LOFAR has had two test facilities. The first one is ITS - Initial Test Station - built in 2004 and the second one is CS1 - Core Station 1 which was built in 2006. Both have been placed in the central core area and have been already dismanteled after they have helped understanding the system and improving it.
|Initial Test Station - Exloo (NL) - 2004|| Core Station 1 - Exloo (NL) - 2006
Radio Map of the Sky at 50 MHz
This is a test image made with LOFAR Core Station 1 (CS1). It also has some highly encouraging features. For instance, the calibration is good enough to completely remove the bright source Cas A (20.000 Jy) from the field centre. This makes it possible to see about 30-40 fainter 3C sources (~50 Jy). The image noise level is ~3 Jy, still 6 times higher than the expected thermal noise. Much of this is caused by the incomplete subtraction (on the right) of the second-brightest source, Cygnus A (5000 Jy), which gets very close to the horizon during the observation.
The CS1 array consists of only 16 "stations": 15 individual dipoles and one phased array of 48 dipoles (half a standard LOFAR station). The maximum baseline was ~500m. The total bandwidth was 450 kHz, distributed over 3 subbands between 45 and 60 MHz. The total observing time was 29 hours. The primary beam of the 48-dipole station was pointed at Cas A, which was therefore observed with much greater sensitivity than the rest of the sky.
The calibration consisted of solving simultaneously for the complex gains (JJones) in the direction of the two brightest sources, Cas A and Cygnus A. The contributions of both sources were subtracted from the data. Before making an image from them, the residuals were corrected for the complex gain in the direction of Cas A. This procedure gives the best image quality for sources close to Cas A, but it is still surprisingly good elsewhere.
Detection of a pulsar with CS1
On 14th June 2007, 6 HBA tiles of the LOFAR CS1 station were pointed at the zenith as the radio pulsar B0329+54 passed overhead. The signals from the tiles were added incoherently and the source was observed for 15 minutes while it remained completely in the beam. A total of 48 subbands each of 0.15625 MHz wide, were collected in the frequency range 170-230 MHz. The left hand plot shows the frequency versus pulse phase, with the greyscale showing the pulse intensity. The pulsar is seen in almost all subbands and displays the expected change in arrival time due to dispersion in the interstellar medium. The total dispersion delay across this frequency range is about 1.8 seconds or 2.5 pulse periods. Only a couple of the subbands are affected by interference.
The right hand plot shows the sum of all subbands once the dispersion delay has been corrected for, and shows an excellent signal to noise. We note that this detection uses only 2.5% of a single station's collecting area!
Credit: Ramesh Karuppusamy & Ben Stappers
ITS All Sky Survey Map with detected 3C Sources
As shown above the ITS is capable of making
snapshot images of the sky above the site. If such snapshots are
taken at different times of the day, a different part of the complete
sky is imaged. These images can then be combined to produce an all
sky map as shown here. This map is composed of 86 snapshots using
all available RFI free channels between 29.5 and 30.5 MHz. To make
the weak sources on the map visible, the two strongest sources,
Cassiopeia A and Cygnus A have been subtracted from the individual
snapshots after calibrating the array. All these steps have been
done automatically by a computer without human interference, showing
that we're able to make an automated data reduction pipeline. The
detected sources from the 3C catalog have been indicated. There
are more sources which are probably detected, but not very clearly.
These are not included here.
credit: Stefan Wijnholds (ASTRON)
On June 22nd, 2004, an ITS observation of Solar and Jovian bursts was carried out jointly with the (NANÇAY Observatory, France).
The observation was performed in sync-stop mode, such the data recording could be controlled by an external trigger; this trigger was derived from the real-time display at Nançay, such a data snapshot was taken when a Jupiter burst took place. For each snapshot 6.7 seconds of digitized time-series data (at 80 MHz sampling frequency) were recorded, transfered to ASTRON and analyzed offline.
The image to the right shows
a 28.05882 MHz map centered on the position of Jupiter during a
credit: Lars Baehren (ASTRON)
Lightning over ITS
The weekend of July 17.-18. 2004 experienced the passing of a large storm system over the Netherlands. A significant amount of thunderstorm discharges were recorded over the area of the Drenthe province, 'local' lightning activity peaking between 18:30 and 19:00 hours. Analysis of chance observations (consisting of 0.419 seconds digitized time-series data) revealed prominent short-time broad-band radio emission features. Follow-up observation were carried out on several occasions; the image to the right shows the typical radio emission signature by which electric discharges in thunderstorms can be identified in dynamic spectra. Beamforming of the raw data yields temporal-spatial resolved maps, some cases also allow distance determination.
credit: Lars Baehren (ASTRON)
30 MHz map of the sky taken with the Initial Test Station (60 antenna's operating at 10-40 MHz, resolution of ~3 degrees at 30 MHz).
Integration time is 6.7 seconds, total bandwidth 1.05 MHz selected from the clean channels in the 24-31 MHz frequency range. The brightest discrete source Cas A (3C461) has been subtracted. Other well known discrete sources are also indicated. The diffuse emission is due to the Milky Way. The resolution of the final array will improve by a factor of ~250,000 (with 100 km baselines) compared with this ITS image and the sensitivity will go up by at least 7 orders of magnitude (more collecting area and longer integration time plus broader bandwidth).
credit: Michiel Brentjens (ASTRON)
observes a solar burst
The observation (click on the picture to the right/left to start the movie) have been done during a demonstration in Amsterdam. When the movie starts, it shows the "normal" sky with Cas A as brightest source on a region of diffuse emission from the galactic plane. At about 10:30 a bright source appears at the southeastern horizon becoming gradually stronger thereby overpowering the other sources in the sky. At about 12:00 its power decreases and Cas A becomes visible again. A check on the location of the source indicated that it could be identified as the sun. Our observation is consistent with measurements from the SOHO satellite which observed a solar burst at that time.
credit: Stefan Wijnholds (ASTRON)