The Radio Observatory of ASTRON invites the world-wide community to submit Expressions of Interest to define, prepare and perform large survey programmes with Apertif, the new Phased Array Feed receiver system for the Westerbork Synthesis Radio Telescope. The WSRT and Apertif serve as a scientific and technological pathfinder for the Square Kilometre Array.
The submission of Expressions of Interest by the community is the initial step toward selecting and defining large surveys that will take full advantage of the strengths of the WSRT equipped with Phased Array Feeds. Submitting an Expression of Interest offers the survey science teams with the opportunity to become involved in defining the surveys and derived data products. It also provides the survey science teams with the advantage of prolonged proprietary access to the commissioning data, as well as an optimal positioning for an expedient scientific exploitation of the survey data.
Expressions of Interest should be limited to 3 pages of text (excluding references), while there is no limit for additional figures and tables. Minimum font-size is 11pt. Submission is through the WSRT NorthStar web tool at http://apertif.astron.nl/proposal which will be activated on Friday, 10 September 2010. The deadline for submission is
The Westerbork Synthesis Radio Telescope consists of fourteen 25-metre dishes, configured as an east-west array, 2.7km long. Earth-rotation is used to fill the UV-plane. The four eastern dishes are movable along short railway tracks to improve the UV-coverage. This configuration provides an angular resolution of 13 arcseconds at 1420 MHz. The equatorially mounted dishes and some redundancy of the baselines provide excellent polarisation characteristics and the highest achievable dynamic range in imaging. After more than 40 years, the WSRT will be transformed into a superb survey facility, operated under a new model with direct engagement of the community through active survey science teams.
Apertif will enlarge the instantaneous Field-of-View for twelve of the fourteen 25-metre WSRT dishes to 8 square degrees and provides 16384 spectral channels over a 300 MHz contiguous bandpass to be tuned anywhere between 1000 and 1750 MHz, increasing the survey speed of the WSRT by a factor 20. Further technical details of the Apertif system can be found below.
While 15-25% of the available observing time will be allocated to smaller 'open-time' programmes up to several hundred hours, Apertif surveys are expected to extend over a four-year period, consuming a total of several thousand hours per survey. It is stressed, however, that no a priori restrictions are imposed on the size and the scope of the proposed surveys. To justify the large amount of observing time dedicated to a survey, the following considerations need to be addressed in the Expression of Interest:
Large surveys with Apertif should enable research of the highest scientific relevance. Surveys with Apertif are expected to yield early and intermediate scientific results based on a partially executed survey. EoI's should contain an estimated timeline of the anticipated results.
The data collected by the survey science teams may serve as focused study on particular scientific questions. The data and derived data products, however, should have continued value for the global community to investigate the broadest possible range of scientific topics, long after the aims of the original survey are complete.
To allow for the most efficient use of the available observing time, Apertif surveys are encouraged to consider accommodation of commensal measurements of observables that do not pertain to the direct scientific goals of the proposed survey.
The WSRT array contains four movable telescopes to adjust the configuration of baselines. To maximise observational and operational efficiency, surveys should be carried out with no or a minimal number of telescope re-locations. The need for multiple configurations should be motivated in the EoI. Without well-motivated requests by the community for multiple configurations, observations will be carried out in the standard 'maxi-short' configuration.
The Apertif correlator provides 16,384 channels over a contiguous 300 MHz bandwidth which corresponds to a channel width of 18.3 kHz or 3.9 km/s at 1420 MHz. Alternative correlator configurations such as a spectral 'zoom-mode' with narrower bandwidth and higher spectral resolution, can be considered and should be specified in the EoI if desired.
After a brief period of validation and quality assessment, the data will be deposited in an Open Access archive. The EoI should address which data products (cubes, images, spectra and catalogues) will be deposited in the archive.
To maximise the scientific return of the Apertif surveys, ancillary imaging or spectroscopic data at complementary wavelengths or from other facilities may be required. The survey science teams are encouraged to address in the EoI how these ancillary data will be collected or how access to existing data bases might be arranged, e.g. by means of extended collaborations.
Survey teams are encouraged to express their willingness to engage in a broader collaboration with other teams proposing similar surveys.
An Expression of Interest should focus on the general concept of the proposed survey, provide a short summary of the relevant technical details and the observing strategy (required sensitivity, spectral resolution, sky coverage etc), address all considerations mentioned above, and describe to what extent the survey team expects to contribute, in terms of human resources, to the preparation of the survey, as well as the commissioning process.
Apertif is the upgrade of the WSRT with main aim to increase the ﬁeld of view of the
WSRT. This is done by replacing the current single feed Multi-Frequency Frontends with
Phased Array Feeds (PAFS). Each of the PAFs consists of 121 Vivaldi elements and will
detect the radiation ﬁeld (in dual polarisation) in the focal plane of each dish over an area
of about 1m2. Because of this, for each dish many beams can be formed simultaneously
making it possible to image an area of about 8 degree2 on the sky, which is an increase of
about a factor 30 compared to the current WSRT. The other major difference with the
current WSRT is that Apertif will only operate from 1000-1750 MHz, in contrast to the
current MFFEs which cover a much larger wavelength region. The lower frequency limit is
set by strong RFI due to GSM signals just below 1000 MHz.
The low-noise ampliﬁers of Apertif are not cooled, hence the system temperature (50-55K)
is higher than that of the current WSRT (30-35K). However, this is to some extent
compensated by the fact that PAFs have a much higher aperture efﬁciency (75% vs 55%).
The noise of Apertif spectral line data will be 10-20% higher than of WSRT data. Note that
the total bandwidth of Apertif is 300 MHz vs 160 MHz of the current WSRT. Therefore for
continuum observations Apertif has better sensitivity. The noise levels given in the table
below are calculated from the current experience with the WSRT and include many real-
life effects such as shadowing, solar interference etc. Because of the layout of the WSRT
array, the noise in images tapered to 30 arcsecond resolution is only 20% higher than in
full-resolution images. It has not been decided whether PAFs will be installed on 12 or on
13 WSRT dishes. One WSRT dish will remain operational at all current WSRT bands, for
example for VLBI.
Due to the E-W layout of the WSRT, a full 12-h synthesis is required to obtain full coverage
of the uv plane. Shorter integration times can be achieved by mosaicing a larger region of
the sky while revisiting a given position repeatedly. However, large projects can make only
limited use of this mosaicing mode because of the stress this would otherwise put on the
telescope structure (this year we celebrate they are 40 years old!!!).
Nominally, the PAFs will be used to form 37 beams for every dish. However, the system
allows to make a different number of beams at different frequencies, e.g. more beams at
the high-frequency end of the observing band, and fewer at the low-frequency end.
|Prototype of the Apertif Phased Array Feed, currently installed in one of the dishes of the Westerbork telescope.|
|Field-of-View||8 square degrees|
|Number of beams||37 (nominally)|
|Angular resolution at 1.4 GHz||13 x 13/sin(δ) arcsec2 (EW by NS)|
|Shortest spacing||36 metre|
|Number of dishes||12 or 13|
|Dish diameter||25 metre|
|System temperature||50 - 55 Kelvin|
|Frequency range||1000 - 1750 MHz|
|Instantaneous bandwidth||300 MHz|
|Number of channels||16384|
|Noise spectral line (12 hr)||0.5 mJy/beam over 16 km/s|
|Noise continuum (12 hr)||10 μJy/beam over 300 MHz|
|Noise uniformity over Field-of-View||10-25%|
|Primary beam size of one beam||33 arcmin|
More information on the Westerbork telescope and further technical details of the Apertif system can be found at
For further questions, please contact the Principal Investigators for Apertif:
Tom Oosterloo (oosterloo [at] astron [dot] nl)
Marc Verheijen (verheyen [at] astro [dot] rug [dot] nl)