Update on the POLFAR project

As we reported last year after the Polish announcement, three new antenna stations for International LOFAR Telescope are to be constructed in Poland. At the end of 2013, POLFAR received a grant from the Polish Minister of Science and Higher Education for the construction and equipment of three international LOFAR stations as part of their national research infrastructure investment. Today we have the press release from ASTRON regarding the announcement.

Today the contract was signed for the POLFAR construction work. Specifically, the Netherlands Institute for Radio Astronomy (ASTRON) and the Polish LOFAR consortium (POLFAR) signed a contract for the construction of three new antenna stations for the International LOFAR Telescope (ILT) in the north, west and south of Poland. The signing of the contract took place at the University of Warmia and Mazury in Olsztyn in Poland in the presence of representatives of the Polish Ministry of Science and Higher Education and local governments, and representatives of the Polish astronomical and space sciences communities.

The new LOFAR stations will be located in Łazy (in southern Poland, operated by the Jagiellonian University in Krakow), Bałdy (in northern Poland, operated by the University of Warmia and Mazury in Olsztyn), and Borówiec (in western Poland, operated by the Space Research Centre of the Polish Academy of Sciences). The formal agreement between the POLFAR consortium and ASTRON now marks the start of the preparations for the roll-out of these new stations.

The new map of the ILT showing the POLFAR stations (source: ASTRON)

The International LOFAR Telescope has 38 stations in the Netherlands, six in Germany, and one each in France, Sweden, and the United Kingdom. Connecting the three new ‘POLFAR’ stations will add valuable extra sensitivity to the array. And in particular, the Polish stations give ‘baselines’ of up to 1550 km in the array, making the ILT a much more capable instrument for high resolution imaging of detailed structues. The positions of the new stations also literally provide new angles on ionospheric tomography.

All components for the LOFAR stations, such as the manufacturing of thousands of antenna elements and electronics, are to be contracted out to industry. The construction of the three new stations will start immediately and is estimated to be completed before the end of 2015. 

KAIRA film

At the LOFAR Science Workshop 2014 in Amsterdam, we premièred the KAIRA film. This was produced by SiteEye in conjunction with Sodankylä Geophysical Observatory, U. Oulu, ASTRON, EISCAT, and many others. (Versions with subtitles in many languages will be posted later.)




Enjoy!

UPDATE!
The film is also available with Finnish subtitles (LINK). 

LOFAR station Norderstedt

Guten Tag! This is Germany calling!

At the Hannover Messe (the world's largest industrial fair), ASTRON, the University of Hamburg and Bielefeld University will sign tomorrow a contract for construction of a new German LOFAR station for the International LOFAR Telescope (ILT).

The new station will be located at Norderstedt, close to the city of Hamburg.

Click map to enlarge (image based on an original by ASTRON)

The additional of another station to the ILT network is important not just for the resources it provides to local education, research and industry, but because of its important contribution to the ILT itself. Each LOFAR station adds valuable collecting area, thus allowing astronomers to detect ever-fainter signals. However, the placement of the new station is such that it provides critical intermediate baselines, thus improving the imaging capabilities of the ILT.

But there is also the knowledge factor as well. Each new station gives an opportunity for new researchers and new expertise to join the strong ILT community. This is vital for the local knowledge base and the advancement of science for participating regions and countries.

In the case of the new German station, researchers at the University of Hamburg, led by Prof. M. Brüggen, specialise in studying the formation and evolution of clusters of galaxies from the early Universe to the present era. The group at Bielefeld University, led by Prof. D. Schwarz, studies the distribution of galaxies on the largest observable distances in the Universe, which carry imprints from the era of cosmological inflation.

Signing of the contract will take place tomorrow at the Holland High Tech Pavilion at the Hannover Messe, in Hall 2 booth D10 from 12:00 - 12:30 hours (CEST).



Link:
https://www.astron.nl/about-astron/press-public/news/contract-new-lofar-antenna-station/astron-university-hamburg-and-biel

Size comparison

There are three large radio telescope facilities in Finland: Metsähovi, Sodankylä and KAIRA. To give an indication of the physical sizes of these systems, we've drawn them to scale in the following diagram, and have listed the physical sizes of the arrays in metres. Both Metsähovi and Sodankylä are steerable parabolic dishes. And KAIRA, as regular readers will know, is a twin phased-array system.

Because the High-Band Antenna (HBA) array at KAIRA operates at different frequencies to the Low-Band Antenna (LBA) array, you cannot count them together for the purposes of a size comparison. They can, however, still operate together (for example, using the Mode-357) -- albeit with reduced sensitivity.

VLBI with KAIRA, LOFAR and the LWA

As we write this, KAIRA is carrying out its first VLBI experiment. We are recording 122 beamlets from RCU mode 4 (LBA, but with 30-80 MHz filters). The subband statistics, beamlet statistics and antenna cross-correlation files are being recorded, but it is the raw beamlet data that is significant here, as it can be used to correlate with the other VLBI stations involved. This particular experiment, organised by Olaf Wucknitz, is being done together with LOFAR stations Effelsberg (DE601), Jülich (DE605), Onsala (SE607) and the LWA in the United States. The map shows the location of these sites:

Sites used in this VLBI experiment. (Background map: ESRI/Penn)

Although this is not the first international experiment with KAIRA, it is the first with recorded raw beamlet data and, therefore, our first attempt to get radio interferometric fringes. Although this is a very ambitious project, if we do manage to get this to work it will be a major technical success.

Updates:

03:26 UTC — In the end, DE605 did not take part. DE601, KAIRA and the LWA reported in okay. No news yet, from SE607.

05:49 UTC — SE607 have now reported it. Their observations went well too.

ERIS 2013

Registration for the 2013 European Radio Interferometry School – ERIS 2013 — is now open. This is the Fifth European Radio Interferometry School and it will take place in Dwingeloo (The Netherlands), in the week of 9-13 September 2013.  ERIS will provide a week of lectures and tutorials on how to get scientific results from radio interferometry. Topics covered include:

• Calibration and imaging of continuum, spectral line, and polarization data
• Low frequency (LOFAR domain), cm-wave (e-MERLIN domain), decimetre-wave (HI/OH domain), high frequency (ALMA/IRAM domain), and very long baseline interferometry
• Extracting the information from astronomical data and interpreting the results
• Choosing the most suitable array and observing plan for your project

A preliminary programme is posted on the ERIS webpage: http://www.astron.nl/eris2013/programme.php

Participants are expected to bring fairly recent Linux or MAC o/s laptops with tens GB disk space. Instructions for installing data reduction packages and downloading data will be provided nearer the time of the event. Most examples will be drawn from m-, cm-, and mm-wave instruments such as LOFAR, WSRT, JVLA, EVN, e-Merlin, and ALMA.

Registration will be open until April 1st 2013, although the conference organisers say that the event is likely to be very popular and that people should register as soon as possible to secure a place. http://www.astron.nl/eris2013/registration.php

Link: http://www.astron.nl/eris2013/

Routine display of all-sky images

For those of you who have been wondering about the strange plots on the right-hand side of our weblog page, here is the explanation.

These are all-sky radio images, taken using KAIRA. Our array is capable of imaging the entire sky instantaneously. In some ways, you can think of it as a fish-eye lens for radio astronomy. Because of the typical observation mode, we can taken these images regularly and put them on the web for everyone to see. In fact, we are the only LOFAR-based station that we know of that does this on a regular basis. Typically, the images are updated every 9 minutes or so. Here's a recent example:

Along the top of the image is the date (top left) and time (top right). The times are given in Coordinated Universal Time (UTC). Along the bottom are some details of the observation. The "mode" is the "RCU mode" (RCU = Receiver Unit) which specifies which filters are being used and which antenna array is selected. Typically, this is Mode=3 for our low-band antenna array observations. "Sb" is the subband (or receiver channel). Each receiver unit splits the signal up into 512 subbands which are sampled and processed. These all-sky observations are only one subband. The equivalent frequency of this subband is shown at the bottom right.

Around the edge of the plot are the cardinal points (North, East, South, West). Although it might seem that East and West are incorrect, this is actually what you expect when you look up. Imagine lying on your back, looking up. If North is above your head, then East is on your left and West to the right. This is also what you see on conventional star maps.

Because the images are regularly updated, you can watch the radio sources change position with time. The sequence below shows four images, separated by approximately one hour each. As you can see, the position of the radio objects move. This is because the Earth is rotating in the opposite direction. As a result, they appear to be moving around the north celestial pole.

The amount of time it takes for the sources to complete one full circuit is one sidereal day (approx. 23 hours 56 minutes). This is due to the mix between the rotation of the Earth and the orbit of the Earth around the Sun. This also means that for a given time of day, the radio sky will appear at a different position at different times of the year.

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.

Path to SKA-low Workshop

This week, there is the "Path to SKA-low Workshop". SKA is the Square Kilometre Array (see our two previous articles: LINK1, LINK2), and this important meeting focuses on the aperture arrays that will be used in this up-coming radio telescope.

As part of the SKA's Aperture Array Verification Programme (AAVP), there will be reports on the latest science and system results from low-frequency pathfinders (LOFAR, MWA, KAIRA, etc.), discussions on the major system design challenges, and progress reports on the crucial technologies and techniques. Not only will KAIRA be contributing its experience to the discussions, but we hope to learn some important lessons to assist with the EISCAT_3D project.

The workshop is being hosted by the International Centre for Radio Astronomy Research (ICRAR), Perth, Western Australia from the 6th to 9th September, 2011.

Link: http://www.icrar.org/news/ska-low

RadioNet - Advanced Radio Astronomy in Europe

The earth's atmosphere lets through several broad regions of the electromagentic spectrum. These are referred to as 'atmospheric windows'. In addition to the optical and infra-red spectra, there is a broad region of radio frequencies, ranging from the ionspheric cutoff (1 to 10 MHz) up to the slow deline in transmission due to atmospheric absorbtion (100 to 1000 GHz). The edges are not sharp and increasing sensitivity and new engineering techniques have meant that we've been able to push out into regions and explore this large span of the spectrum.

The radio telescopes that observe within this region are as diverse as they are advanced and new projects and facilities, ranging from LOFAR and SKA, through to projects like ALMA, JCMT and IRAM.

RadioNet is a collaborative effort to build on the radio observatory expertise in Europe, from 10 MHz to 1 THz. It involves 26 partners contributing effort to 18 different work packages (5 Networking Activities, 4 Joint Research Activities and 9 Transnational Access programmes). It aims to support the radio astronomy community and to improve the capabilities of the major radio astronomy facilities in Europe and beyond. It also aims to ensure that European researchers have access to the radio astronomical facilities they need to undertake the science they wish to pursue.

How do KAIRA and LOFAR work? — Part 11 : Ambiguities

As we’ve seen over the past few instalments of this series, as the direction in which the radio waves approach the telescope changes, the received signals will go out of phase. This effect reduces the sensitivity of the telescope in that particular direction. Additionally, by adding a delay to the electrical signals (whether the analogue electrical signals or the digitised version) the direction in which the telescope is sensitive can be moved.

But there is a problem.

As the incoming direction changes, the signals go out of phase, and thus cancel. However, if the direction continues to change, the signals can actually start to come back in phase again. The peak of one wavefront signal comes through one receiver and adds together with the peak of a different wavefront signal that has come through the other receiver. Click on the following image to get a full-sized version and you'll see what we mean.


At first appearance this might seem like a good thing, as the telescope seems to be sensitive to several directions at once. But it is not. Instead, this leads to an ambiguity. If a signal is received, it is impossible to tell whether that source of that signal is at the ‘on axis’ direction of the telescope, or is in a region off to one side.

These directions of sensitivity that are not in the intended pointing direction of the array are referred to as ‘sidelobes’.

However, there is a way to alleviate this. This is done by adding more receivers to the array. In our example, let’s consider adding a 3rd receiver between the original two. Now, even though there may be ambiguity on the signals received by the first two receivers, the middle receiver is out of phase with the others.

The addition of additional receiving elements helps reduce the sensitivity of some of the unintended directions. This effect is sometimes referred to as ‘suppressing the sidelobes’. If we were to sketch a graph showing an approximation of the sensitivity of the system to a particular direction as we look at angles to the left and right of the primary pointing direction, it would look something like this:
Adding more and more receivers improves this even further. As more receivers are added, it will have the effect of suppressing more of the sidelobes and thus reduce the ambiguity of the telescope.

How do KAIRA and LOFAR work? — Part 10 : Multi-beam operations

As we saw in the last web log entry in this series, digitising the signals gives some distinct benefits when it comes to adding a delay. Apart from being able to do it very precisely, you can change it quickly. However, digital data has another massive advantage.

It can be easily copied.

So, if you make a copy of your digital data, you can process each copy however you want. If you want to filter it differently (for example, to ‘listen’ at a different frequency) you can. However, you can also make a copy and apply different sets of delays to it. This means that if one copy of the data has one set of delays and another copy has a different set for the different antennas involved, then these two copies will be sensitive to different directions.

In other words the telescope will be looking in two different directions at once!

This technique of introducing delays and combining the signals to change the direction in which the telescope is sensitive is known as ‘beamforming’. And making digital copies and delaying them differently to sensitive in several directions is called ‘multiple beamforming’ (or sometimes ‘forming multiple beams’).


In the above diagram (which you can click to see a larger version), each received signal is digitised and split into two copies. The copies are fed into the different digital delay units (D1, D2, D3 & D4). Let’s say that D1 and D3 have no delay at all. The added signal from these will be sensitive to something from directly overhead. In this case, the original radio signal was coming in obliquely; it adds out-of-phase and no output is seen. On the other hand, by adding a delay to D2 (but not D4), the added signal from these will be sensitive to the oblique radio wave. It adds in phase and a strong output is seen.

Because the data is digital, you can copy and delay it as much as you want and there is no degradation of the signal. If you want to be sensitive in more directions, simply add more copies, delay units and adders. The limits are not related to the signal, but rather to issues of equipment, memory, power, space (... and ultimately the project budget).

In practice, systems like KAIRA and LOFAR are also limited by the amount of digital signal processing equipment. At the time of writing this, LOFAR is routinely operating with up to 30 or so different beams. However, some experimental observations have been carried out with over 250 beams.

How do KAIRA and LOFAR work? — Part 9 : Digital beam steering

As we saw in the last web log post on how it all works, we can adjust the cable length and thus the delay in the electrical signal to determine which directions will add in-phase (and thus be stronger) and which will add out-of-phase (and hence cancel each other out).

Although this is sometimes done by adding more cable, it doesn’t have to be. The only thing that is important is that there is the correct delay.

So, instead of using cable, we can digitise the signal and then save the data, wait a little while, and only then load them back again to feed into the adding electronics.If you are constantly taking digital data and storing them, then it is exactly the same as writing numbers into sequential memory. And if you are reading them back out at a fixed delay, then it is the effectively same thing: reading numbers from sequential memory.

As long as those data are always being written into the memory at a nice fixed rate, and are being read back out again at the same rate, all you need to do is adjust the offset between the write position and the read position and you are controlling the delay.

The limitations are that the accuracy at which you represent the incoming wave is a function of the speed at which it is being sampled. Fast memory and fast sampling requires some sophisticated electronics. There is also a limitation that the total amount of delay you can add is a function of the speed at which you are writing/reading and the total amount of memory you have.

However, with modern digital electronics, this sort of memory is relatively cheap. Additionally, changing that write-read offset (and thus the delay) can be done very quickly. As a result, you can change the signal delays, and hence the pointing direction of the entire array very quickly. To go from one side of the sky to the other takes something in the order of a millisecond.

Try doing that with a 100m radio dish!

Alternatively, you can gradually adjust the delay and move the sensitivity of the array quite slowly. This is ideal for tracking celestial sources as they appear to move slowly across the sky due to the Earth’s rotation.

However, despite the power of digital signal processing on changing the directional sensitivity of the phased array, there is another benefit as well. Stay tuned!

How do KAIRA and LOFAR work? — Part 6 : Squinted vision

But what if the radio signal is not coming in from directly overhead? Well, as that signal goes away from the vertical position, the result will decrease. Not only will the reflected signals miss the receiver, but they will be adding out of phase as well.

At some angles, the radio waves from different parts of the system interfere. This can result in zero signal at certain angles where the radio waves from the different parts arrive out of phase, and enhanced signal at other angles where the radio waves arrive in phase.

So if you have a set of reflectors that are carefully adjusted to give good signal addition in a particular incoming direction, and then your radio signal moves, you have to tilt the entire set of reflectors and the receiver to follow it. That is, you change the direction in which your system is most sensitive. This is the reason why radio dishes are usually made so that they can be steered.

But this presents problems: as the size of the system gets bigger, you need more and more steel for the backing structure, bigger and bigger motors and better and better controllers to cope with structural deformations at different angles, wind loading and so on.

The elevation motors on the Kiruna 32m antenna.

An azimuth motor drive and gearbox
(grey+red) on the Kiruna 32m antenna.

As a result there is a limit as to how far you can go. Thus the largest fully-steerable dishes are typically 100 metres in diameter, such as the Effelsberg Radio Telescope in Germany.

The Effelsberg 100m radio telescope, Germany

This sort of size is about the limit at which structural and cost considerations make further enlargement prohibitive. Let’s face it, a large moving steel structure is complicated, difficult to maintain and very expensive to build. But there is an alternative...

(For those of you who are patient enough to have followed this so far, we’ve now covered all the pre-amble and can get on with the interesting phased array topic later this week!)

Photographs: Derek McKay-Bukowski

How do KAIRA and LOFAR work? — Part 5 : The parabolic dish

As we saw in the last instalment, adding a reflector increases the received radio signal. And, we can continue to add more reflectors, each at the correct position and angle, to continue increasing the power of the combined radio wave.
If you do this enough, and imagine it in three dimensions, rather than two, you should understand the basic principle behind the traditional radio dish.


And, finally, for those trying to guess the faded dish photograph behind the last diagram here is view of the same structure, but from a slightly different angle.

This is the 25m dish at the Chilbolton Observatory in Hampshire, England. Next week, we’ll be considering directional sensitivity and starting to get down to the gritty detail of phased arrays!

How do KAIRA and LOFAR work? — Part 2 : What is a LOFAR system?

Before we get started on the explanation, let’s take a moment to quickly describe ASTRON's LOFAR project on which KAIRA is based. The LOFAR (LOw Frequency ARray) radio telescope is a distributed system; namely, it is not all located at a single site. Instead, it is split into dozens of separate locations called “stations”. Most of these stations are in the north of the Netherlands, but there are also a few scattered further afield in Germany, France, Sweden and the United Kingdom. (KAIRA is effectively a single LOFAR station, but with some interesting features and a few special extras.)

Typically, each station has two fields containing arrays of antennas. These are referred to as the Low-Band Array (LBA) and High-Band Array (HBA). The “low” and “high” refer to the radio frequencies that these arrays are designed to receive. The LBA is optimised for the frequency range 30–80 MHz and the HBA for 120–240 MHz. The gap between the two bands is where FM radio is broadcast. It is pointless trying to listen to faint radio signals from the depths of space, when these powerful radio transmitters are nearby. Thus, LOFAR ignores this part of the radio spectrum. And, rather than designing one type of array which straddles this gap, it has been split into the two arrays, with the LBA and HBA being specifically tuned to the respective parts of the radio spectrum.

Aerials of a LOFAR Low Band Array.

Tile antennas of a LOFAR High-Band Array

Apart from the different antenna designs, the arrays are also arranged slightly differently. The LBA is made up of a scattering of spindly aerials that look like wire pyramids. The HBA comprises large flat black-plastic tiles packed sided by side. There are some good reasons for these differences, but ultimately each of them is a phased array receiving radio waves from space.

And we’ll continue with that tomorrow!

Photos: Derek McKay-Bukowski, STFC

How do KAIRA and LOFAR work? — Part 1 : Introduction

KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array) is a radio receiver system. It can measure very faint radio signals and determine very precisely which directions those signals are coming from. It uses LOFAR antennas and signal processing to accomplish this. If you ignore the details of how the data are processed and interpreted, you can safely say that antennas of LOFAR and KAIRA are doing the same sort of thing.

LOFAR (LOw Frequency ARray) is a radio telescope, designed by the Netherlands Institute for Radio Astronomy — ASTRON. Unlike the so-called traditional radio telescopes, which use large parabolic dishes, LOFAR has fields of antennas, which do not move. Instead, it uses electronics and computing to change which way the telescope is looking in the sky. With no moving parts, one is often asked how it works. The problem is that the answers often come in two extremes.

The first is that there is some glib statement that simply reiterates how the 'signals from all the antennas are combined to emulate a large telescope and that the telescope uses digital technology to change the direction that it points in the sky'.

The second is a full-blown lecture course that starts 'Consider electromagnetic radiation from direction s from a small elemental solid angle dΩ, at frequency υ, within a frequency width dυ'. From there it quickly degenerates into page after page of equations.

For the next few weeks we’ll be posting a series of articles to the KAIRA web log in addition to our regular photographs and reports. These articles will steer the middle ground between the two extremes. Of course the entire topic can be extremely complex, and there are many complications and assumptions that we’ll have to skip over to avoid getting bogged down in the detail. (For the experts out there, please indulge us these simplifications.) And yet, we intend to go into sufficient detail, step-by-step and without mathematics, to explain with words and illustrations how KAIRA and LOFAR will accomplish their amazing observations.

SKA — Square Kilometer Array (Part 2)

This article follows on from yesterday's discussion of the Square Kilometre Array (SKA) project.

The SKA will operate over a wide range of frequencies (70 MHZ to 10 GHz) and, like KAIRA, it will be split into different antenna arrays to cover these different frequency bands. The reason for this is that there is no single antenna design that can cover the entire frequency range efficiently.

As a result, the SKA will comprise arrays of three types of antenna elements. These arrays are referred to as the SKA-low, SKA-mid and SKA-dish arrays.


The SKA-low array uses simple dipole antennas to cover the frequency range from 70 - 200 MHz. These will be grouped in 100m diameter stations each containing about 90 elements. This system is very similar to the existing LOFAR Low-Band Array layout.

An artist's impression of the SKA-low dipoles. (Image:
SPDO/Swinburne Astronomy Productions)

The SKA-mid array is again LOFAR-like. It will most likely comprise more delicate antennas, assembled into 'tiles', which are then configured into arrays. These tiles cover the medium frequency range from 200 to 500 MHz, with tiles clustered together into circular stations.

An artist's impression of the SKA-mid tile cluster.
(Image: SPDO/Swinburne Astronomy Productions)

The SKA-dish array will have several thousand antennas in the more traditional 'dish' form to cover the frequency range 500 MHz to 10 GHz. The plan is to equip these dishes with focal plane arrays at their focus to increase their field-of-view (a limitation of traditional dishes).

An artist's impression of the SKA high-frequency dishes.
(Image: SPDO/Swinburne Astronomy Productions)

There are lots of links on the Internet about the SKA project. However, the official project web site is: http://www.skatelescope.org/

SKA — Square Kilometer Array (Part 1)

We've just been to the Square Kilometre Array (SKA) Concept Design Review, held in Manchester in the United Kingdom. Attendance has been as an observer, but it has certainly been very useful to meet with the SKA community. There is a degree of overlap in technologies between the SKA, LOFAR, EISCAT and KAIRA projects and there are many opportunities for fruitful collaboration.

SKA will probably be mentioned a lot in the web log as the months go by. It is a big project, and sits firmly in the scientific vision of most groups undertaking radio astronomy. But what is it?

SKA, as the name suggests, is an array of antennas with a total collecting area of 1 square-kilometre. 1 square kilometre is 1 million square metres, so this is big compared to the 4800 m2 of KAIRA, and it will even outstrip LOFAR and EISCAT.

The antennas will be grouped into stations, with a large number of these stations clustered together into a 'core' (in the same way that LOFAR has a 'core' of stations that are close together).

Artist's impression of how clusters of tile-antennas in the central region
of the SKA might look. (Image: SPDO/Swinburne Astronomy Productions)

These core stations will cover an area some 5 km or so. The furthest stations will be scattered out to distances of several thousand kilometres from the core.

Layout of the central region of the SKA. (Image:
SPDO/Swinburne Astronomy Productions)

This requirement for a large area, as well as a desire to build it in the southern hemisphere (for astronomical reasons) and a need to keep away from interfering radio signals has limited the possible locations where the SKA can be built. Currently there are two candidate sites: one is Australia and one in South Africa. The decision on this will be made in the next few years.

Tomorrow we'll have a closer look at the SKA antenna systems themselves.

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! ;-)