Friday, April 30, 2010

Ground Control

The RF-container was placed in its final position last week and it is now being fitted ready to start accepting cables and electronics. One of the first things to be sorted out is the electrical work, including not only the introduction of power, but also in grounding the cabinet.

This first picture shows the RF-container, now in place. Also featured is the trusty observatory Land Rover that has faithfully carried the staff and a lot of cables, tools and equipment back and forth between the main observatory building and the LOFAR field.

This picture is of the base of the RF-container showing the earth strap and ground-pin cover. Notice how the RF-container is up on rubber blocks.

Thursday, April 29, 2010

Hana Gets "Hands On"

From the University of Portsmouth news, PhD student Hana Schumacher getting her hands dirty building LOFAR.

Cable Rivers

The folks at Chilbolton are seriously into cables at the moment! Here's the latest on the cables.

The cables for both the HBA and LBA fields are done in "river" mode. Each antenna has two cables. These go down into the trench and head off in the direction of the RF-container, where the processing will occur. As they pass through the trench, new cables come in from other antennas. Each new "stream" adds to the "river" and the bundle of cables slowly builds up.

This scheme requires care. The cables must be put into the trenches in the correct order and the covering soil cannot be added until the full river has been built up. Managing the bundle of cables as it nears its destination also becomes difficult. The become difficult to handle and must the meticulously checked to ensure that no small stones are caught between the strands.

Here is the start of stream, with a pair of cables (one per polarisation) heading off down the trench.

Half way across the array and already the cable group has become quite substantial. In this section, there are 128 cables in the bundle. Jon Eastment is checking that everything looks OK.

Wednesday, April 28, 2010

Studying Pulsars with LOFAR

Another Discovery News article about LOFAR - Shedding New Light on a Pulsar Mystery, by Nicole Gugliucci.

LBA cable trenches open for business

Yesterday at Chilbolton they made the first incision for the LBA trenches. Unlike the HBA trenches which lie on a regular grid, the LBA aerials are scattered across the field in a pseudo-random pattern. This helps ensure good beam properties, but it also makes the cable trenches a bit of a nightmare to dig. However, the ground crew have demonstrated incredible finesse in negotiating the heavy machinery through the labyrinthine trench plan.

In this picutre you're looking down the line of the first LBA cable trench.

It looks like a giant Kanji character scribed into the ground. However these are the feeder trenches from the LBA aerials back to the RF-container.

Tuesday, April 27, 2010

HBA Cable Graves

In the foreground of this photograph you can see the so-called "cable grave" for the High Band Array (HBA). Because it is important to control the cable lengths for the LOFAR radio telescope, each cable is made to a specific length. And to ease the manufacturing process, these lengths are standardised to a small number of set lengths. Because of the positions of the HBA tiles, there can be several tens of metres of excess cable which needs to be carefully zigzagged back and forth before the remainder of the cable goes into the RF-container.

In the photograph, Harry Smith and Dave King are adding another cable to the cable grave. In the background, the ground works crew are adding a layer of sand to protect the recently laid cable.

Free-free emission

An electron will radiate when it is decelerated, so electromagnetic waves carry energy away from the electron. In space, a free electron that passes near a charge feels a small perturbation, and becomes more stable by emitting an electromagnetic wave. After emitting this wave, the electron will be moving slower, since it has lost energy. This form of emission is often referred to by its German name, Bremsstrahlung, which means braking radiation. Since the electron was free before it emitted the electromagnetic wave (it was not trapped in an atom) and is still free after emission has occurred, it is also known as free-free emission, and we will use this name here.

Below is a sketch of an electron (blue circle) passing near an ion (red circle) and losing energy by free-free emission (green wave).

For free-free emission to occur, a fast electron must pass close to another charged particle. This form of radiation will therefore occur more often in regions of high density, because the electron will have a higher chance to come close to a charged particle that will perturb it. However, for free-free emission to occur, the gas must also be ionised, so that the electrons are free from the protons, rather than bound in a hydrogen atom. The amount of free-free emission depends on the temperature of the ionised gas and it is a type of thermal emission, and because the emission is continuous with frequency, it is described also as continuum emission.

Clouds of ionised gas near regions where young stars are forming will emit free-free radiation. It can therefore be used to trace this gas and to study the surrounding regions to where new stars are born, since it gives information on the temperature and density of the gas. As an example of such a region, below is an image of the Carina nebula, courtesy of the Hubble Heritage.

Saturday, April 24, 2010

What physical processes do we observe with LOFAR?

LOFAR stands for LOw-Frequency ARray, and it is a radio telescope. This means it is sensitive to electromagnetic waves whose wavelengths are very long, typically longer than one metre, corresponding to frequencies of 250 MegaHertz (MHz) or less. Other examples of electromagnetic radiation are optical and infrared light, X-rays and gamma-ray radiation. All of these
have much shorter wavelengths (higher frequencies) than radio emission.

The waves detected by LOFAR are of the same type as those used by FM radio. When you tune to 92-95 FM for Radio 4, that's actually tuning the receiver in your radio to 92-95 MHz. In fact LOFAR has a gap between 80-120 MHz. It does not observe at these frequencies since the signal from the FM radio stations overwhelms all other signals, making it virtually impossible to detect any emission from space.

Electromagnetic waves are radiated when a charged particle is accelerated. An example is an electron that is suddenly accelerated by a change in an electric field or a magnetic field. It will radiate some energy away in the form of electromagnetic waves.

In a series of 3 blog posts over the next couple of weeks I will summarise three important physical processes that can be observed with LOFAR to study astrophysical objects outside of our Solar System: free-free emission, synchrotron radiation and the hydrogen 21cm transition.

Planting the cables

From each place where an antenna is located, a trench is dug into the ground. It leads into other trenches and gradually these make their way to the RF-container, where the signal is combined. The placing of the cables is critical; the exact lengths must be measured so that the exact timing of the signal arrival is known. This is because LOFAR uses software and timing between the signals from different antennas to "point" to different parts of the sky.

The cables must also be protected - ultimately, they will be safely buried underground. But for a short time they will be in open, exposed trenches, so this is a critical phase of the installation.

Below, Harry Smith is positioning the start of two HBA cables. These are measured to ensure they have sufficient ground clearance. They are then sealed and attached to the survey peg.

The cable laying is a carefully coordinated operation between the different groups involved. Here the digger-crew wait for the all-clear from the cable-team before proceeding to open the next section of the trench.

Friday, April 23, 2010

Trench Warfare

At the Chilbolton site, digging has now started for the cable trenches on the HBA field. Unlike the LBA, the HBA (High-Band Array) is arranged in a regular grid pattern (as shown below - click to enlarge the diagram).

There are a total of 11 "columns" which have varying numbers of cable exit points where the HBA tiles will be connected. The main trench is dug by a mechanical digger. Feeder trenches then are cut into the main trench to give access to the cable exit points. Cables are then fed down into the trench from the surveyed marker pegs and bundled along the bottom of the
trench channels.

In the picture, a digger carefully backs across the site as it digs in another
HBA cable trench.

Pre-Installation Cable Testing

An international LOFAR station is made up of two fields of 96 antenna elements each (96 LBAs and 96 HBAs). Given that each antenna can receive in two polarisations which both need their own cable, this makes for a total of 384 cables between the antennas themselves and the RF-container. Together, these cables have a combined length of 36,360 metres (just over 22.5 miles).

The cables will be buried into the ground before the antennas are deployed. However, before doing this, each one needs to be tested for its electrical characteristics. It is painstaking work, but this verification work is extremely important.

In the picture, Mike Willis and Harry Smith are very happy to have completed the last of the lab testing of the 384 cables.

Thursday, April 22, 2010

Placing the Container on the Foundation

On Monday, the RF container was moved from its location in the main compound of the Chilbolton site over to the LOFAR field. There, it was placed on the recently cured concrete pad. This container is now ready to accept the cables that will connect the antennas to the main distribution panel that is contained within the container. In the photograph, the container is being lowered down onto its final resting place. The cable is the main power cable that will supply electricity to the equipment.

Wednesday, April 21, 2010

Pouring the Container Foundation

On April 14th at Chilbolton, the base concrete was poured for the LOFAR RF-container.

The container is a box which will house most of the LOFAR electronics located at Chilbolton and is used to digitise the data, break it up into separate spectral channels and then transmit it first to the main site building and then on to the Netherlands for processing.

All the signal cables from the antennas in the field will feed into the container. They enter through tubes that go under - and then rise up through - the concrete pad.

A LOFAR Container:

The container itself is called an RF-container, because it is shielded against RF (radio frequencies). Electronics emit significantly at the very frequecies we want LOFAR to detect, so shielding them from the antennas is very important.

Friday, April 16, 2010

Video of the LBA Test Installation

Now available on YouTube is a short video of the LOFAR-UK LBA Test Installation (as previously described on the blog).

Hopefully the weather will be a bit more pleasant for the full installation next month.

LOFAR hits the tabloids

An article about SETI with LOFAR appeared in The Sun on Wednesday: "New Scope to Listen for Aliens".

Wednesday, April 14, 2010

LOFAR at NAM2010

The UK National Astronomy Meeting (NAM) is happening this week in Glasgow. There were three talks about LOFAR in the "Pathways to the SKA" session yesterday. Steve Rawlings from Oxford talked about precurser SKA telescopes, causing some interest in the planned "student army" who will be helping to construct the LBAs next month (you'll here more about that here I'm sure!). John McKean from ASTRON talked about early results from LOFAR commissioning, and Alan Penny from St Andrews talked about using LOFAR for SETI.

There has also been some press interest from the talks, with an article at Discovery News: "Seeing in a New Light (and Searching for ET) with LOFAR", by Nicole Gugliucci.

Monday, April 12, 2010

Panorama of Ground Works at Chilbolton

Click on the below image for a panoramic view of the ground works in progress at the Chilbolton site. Progress is still good in advance of the LBA (low band antenna) installation, which is expected to take place next month.

Testing the HBA Anchors

On 8th April 2010, Chilbolton received a visit from Autonational, the Dutch company who will be doing the installation of the HBA (High Band Array). The HBA tiles need to be anchored to the ground to prevent any movement. This is done by driving a metal peg vertically down into the ground using an electric or hydraulic hammer. When tension is then put on the peg, it tumbles into a horizontal position to lock into the ground base. Each anchor can take a load of over 200kg. Because the different LOFAR sites vary considerably, it is necessary to test each one before the installation commences, to ascertain whether pilot drilling is required, what type of mechanical hammer is needed and what the best target depth is. The Chilbolton site is on a chalk base and it seems that it will be heavily affected by the ground water content at the time of the HBA anchoring. We will be using hydraulic hammers and driving the pegs 600mm below the surface.

In the below photo, Henk Paarhuis drives in a test anchor on the northwest edge of the HBA field. Eric Petrusma assists by keeping the anchor cable clear of the hammer bit.

Wednesday, April 7, 2010

LBA field survey complete

Field preparation at the Chilbolton site has continued according to schedule. Both the LBA and HBA fields have now been cleared and the LBA field has been prepared and surveyed ready for placement and trenching. The survey positions are measured using the European Terrestrial Reference System 89 (ETRS89). Because this system is calibrated using VLBI-Geodetic measurements, it turns out that the Chilbolton 25m dish was in fact one of the contributors to the standard reference frame, which we are now using. So that's a nice local link!

As we have now completed the survey of the LBA field the coordinates have been sent to ASTRON, who will use the measurements to work out the exact positioning of the 96 aerials.

The image below shows surveying equipment at the Chilbolton LOFAR site. In the background are two heavy diggers involved with the soil clearing operations.

Tuesday, April 6, 2010

TKP Project Up and Running

The Transient Key Project (as described in our last blog entry) started test observations this week with LOFAR. An important milestone for the project.