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Articles on this Page
- 02/23/15--16:00: _Comet Lovejoy
- 02/24/15--16:00: _Sneeuwpret
- 02/25/15--16:00: _Today's colloquium:...
- 02/26/15--16:00: _Symbionos: Test of...
- 03/02/15--16:00: _Near-Earth asteroid...
- 03/01/15--16:00: _LOFAR data goes public
- 03/03/15--16:00: _Meetings are better...
- 03/04/15--16:00: _Digging up LOFAR in...
- 03/05/15--16:00: _Of Beams & Blue Rib...
- 03/08/15--17:00: _How Truly Great Inv...
- 03/09/15--17:00: _Dutch Astronomy in ...
- 03/10/15--17:00: _SNOW-FAR
- 03/11/15--17:00: _Today's Colloquium:...
- 03/12/15--17:00: _SETI reloaded: Next...
- 03/15/15--17:00: _Today, official ope...
- 03/16/15--17:00: _Today's colloquium:...
- 03/17/15--17:00: _Immersed grating De...
- 03/18/15--17:00: _Today's colloquium:...
- 03/19/15--17:00: _WSRT in the snow.
- 03/22/15--17:00: _The Low Noise Tile ...
- 02/23/15--16:00: Comet Lovejoy
- 02/24/15--16:00: Sneeuwpret
- 02/26/15--16:00: Symbionos: Test of Simultaneous Observations
- 03/02/15--16:00: Near-Earth asteroid flyby
- 03/01/15--16:00: LOFAR data goes public
- 03/03/15--16:00: Meetings are better when people stand up
- 03/04/15--16:00: Digging up LOFAR in Finland
- 03/05/15--16:00: Of Beams & Blue Ribands II: Depths and DoFs
- 03/08/15--17:00: How Truly Great Inventions are made
- 03/09/15--17:00: Dutch Astronomy in the 20th Century
- 03/10/15--17:00: SNOW-FAR
- 03/15/15--17:00: Today, official opening of the new ASTRON building
- 03/16/15--17:00: Today's colloquium: Four years of detecting cosmic rays with LOFAR
- 03/17/15--17:00: Immersed grating Demonstrator Delivered to NOVA-ASTRON
- 03/19/15--17:00: WSRT in the snow.
- 03/22/15--17:00: The Low Noise Tile - Bandwidth extension
© Albert van DuinComet C/2014 Q2 (Lovejoy) was discovered by amateur astronomer Terry Lovejoy on August 17 2014 with a 20cm Schmidt-Cassegrain telescope from Australia.
The last few months this comet has been a nice photographic object, even visible to the unaided eye from a dark location. Due to the cloudy spell we had here the last few months, there were not many opportunities to image this comet. On the 17th of February however, the weather was favorable, a clear sky and little moisture in the air.
So I took the prototype Lynx Astrograph, designed and built by optical designer Rik ter Horst (NOVA-ASTRON), to a dark location near Beilen and exposed 19 images of 60 seconds each at 1600 ISO, with a modified Canon 5D Mk2 and the 280mm F/2.5 (735mm focal lenght) astrograph.
Due to the winsorized sigma clipping combination of the 19 images, and the fact that the position of the comet between the stars is different on every image, the stars look a bit strange after stacking. A different processing method should produce an image with sharp stars as well as a sharp comet, but that will take some time.
This comet is now moving away from us, but it will return in about 11000 years.
© Arnold van ArdenneOu sont les neiges d'antan?
© JGThe bulk of a neutron star's rotational energy is believed to power an ultra-relativistic outflow, called a pulsar wind, which creates a pulsar wind nebula (PWN) when it expands into its surroundings that, when the neutron star is young, is located within the supernova remnant (SNR) created by the progenitor explosion (see the cartoon above). In this case, the evolution of the PWN is sensitive to that of the evolution of both the neutron star and supernova ejecta. Additionally, the association of many Galactic gamma-ray sources with young pulsars strongly suggests that particles are accelerated to extremely high energy in these sources. In this talk, I will demonstrate that one can use the properties of a PWN inside a SNR to measure the initial properties of both the pulsar and supernova ejecta, as well as the spectrum of particles accelerated inside the PWN, and I will discuss the implications of these results on models for the formation of neutron stars and high-energy cosmic ray electrons.
The pictures above show examples of PWNe in SNRs:
Upper: A composite image of SNR G54.1+0.3: the white source near the centre is pulsar J1930+1852, the Chandra X-ray image of the PWN is shown in blue, and the Spitzer infrared images of the gas and dust that condensed out of the SNR are shown in yellow and red. Images courtesy of: X-ray: NASA/CXC/SAO/T.Temim et al.; IR: NASA/JPL-Caltech.
Middle: A gamma ray map of the very high energy gamma-ray source HESS J1640-465 region, which is positionally coincident with the SNR G338.3-0.0. The dashed circle marks the position and extension of HESS J1640-465, the 610 MHz radio contours are shown in black (Castelletti et al. 2011), and the green circle indicates the position of the candidate PWN XMMU J164045.4-463131. Image courtesy of: H.E.S.S. Collaboration/A.Abramowski et al.
Lower: A deep Chandra X-ray image of the SNR Kes 75 (shown in red shades to identify the low-energy emission), with the pulsar J1846-0258 (the bright spot near the center of the image) and the PWN (shown in blue shades to identify the high-energy emission). Images courtesy of: NASA/CXC/GSFC/F.P.Gavriil et al.
© Madroon Community Consultants (MCC)As explained in an earlier AJDI, the Symbionos project investigates the possible symbiosis of LOFAR and GNSS observations, for mutual benefit in terms of the ionosphere. It is a joint project with the Dutch aerospace lab NLR, under ESA contract EGEP ID 89.08.
The picture shows (part of) the special test observations that were done recently. LOFAR was pointed towards a sequence of bright(ish) radio sources close to the the positions of two different GPS satellites, as they moved across the sky. The signals of the latter were recorded by an advanced dual-frequency GNSS receiver in the concentrator node of the LOFAR core.
Since the signals received by both instruments propagate through the same part of the ionosphere, they should be affected by (almost) the same Total Electron Content (TEC). So any differences between them tell us how well LOFAR and GNSS can complement each other in practice.
The LOFAR HBA fields have a diameter of about 5 degrees. They are centered on bright sources from the recently completed MSSS sky survey. Indicated are the brightness of the central source, and the total number of MSSS sources in that field (which will be used for calibration). In this early stage, we steer clear of the complications caused by the Milky Way (yellow) or very bright radio sources like Taurus A.
The Symbionos team was very pleased with the generous help of the LOFAR science support group, especially Luciano Cerrigone, in preparing and executing these rather unusual observations. We are also grateful to George Heald for mining the MSSS survey for suitable calibration sources and Local Sky Models.
© Cees BassaOn January 26th an asteroid by the name of 2004 BL86 flew by the Earth. There was no danger of it impacting, as the closest distance during the flyby was 1.2 million km (3.1 lunar distances). Even though the asteroid is small, it is estimated to be about 325m in diameter, the close proximity meant it was visible as 9th magnitude star, moving between the stars at about 2.5 degrees per hour. At 9th magnitude the asteroid was easily visible in small telescopes or binoculars.
The close approach to Earth allowed radar observations with the Goldstone and Arecibo telescopes. These confirmed that the asteroid has a small moon. The existence of the small moon was reported earlier by Joe Pollock and Petr Pravec based on optical lightcurves of the asteroid.
This composite image of 15 10s exposures shows the movement of 2004 BL86 with respect to the stars. A DSLR and a fast lens were used to take the images. Besides the asteroid two geostationary satellites are also visible.
© ASTRONMarch 2, 2015 marks an important event on the LOFAR calendar. From that day onward, scientific data will be made publicly available through the LOFAR Long Term Archive (LTA).
The International LOFAR Telescope (ILT) operates a state of-the-art array of antenna stations in the Netherlands and other European countries at radio frequencies below 240 MHz, with advanced data processing equipment. Telescope time is available to the world-wide astronomical community through open calls for proposals that are announced every 6 months. Accepted proposals lead to assignments of specific time-limited proprietary data rights to carry out proposed science only. The data are placed in the LTA and remain the property of the ILT.
The table of contents of the LTA is fully public at any time. All data resulting from regular production observing cycles become fully public after the assigned proprietary period(s). Parallel data access rights for science unrelated to the original proposal(s) may be requested from the Director at any time.
The LTA is accessible through the following website:
Additional information on how to use the LTA can be found on the LOFAR wiki:
© Madroon Community Consultants (MCC)It is well-known that meetings are shorter, and arguably more efficient, when people do not loll about in chairs(*). For one thing, it prevents them from fiddling with their little laptops, so they actually participate in the proceedings. For another, they are kept more alert by the blood that is pumped vigorously to the brain. But most of all, since there is no option to just sit it out, they are all keen for the meeting to reach its conclusion, and will actively work to facilitate that.
The LOFAR/WSRT operations group, under COO Roberto Pizzo (right) meets briefly every morning in the telescope control room in Dwingeloo. Continuing towards the left: Michiel Brentjens, Manu Orru, Jur Sluman, Luciano Cerrigone, Sander ter Veen, Pieter Donker and Teun Grit. Obviously, this is only a small Italian-Dutch subset of the proudly international team that makes these wonderful telescopes available to you, the user.
(*) The editorial staff of the prestigious French newspaper Le Monde used to meet standing up, until they went soft. Being mutiliated in the Resistance was the only acceptable excuse for sitting down, and only on tuesdays.
© Anna ScaifeHi Jan
Thought you might like these? Examples of practical bandpass calibration from KAIRA :-)
It's generally ok for the elastic bungee to be below the snow line, but as soon as the wire gets covered even slightly then the bandpass drops. Then it's a case of digging it out, but of course you need to dig your way to the antenna first!
I'm out here with Poppy Martin and Derek McKay-Bukowski for the next week, so there may be more pictures to follow...
© Rhodes University / SKA SA / NRAOLast year we presented a new world record dynamic range (DR) map of the field around 3C147 made from JVLA C- and D-config L-band data. It is about time to refresh this result. The map above boasts a modest gain in nominal DR (just under 5.1 million to 1). Such DR is difficult to capture in a single static image; the one above is rendered in a log-scale colourmap from 0 to 10 mJy. The map is about 3 degrees across, and we can see sources imaged well into the second sidelobe of the primary beam (PB).
Most of the gain in DR is due to all of the available bandwidth now having been processed, or about 640MHz in total (the rest of the 1GHz observable bandwidth being lost to RFI), as compared to 192MHz in last year's map. In technical terms, however, the new map represents a completely new calibration pipeline, one that fully incorporates a wideband PB model into the calibration, and corrects for direction-dependent effects (DDEs) while solving for orders of magnitude fewer degrees of freedom (DoFs). For an apples-to-apples comparison, we also present versions of the 192 MHz image made with the new-style pipeline and the old pipeline (rendered in linear scale close to the noise).
The crucial difference in the new pipeline is that it includes a full-Jones PB model (in this case, produced by Walter Brisken's cassbeam software, though any model pattern can be plugged into MeqTrees via FITS cubes), accounting for the rotation of the PB pattern on the sky, as well as scaling of the PB with frequency. In addition, we apply the MT-MFS algorithm during deconvolution to recover apparent source spectral indices, and explicitly correct these for PB frequency scaling. The 2014 pipeline used solvable differential gains ("dE's") to absorb all these effects, while applying them to only a subset of "troublesome" brighter sources (marked by crosses in the comparison images). The new pipeline applies a rotating, frequency-dependent PB to all sky model sources during calibration, which takes care of the bulk of the DDEs, while an additional layer of dE's is applied to the "troublemakers" to accommodate some residual DDEs (such as pointing error, and actual deviations from the model pattern). This is illustrated by the three insets on the bottom right: these show sources without any DDE treatment, then with a PB model applied, and finally with a PB model and extra dE solutions.
The immediate benefits of using a proper PB model are threefold. Firstly, all sources, not only those with explicit dE solutions, become largely DDE-free (compare especially the first PB sidelobe in the new-style and old-style images). This, secondly, means that we've taken a big step towards the Platonic ideal of calibration -- our outputs are a source catalog with accurate intrinsic fluxes and spectral indices, and an almost noise-like residual map. By contrast, the dE-only approach absorbs intrinsic source properties into directional gain solutions, thus destroying astrophysical information. Thirdly, the remaining dE solutions need to account for a much smaller effect, and can therefore be made much "stiffer", i.e. with fewer solvable degrees of freedom (in fact, exactly a factor of 60 fewer in the new-style map). The upper right and lower left insets show typical old- and new-style dE solutions for four particular sources. Cutting down on the number of solvable DoFs reduces such undesirable artefacts as source suppression and ghosts. This is plain to see in the upper left insets, which show a small section of the new map, and a difference image w.r.t. the old map. It is clear that the new pipeline consistently recovers more flux.
While a few subtle technical challenges remain (e.g. some deconvolution problems around faint complex sources are now apparent -- though some might call this a luxury problem, since at lower DRs the 3C147 field is notorious for being point source-only!), we don't expect to wring any significant new DR records from this data, since our map is already confusion-limited at the centre of the field. With a few hours of B-config time, we could forge on deeper, but until then, the Blue Riband stands up for grabs at a bit over 5 million.
© Tom OosterlooIt almost seems to be the case in modern science that the only way to progress is through an increasing level of scale, planning and organisation. Although times change, and it is wrong to be sentimental about the past, it is good to realise that most Truly Great Inventions were not made this way.
A case in point is the invention of wireless communication (the Radio), generally attributed to Guglielmo Marconi (although the full story involves many people). Arguably it is one of the most important inventions ever. And very important for us: without radio, there would be no radio astronomy.
The story of Marconi is very interesting. One learns that the real keys to success are vision and persistence. Already at an early age, Marconi saw the enormous potential of the "Hertzian waves", which physicists all over the world were studying: if they could be used for long-range communication, it would completely change the world. However, at the time, most scientists believed these waves could not be used for long-range transmissions: waves propagate in straight lines, which would be a problem with the Earth being a sphere. Marconi, who had average grades at school and never completed university training, did not believe this would make the waves useless. And in 1895 he proved he was right. By using radio waves, he managed to remotely ring a bell which was placed a few kilometers away on the other side of a hill. After this, things moved very fast: in 1901 Marconi achieved the first trans-atlantic radio transmission (which indirectly, 32 years later, led to the discovery of radio astronomy, by Karl Jansky) and not long after, most large ships (including the Titanic) carried radio communication devices. Marconi shared the 1909 Nobel Prize in Physics with Karl Ferdinand Braun (co-founder of Telefunken) "in recognition of their contributions to the development of wireless telegraphy".
The family house where Marconi conducted his experiments, Villa Griffone near Bologna, is now a museum and is a must-see for everybody involved in radio astronomy. The picture above shows the room, in the state it was in during 1895, where Guglielmo Marconi did his experiments and where he demonstrated wireless communication. It is reassuring to see that apart from having vision and persistence, being messy is also good...
© PrometheusPublished this week: De ontdekkers van de hemel: De Nederlandse sterrenkunde in de twintigste eeuw, by science historian David Baneke.
There is something peculiar about astronomy in the Netherlands. Despite its cloudy skies and ubiquitous city lights, NL has been a major astronomical power for more than a century, with giants like Kapteyn, Minnaert and Oort. How come? Who were these people? And why the Netherlands? In The Discoverers of the Heavens, Dr Baneke shows how the story of Dutch astronomy is entangled with the political, economical and cultural history of this remarkable country.
Of course it helped that radio waves from the sky easily penetrate the Ruysdael clouds, and can be studied day and night, right in our back yard. Prof Oort was the first astronomer to grasp the potential of radio astronomy. This led the Netherlands to lead this important(*) new field with world class radio telescopes like the Dwingeloo 25m dish (1956), the WSRT (1970) and LOFAR (2010). Not surprisingly, David regularly visited ASTRON to research his book.
But the rise of Dutch astronomy started with Kapteyn, half a century before radio astronomy. An important factor was the close collaboration with other vigorous scientific communities, particularly in South Africa and (later) the US. In the case of Kapteyn this led to the amusing situation that he was honoured in London, Paris, Berlin, St Petersburg and Washington before the Dutch themselves realised his stature. After Kapteyn, many Dutch astronomers have held influential positions abroad, and vice versa.
The book may be ordered at: http://webwinkel.uitgeverijprometheus.nl/book/david-baneke It is written in Dutch, but that should not be a problem for many astronomers who have spent the most fertile years of their lives over here.
(*) Most of the Nobel (physics) prizes that have been awarded for astronomical subjects were for radio astronomy. Unfortunately, the Dutch tend to be too modest to be noticed in this way.
© Poppy MartinThe Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA; www.sgo.fi/kaira) uses LOFAR technology to form an independent radio telescope situated in Finnish Lapland (69°4'15''N 20°45'43''E). This week we successfully implemented the KAIRA pulsar mode and ran a joint experiment with the EISCAT UHF incoherent scatter radar (www.eiscat.rl.ac.uk/radars/uhf/), based near Tromsø (Norway). EISCAT's UHF radar allows us to measure the total electron content in the ionosphere in any given direction. Variation in the total electron content in the ionosphere causes changes to the observed dispersion and rotation measures of pulsars. By simultaneously monitoring a sample of pulsars using the KAIRA HBA and EISCAT UHF, these observations can be used to study the temporal and spatial variations in the ionosphere.
This method of observing the ionosphere complements observations of the absorption of cosmic radio background due to the ionosphere, made using KAIRA as a riometer (Relative Ionospheric Opacity Meter). Riometry is a well-established technique for observing the D region of the ionosphere and using the broad bandwidth of the KAIRA LBA, local height profiles of the electron density can also be recovered from multi-frequency observations.
Observing conditions at KAIRA are somewhat different to the rest of the LOFAR telescope. It is cold. Very cold. In March the average temperature is -10°C, with lows of less than -30°C, and the average snow cover is 100cm. For the KAIRA HBA on its raised platforms, snow needs only to be swept off periodically; for the LBA, some digging is required to prevent the bandpass response being compromised. There is no high-speed ethernet connection to the KAIRA station and observers generally need to work on site in order to collect their data (and dig for their bandpass). But visitors are not completely isolated, as basic facilities are available a short walk down the road at the customs station on the Finnish-Norweigan border.
© Kevin Koay (University of Copenhagen)Two phenomena observed in the last decade were attributed to the scattering of radio waves in the ionised intergalactic medium (IGM). The first was the observed suppression of interstellar scintillation of compact AGNs at redshifts greater than 2; this was thought to arise from an increase in the apparent angular sizes of high redshift sources due to scattering in the IGM. The second is the frequency-dependent broadening of pulses seen in a fraction of extragalactic fast radio bursts. These phenomena could potentially provide unique probes of the turbulent properties of the ionised IGM, where the majority of baryons in the Universe reside, allowing us to also map and model the distribution of intergalactic ionised gas. In this talk, I will discuss whether these phenomena can indeed be attributed to intergalactic scattering. I will also talk about whether we can expect to detect IGM scattering with current and next generation radio telescopes such as RadioAstron and the Square Kilometre Array, based on observational constraints and theoretical models. The magnitude of scattering in the IGM will have implications for the detectability of extragalactic fast radio transients at MHz frequencies.
Figure: Scattering in the ionised and turbulent intergalactic medium (IGM) can lead to an increase in the apparent angular size of a compact radio source in the background. Since sources that have undergone IGM scattering will appear larger from the viewpoint of the Galactic interstellar medium (ISM), interstellar scintillation will be suppressed in these sources, relative to sources not scatter broadened by the IGM. This idea was initially used to explain observations revealing that compact AGNs at high redshift tend to scintillate less than their low redshift counterparts. While more recent studies have found that this is not the case, there is potential for very sensitive instruments such as the Square Kilometre Array to monitor the interstellar scintillation of compact AGNs, and thus use the resolving power of interstellar scintillation to detect IGM scatter broadening down to ~ 1 microarcsecond scales.
© ASTRONThe Search for Extra-terrestrial Intelligence (SETI) using radio telescopes is an area of research that is now more than 50 years old. Thus far, both targeted and wide-area surveys have yet to detect artificial signals from intelligent civilisations.
In a recent paper, I argue that the incidence of co-existing intelligent and communicating civilisations is probably small in the Milky Way (see Garrett 2015 to appear in Acta Astronautica http://arxiv.org/abs/1503.01336 ). While this makes successful SETI searches a very difficult pursuit indeed, the huge impact of even a single detection requires us to continue the search. A substantial increase in the overall performance of radio telescopes (and in particular future wide-field instruments such as the Square Kilometre Array - SKA), provide renewed optimism in the field. Evidence for this is already to be seen in the success of SETI researchers in acquiring observations on some of the world's most sensitive radio telescope facilities (including LOFAR) via open, peer-reviewed processes.
The increasing interest in the dynamic radio sky, and our ability to detect new and rapid transient phenomena such as Fast Radio Bursts (FRB) is also greatly encouraging. While the nature of FRBs is not yet fully understood, I argue they are unlikely to be the signature of distant extra-terrestrial civilisations. As astronomers face a data avalanche on all sides, advances made in related areas such as advanced Big Data analytics, and cognitive computing (captured in the image above, created by Danielle Futselaar) are crucial to enable serendipitous discoveries to be made. In any case, as the era of the SKA fast approaches, the prospects of a SETI detection have never have been better.
© ASTRONToday, the 16th of March 2015, state secretary Dekker from OCW will officially open the new building. After a visit to LOFAR and Westerbork, the opening ceremony will be held at ASTRON's HQ in Dwingeloo.
In 2012, the construction of the new building started and is now completed. In the picture you can see some photos that were taken during the construction phase.
The new building helps us to keep doing what we do best, making discoveries in radio astronomy happen!
Photos by Ingrid Arling
© AN & LOFAR Cosmic Rays KSPIn June 2011 the first signal from an air shower was detected using LOFAR. Air showers are particle cascades in the atmosphere that are caused by very high energy cosmic rays (>1016 eV). The bulk of charged particles in the air showers causes radio emission, which can be measured as a nanosecond-timescale pulse using the LOFAR antennas.
Over the last four years, the members of the Cosmic Rays Key Science Project have not only made substantial progress in the understanding of the radio emission, but they have also gathered a lot of information about the LOFAR system and its performance. Both items will be summarised in this colloquium, with an emphasis on the current understanding of the radio emission of air showers and how it contributes to studies concerned with the origin of cosmic rays.
The image shows a LOFAR low-band antenna measurement of an air shower above the LOFAR Superterp. Good agreement between the data (shown by the circles) and simulations (shown by the background colours) is illustrated.
© NOVA Optical Infrared Instrumentation groupThe Dutch astronomical community is deeply involved in the instrumentation program for the European Extremely-Large Telescope (ELT). One of its instruments will be METIS, an instrument that will study the universe at "thermal" wavelengths - looking primarily at cooler and heavily embedded objects, like star formation regions, proto-planetary disks, planets, the galactic centre.
One of its key capabilities is high resolution integral field spectroscopy. Due to the criticality of the performance of the disperser in the instrument, an alternative method to disperse the light has been developed in close collaboration with SRON, Philips Innovation Services and TNO.
The advantage of an immersed grating is that the size of the large component (160mmx126mm) is significantly smaller (a factor of 3) than a conventional grating would be. However, the accuracy of its surfaces needs to be better by the same factor of 3. Using lithography techniques, the quality of the component can meet the requirements, and it has now been succesfully integrated. The next steps will be the full characterisation of its performance, which will be done at the NOVA-ASTRON group and at ESTEC in Noordwijk.
The component is transparent for IR-wavelengths, and fully reflective for visible light. The nice colours from the sides of the prism-shaped grating (bottom) are caused by Anti-Reflection and Absorption coatings. More information is available via http://esfri.strw.leidenuniv.nl/
© ESA/ATG medialabRadio pulsars have been known for nearly half a century, and they have become mainstay tools in several different areas of astrophysics. However, we still have only a rudimentary knowledge of how they work. Several of the foundational ideas that have guided attempts to understand their physics are probably incorrect.
Some recent observations and analyses show concretely that a pulsar is at root a plasma machine. In the inner regions near the star, the plasma is so dense and magnetized that EM waves cannot propagate; whereas most radiation that does reach us has been heavily processed by the more tenuous plasma at higher altitudes.
Key to understanding pulsar action, then, is investigating the properties of the so-called "core" radiation that comes from the lowest altitudes within the polar flux tube. This radiation we find propagates as the extraordinary mode and exhibits intensity-dependent aberration/retardation.
© Richard BlaauwWith the winter behind us, the WSRT can look forward to a very exciting summer period. In particular, the roll-out of APERTIF-6 is about to begin, and several of the telescopes have already been renovated in anticipation of the PAF deployment. The telescope operators of the radio observatory are also looking forward to the new challenge of operating this new instrument - we look forward to being part of the many discoveries that are bound to be made.
© ASTRONThe successful design and measurements of Low Noise Tile (LNT) arrays were presented in a series of daily images last year. The next step in the development is to broaden the frequency range, to meet the SKA Mid-Frequency Aperture Array (MFAA) specification of 500 MHz to 1500 MHz.
The frequency range of the present LNT-design has a lower limit of 1000 MHz, caused by a high-pass filter for suppressing RFI from GSM and nearby TV-stations in the Netherlands. A redesign of the Low Noise Amplifier module, including removal of the high pass filter, exhibits no frequency limitation in the Wideband LNT (WLNT) amplifier module. This can be seen in the left picture, which compares WLNT LNA-results with the earlier LNT. It shows a 5 K noise temperature reduction across the original frequency range for the WLNT, due to the removal of the losses associated with the high pass filter. (The effect of the filter in the old LNT-module is seen as a sharp increase in noise temperature below 900 MHz).
The figure on the right shows the measured noise temperatures of the new WLNT modules in a 2x2 array. Again, they are compared to the earlier LNT result. Above 1000 MHz, it shows a similar improvement as in the left picture. One may conclude that the absence of RFI filtering does hardly influence the noise above 1000 MHz. However, at lower frequencies, the presence of RFI only allows array noise measurements at a limited number of frequencies. Furthermore, the impedance of the Vivaldi antenna element below 800 MHz (still the same element as in the LNT, designed for optimum performance in the frequency range from 1000 MHz to 1800 MHz) deviates considerably from the optimum noise impedance of the LNA. This explains the sharp increase in array noise temperature near 700 MHz and at 600 MHz. Nevertheless, array noise temperatures close to 40 K have been measured.
To achieve better performance in the low-frequency part of the band, work is now being done on the design of a new WLNT antenna element, optimized for better noise match and sensitivity over the frequency range, particularly between 500 MHz and 1000 MHz.