Astronomical Association URSA meeting in Inari

The Finnish astronomical association, URSA, held the event called "Tähtipäivät" - "The Star Days" in the Sámi cultural centre Sajos in Inari, 20-23 March 2014. Esa Turunen, the director of the Sodankylä Geophysical Observatory, gave a public lecture in the event. The talk was titled "Avaruusalus planeetta Maa" - "Spaceship Planet Earth". The audience consisted of Finnish professional and amateur astronomers as well as on a number of other interested persons (including myself). Of course, Esa talked about KAIRA as it is, in a sense, the biggest radio telescope on Finnish soil. Hence, today we feature two photos from Esa's talk!

"

The last sweet

In August 2013, we attended the EISCAT symposium in the United Kingdom. At the conference we were all issued with a tin of "rock"... a type of British sweet or candy, where a design is run through the sweet using pulled sugar.

In this case the design was "EISCAT 2013".

Well, I've finally (almost) finished that tin of sweets.

The last one!

Of course the tin was accompanied by a bag of about 500 others, so it was quite a challenge. I suspect I've now got some form of Type-43 Diabetes too.

We had considered posting the sweets back to Mike Kosch (the coference organiser). We'd send them one at a time. One each week. After week. After week.

But no... we just ate them all.

(Hmm... however, I wonder if the other bag of 500 sweets is still around. When I last heard it was in Tromsö, Norway.)

In any case, I guess it really is 2014 now.

Storm damage at MAARSY

Ralph Latteck from IAP sent me some pictures of the damage at MAARSY caused by the same winter storm that disintegrated one of KAIRA's HBA tiles and fell down an antenna mast of the EISCAT Tromsø ionospheric heater.

Even though MAARSY is right by the seaside in Andoya, it survived the storm with only minor scratches. The main damage was in the form of twisted cable trays and a few bent antenna elements. While the some of the cable trays look pretty mangled, only a few of the cables feeding the antennas were severed, allowing the radar to continue operations pretty much normally. Good thing that the antennas were fairly rugged and firmly weighed down with slabs of concrete.

One mangled cable tray. The picture also shows the new configuration of the array with the dual polarization capability. This is helpful when observing the ionosphere, where the 53.5 MHz radio wave experiences significant Faraday rotation.   

The worst of the damage. The cable trays have caught some wind, causing two antenna elements to bend out of shape.

This was also a reminder for me that I should write a posting about MAARSY at some point. It is truly a remarkable instrument. Not only is it a state of the art instrument for observing the mesosphere, stratosphere, and troposphere, it can also be used to observe meteors and the ionosphere.

Storm damage to the EISCAT Heating Facility

The storm that caused the KAIRA damage recently, probably also caused damage at the EISCAT site at Ramfjordmoen. This Heating antenna mast (which is also four coaxial feed lines) to tilt as one of the antenna-fixing wires broke.

The antenna was sound electrically on 23 February but was found faulty on 11 March, and a ski tour of inspection on sunny Sunday 16th March found this.

We suspect that the holding cable was also weak due to faulty construction about 30 years ago based on a similar failure once before in 1995, and that the extra forces from the wind finally broke the connection.

The second photo shows the broken insulating cable hanging from the wooden support mast. It will take some time to rebuild this. It means that two out of twelve Heating transmitters cannot be used on this low frequency (4-5.5 MHz) array until the summer.


Text and photographs by Mike Rietveld

North Celestial Pole

In a LOFAR system, a "beamlet" is a pointing direction and a frequency channel (which is called a subband). In some ways, you can think of it as a  "radio pixel" on the sky. These can be static, pointing at a fixed direction, or tracking, where they follow a particular astronomical object as it rises, travels across the sky, and then sets.

This rising and setting, and the motion of the stars generally, is dominated by the rotation of the Earth. Although some objects move about in the sky, due to orbits or proper motion, it is the 24-hour cycle of the Earth that is the dominant effect.

However, there are two points on the sky which do not move very quickly nor very much (although they do still move!). These are the Celestial Poles. These are the points at infinity that are projections of the Earth's rotation axis. Check the Wikipedia article for a full description.

So, in principle, if you were to point a beamlet at the North Celestial Pole (which is the one we see from KAIRA), then you would expect the power level measured to be absolutely flat. There is radio power coming from that direction, but it should remain constant, as it is only rotating within that radio pixel.

It is sort of flat... but not quite!

Here's an example:

The variations come from several things. Tall vertical spikes are radio-frequency interference. And the fuzzy-noisy patches can be from scintillating sources going through the side lobes.

More generally, gradual variations can be from larger areas of Galactic emission and discrete sources passing through the sidelobes of the beam pattern. And then there are subtle variations caused by snow on the antennas, temperature variations, receiver stability, etc.. These are much smaller, but are still present.

As a result, it can be quite a challenge to calibrate data from the instrument.

Such is the fun of radio astronomy!  :-)

KAIRA in the snow

Today we just have a photograph of KAIRA in the snowy conditions typical for the early part of the year. After all the trauma of last couple of weeks, let's hope things remain calm now.

KAIRA in the snow (Photo: D. McKay-Bukowski)

Mapping the RCU number to the LBA/HBA number

Each polarisation from each antenna is processed by a separate channel. The electronics on this channel is called a Receiver Unit (RCU). KAIRA has 96 RCUs (numbered 0..95).

When looking at the signals coming into the system, they are labelled by their RCU number in the first instant. One needs to map this to the antennas to work out which antenna/polarisation is being shown.

The polarisation mapping is straight forward... even numbers for X-polarisation and odd numbers of Y-polarisation. However the match between the RCU numbers and the antenna numbers is a bit more confusing. Yes, there is a logic, but it is not obvious. Sometimes it is simply easier to look it up!

So, here is the mapping of RCU to antenna number for KAIRA (the scheme is different for other LOFAR stations).

For where those antennas are on the actual site, refer to the array layout map.