Monday, May 31, 2010
Thursday, May 27, 2010
This photograph was taken at Chilbolton this morning, after the first two LBA antennas were installed yesterday. There was a light drizzle of rain to settle the dust and put some drops over the new antennas.
But two antennas do not make a radio telescope and even once the field is complete with all 96 LBA antennas, there will be a lot more work to do.
However, it is a nice opportunity to pause and reflect on how far the installation team working at Chilbolton (as well as all the other people working hard to bring LOFAR-UK to reality) have managed to get.
The pathway to the RF-container being installed.
The completed path.
During the installation process, the cables are exposed and vulnerable. As the backfilling work occurs, it is possible that a tool, rock or other object causes the cables to be crimped, abraded or even cut completely. And, of course, now that the burying has been completed any discovered fault constitutes a major problem.
To test the cables, the same vector network analyser is used. However, instead of testing them individually, they are tested in pairs. A signal is sent from the RF-container patch panel out one cable. At the field end, a small loop back cable is placed to direct the signal out of the first cable back down the second cable. Then at the RF-container, the other end of the vector network analyser is connected to the second cable and the round trip signal degradation is tested.
Harry Smith attaches the loopback to a pair of antenna cables on the HBA. As part of this process, the connectors and cables are visually checked and inspected and afterwards they are re-sealed and secured.
Meanwhile, back in the RF container, Jon Eastment looks at the results of the cable-pair under test.
Wednesday, May 26, 2010
Yesterday, a small group of us - astronomy staff and students from Southampton, Portsmouth and Hertfordshire - spent the day learning the details of the LBA installation process from the experts, Derek McKay, Mike Willis and Harry Smith, so that we can act as "team leaders" during the main installation. To begin with Derek McKay took us through a test build, running through the entire procedure step by step, on the test range away from the main LOFAR-UK site. Drilling into grass and soil to insert the custom-made "Chilbolton pegs" for attaching the dipole wires lulled us into a false sense of security about how easy the process was going to be... Here Bob Nichol drills a hole during the test build, while Harry Smith and Derek McKay look on:
After lunch, the plan was to run through the procedure for real, using one of the newly compacted LBA antenna pads. There are a number of steps in the process, which need to be carried out very carefully, with quality assurance checks at several stages. The main stages involve laying out the ground sheet (whose main purpose is just to prevent weeds from growing round the antenna), laying the ground plane (the metal grid that helps reflect radio waves towards the dipole wires), drilling holes and hammering in pegs to attach the dipole wires, setting up the antenna pole, and then finally attaching the LNA (low-noise amplifier) and dipole wires - here demonstrated by Martin Bell and me:
All went well until we reached the stage of drilling holes - the carefully honed procedure involving ensuring an accurate 45-degree angle for the pegs had to be scrapped when it became clear that there were impenetrable rocks that needed to be avoided in some parts of the LBA field. Luckily, introducing a bit of flexibility about the peg angle doesn't cause a problem for getting the correct tension in the dipole wires, and so after a bit of discussion we managed to complete the first LOFAR-UK LBA antenna, which will form part of the final station - an excellent achievement for an afternoon's work! This is the first completed LOFAR-UK LBA antenna, and (most of) the installation team: Bale Granville, Harry Smith, Hana Schumacher, Martin Bell, Mike Willis, Derek McKay, Judith Croston, Bob Nichol (in front). The second picture also includes Martin Murphy, Martin Hardcastle, and Mark Andrews.
We then went on to build a second antenna, to try to speed up the process and to give us all some more practice. By the end of the day we had two completed LBA antennas, a group of (hopefully!) well-trained team leaders, and a refined procedure for the main installation. And we're looking forward to getting the other 94 LBAs installed in a few weeks' time!
Admiring our work: Mike Willis, Harry Smith, Martin Bell, and me.
Pictures are courtesy of Martin Hardcastle.
The sub-contractors from Coral Constructors Ltd start backfilling the last stretch of LBA trenches.
The LBA field completely filled in (with the rest of LOFAR Chilbolton in the background).
In the picture Dave King (left) and Mike Willis set down a stack of LBA ground planes.
Tuesday, May 25, 2010
Martin Bell pulls back some of the final cables across the LBA grave. This is the last time they will look all neat and tidy.
Now that the cables have been all patched in, the cables in the grave are "shuffled about" to make them sit better and reduce stress points. This does make them look rather messy, but it does help ensure cable integrity.
The next step is the covering of the cables with the protective sand layer.
Then the crew move their way up towards the RF container, following the feeder trenches.
Finally, the sand covers all the cables. Warning tape is placed over the area (should anyone excavate in this area in future). Then all the graves are backfilled.
Many people are contributing enormously to this effort, and yet to many of you their names remain unknown. These heroes of the trenches (and the telecons, progress reports project plans, press releases..) are the people who are allowing us to build this station on a very tight budget in very lean times. One such hero, pictured here stepping through the trenches with his eyes closed, is Derek McKay. Derek is a senior STFC scientist, with a strong background in radio astronomy, who is being seconded by SEPNET to - amongst other things - commission the station. His efforts to date have been outstanding - many thanks Derek!
Monday, May 24, 2010
Great progress has been made on the groundwork. The HBA (high band antenna) field groundwork is almost complete and all the positions for the HBA tiles have been marked.
The LBA field trenches have now all been backfilled and next will begin the construction of the pads for the LBA antennas.
A significant landmark was reached last week with all 384 signal cables (the cables which connect the 96 LBA and 96 HBA antennas to the container - there's two per antenna) being connected. This was an enormous task!
LBA aerial installation
This is planned to occur over a week in early June, using volunteers from Oxford, Southampton and Portsmouth Universities. Final plans being worked out this week.
HBA aerial installation
This will start towards the end of June when the contractors will pre-drill for the ground anchors. Tile placement will take two weeks using an 18 ton digger with a lifting attachment as a crane and an all terrain forklift to unload and deliver tiles to the unfolding station.
Overall, the project remains on schedule and we are still on target for the September launch date.
The first step is to apply the sand. A narrow bucket is attached to the digger and builders sand is scooped from the dumptruck into the trench. This is then carefully spread by hand to ensure that the cables are properly protected.
Once the sand is in place, the wide bucket is attached to the digger and the piles of chalk and soil are used to back fill the trench.
Friday, May 21, 2010
Wednesday, May 19, 2010
LOFAR-UK has been so lucky to have such a group of dedicated workers for whom nothing is too much to ask. They are there every day at the crack of dawn, have been accommodating at every turn, and have always been prepared to go that extra mile to get the job done. Furthermore, the quality of the work so far has been first class. Trenches have been spot on, placement has been exact and negotiating the complex field has never been an issue.
When all the work is done, the antennas are in place and the landscaping is all sorted, it will be easy to forget the hard work done in reaching that point. However for our radio telescope, every measurement, every observation and every scientific discovery will only have been possible because the trenches were right, the cables were safe and the fields were level.
Mark Andrews, John Murray and Martin Murphy of Coral Constructors Ltd. Well done lads. Keep up the good work!
Tuesday, May 18, 2010
Dave King prepares to insert the last cable into the patch panel and then complete the cable-dressing.
Looking down from the HBA panel, into the duct from whence the cables arrived from the antenna field. Look at how little of the 200mm duct was used in the end... how's that for neat? In fact, there's an open challenge to any other LOFAR station to do better!
All done. The HBA (top) and LBA (bottom) patch panels, fully populated and ready for the next phase of the LOFAR construction process.
Looking down a trench, there are numerous cables stacked up, waiting to be routed through the LBA cable grave.
Harry Smith working a new line through the LBA cable grave. Like the HBA, the LBA also has a cable grave to take up cable excess before it goes into the final RF container.
Brian Finegan prepares to feed the final cable pair through the duct and into the RF container.
Monday, May 17, 2010
It may look good, but there is more work to be done on this section of the HBA field.
Having packed the surface, a digger prepares to roll over it. We intend to use this vehicle for the HBA tile placement, so it is important to assess if driving it across the finished surface causes any significant damage.
A hydrogen atom consists of a proton and an electron, and both of these particles have a quantum property called spin, which is related to their angular momentum. It is a similar concept to rotation around the axis, for example of the Earth. The proton and electron can have a configuration where their spins are pointing in the same direction (parallel) or pointing in opposite directions (anti-parallel). The first configuration has a slightly higher energy than the second configuration.
When a hydrogen atom makes a transition from the parallel to the anti-parallel state, it will emit some electromagnetic radiation with a wavelength of 21 cm (1420 MHz). Alternatively, a hydrogen atom in the anti-parallel state can change to the parallel state by absorbing electromagnetic radiation with a wavelength of 21 cm. This transition is extremely weak, but the masses of hydrogen in galaxies are so large that it can be detected in nearby galaxies. Due to the fact that it occurs at a very specific frequency (or wavelength) this type of emission is known as line emission in contrast to continuum emission, such as free-free and synchrotron radiation.
LOFAR will use this physical process to study the ‘Epoch of Reionisation’. In the early Universe, the vast majority of hydrogen in the Universe was in the form of neutral atoms unlike nowadays, when it is mostly ionised plasma so the protons and electrons are separated. As the first stars switched on, they produced ionising radiation that began to separate neutral hydrogen atoms into electrons and protons (also referred to as ions). We use the prefix ‘Re-‘ in Reionisation because shortly after the Big Bang the Universe was ionised, and it later cooled down and electrons and protons joined to form neutral Hydrogen atoms. Hence the epoch of Reionisation is the second epoch when the Universe was ionised.
The epoch of Reionisation occurred when the scale of the Universe was significantly smaller, approximately one seventh of the present scale or smaller. The radiation at 21 cm has scaled up with the Universe, and reaches us with a wavelength of about 1.5 metres or longer (corresponding to a frequency of 200 MHz or less) and it is therefore observable with LOFAR. Studies of the 21 cm line can yield information on the density of neutral hydrogen and its distribution in the early Universe.
The figure below shows the signal of the 21 cm line from the epoch of reionization which LOFAR is expected to measure. The colour scale shows the difference in the observed intensity caused by regions with a high or low content of neutral hydrogen: light regions have the most neutral hydrogen compared to the average, dark have the least (remember the average changes with epoch, so we are looking at the contrast). The vertical axis shows the physical extent of the regions, the units are megaparsecs, Mpc, which correspond to approximately three million light years. The horizontal axis corresponds to observed frequency (in MHz), with lower frequencies looking at earlier epochs when the Universe was younger. This image was produced by Garrelt Mellema using a simulation of the young Universe by Ilian Iliev.
Against a strong source of radiation, for example synchrotron emission from a background source, the neutral hydrogen will leave an imprint of absorption due to the transition at 21 cm. Below is a simulation of the spectrum of such a strong source. The horizontal axis is frequency, in MHz, the vertical axis is flux density (another measure of intensity). The source is at redshift 10, and at this redshift the 21 cm line appears at 129 MHz. The absorption from neutral hydrogen can be seen as strong dips in the spectrum to the right of 129 MHz. The solid line serves as a guide to the eye, it shows what the spectrum would have looked like without any absorption. Only neutral hydrogen between us and the source can cause absorption, and hence the spectrum is only affected at frequencies higher than 129 MHz. Image credit: C.L. Carilli, N. Gnedin, S. Furlanetto, F. Owen.
Friday, May 14, 2010
First a thick layer of protective sand is spread across the cables. The crew work the cables gently to ensure that the sand penetrates between all of them.
Here is the finished sand cover. This protects the cables against flint and other sharp rock as the spoil is back filled into the grave.
With all cables completely covered, the digger moves in to bulk fill with previously excavated material. And that is it! The HBA cables are fully covered and the crew can walk through the area without the risk of falling into the hole or collapsing a trench wall.
There have been many people involved in the laying of the HBA cables, and here are a just a few of them. From left to right: Dave King, Jon Eastment, Mike Willis, Alejo Martinez-Sansigre and Harry Smith. To these few, and everyone else involved in this stage of the project: thanks and well done!
Thursday, May 13, 2010
The type-1 is applied to the surface. Although it is only a light grey, against the stark white of the chalk it does seem very dark.
Using a laser level, the type-1 is scraped to the correct height. On the edge of the field a rotating laser allows the surface height to be accurately measured. The skill of the digger operator in achieving this was quite remarkable.
Using the roller, the type-1 is compacted to the final finish.
Wednesday, May 12, 2010
(as shown below - click to enlarge the diagram).
This gives the resulting telescope great beam properties. However, it also makes it difficult to install.
Here, Griffin Foster (U.Oxford) and Mayaane Soumagnac (ENS-Lyon, France) start setting up the cables for one of the LBA aerial locations. There is a six-page document describing the procedure, as many of the steps done in this initial phase are critical to subsequent phases of the installation.
"You are in a maze of twisty little trenches, all alike". Here the LBA team are busy placing the LBA cables in place. From left to right: Harry Smith, Griffin Foster, Mayaane Soumagnac, Danny Price and Jack Hickish.
Our equivalent of Ariadne's Thread: countless cable trenching plans, diagrams, lists and tallies are used to keep track of the deployment within the LBA. However the ultimate is this dual-core backpack-computer, with headset, scanning gun, barcoded-trench plan, LBA antenna checking software, text-to-speech synthesiser and rocket launcher! (Okay, okay... maybe not the rocket launcher.)