Sweden)
• DE604 (LOFAR station at Potsdam, Germany)
• UK608 (LOFAR station at Chilbolton, UK)
• FR606 (LOFAR station at Nançay, France)

 Putting them on the map, the layout is as follows:

The observations are being carried out at approximately 224 MHz (thus allowing us to use the Kiruna receiver station). The ESR is at a different frequency, but is providing concurrent observing. And both KAIRA and UK608 are using special observing modes. Kiruna and the ESR are conventional, mechanically-steered, parabolic dishes. The others are all LOFAR-design phased arrays.

People often ask if KAIRA is part of the LOFAR network. The answer to this is: "not directly". Although there are plans to one day link KAIRA directly to LOFAR via a high speed link, we are currently running as an independent station. While this gives us the flexibility to pursue our own science programme, it certainly does not preclude us from participating in large, important, observing experiment — like this one.

Oh... and we should also point out that although this particular experiment is being run by ASTRON, this work is being done by ASTRON staff at the KAIRA site!

Stay tuned as the week goes on as, no doubt, we will have some very interesting things to report!

Fixing cable shielding problems

Last week we reported on the work that we were doing on the LBA array bandpasses. We had noted a few problems and have been looking into the cause of them. It turns out that some of the cables with joins did not have a correct ground shielding. The break in the shield occurs at this join location and is caused by resin from the water-proofing forming an insulator gap in the connector threads.

To fix the problem, we need to very careful remove the housing and clean out the resin. The cable connectors can then be re-fitted and tested.

A cleaned and reconnected LBA cable join.

Carrying out this work is extremely time-consuming. Each cable join is taking between 15-30 minutes to clean out and test. Still, it is important work, and it is essential in order to get the entire LBA operational.

Tiled panorama of the HBA

Just a nice photograph to end the week. This is actually a sequence of photographs taken by Thomas Ulich from the top of the High-Band Antenna array. In the background are the mountains of the surrounding landscape, radiant in their autumnal colours. Click on the image to see an enlargement.

Tiled panorama of the KAIRA HBA array. Photo: Th. Ulich.

Have a nice weekend!