Call for Expressions of Interest for Apertif surveys

Call for Expressions of Interest for Apertif surveys

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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 which will be activated on Friday, 10 September 2010. The deadline for submission is

2010 September 22, 12:00 CET (10:00 UTC)


About the WSRT and Apertif

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.

Considerations and recommendations for the EoI

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:

Scientific potential

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.

Legacy value

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.

Acceptable array configurations

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.

Desired correlator configurations

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.

Data exploitation

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.

Ancillary data

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.

Procedures and timeline

The following process will be observed towards selecting, defining and implementing the surveys. Further details on the Commissioning and Operations phases will be provided in a forthcoming Call for Survey Proposals.

1. Selection and definition of surveys (2010-2011)

Call for Expressions of Interest (deadline: September 22, 2010)

Submitting an Expression of Interest is a requirement for becoming involved in the definition, preparation and execution of the surveys, and has the advantage of obtaining prolonged and exclusive access to the commissioning data. Abstracts of the EoI's will be made public after the submission deadline.

Evaluation of EoIs (September-October 2010)

The EoI's will be evaluated by a small committee and a non-ranked selection of proposed compliant surveys will be made. Attention will be paid to all above mentioned considerations and with the idea that only a few optimised surveys will eventually be carried out, possibly in a commensal mode.

Workshop (November 22-23, 2010)

Authors of the selected EoI's will be invited to attend a joint Apertif surveys workshop. The aim of this workshop is to facilitate voluntary collaborations and to discuss in depth the various trade-offs that allow for maximum commensurability and scientific return of the surveys. The goal is to define a framework of a few non-overlapping surveys to be worked out in more detail in the months following this workshop.

Call for Survey Proposals (deadline: spring 2011)

Following the workshop and several months of proposal preparations, final proposals will be submitted to the Programme Committee (date to be decided). Only those survey teams that have submitted an Expression of Interest and have participated in the workshop are eligible to submit a final proposal. Surveys will have flexible starting dates, pending the progress of survey-specific preparations and commissioning.

2. Preparing the surveys (2011-2012)

Building the data calibration and reduction pipeline

ASTRON will provide the basic calibration and data reduction pipeline with involvement of the survey science teams to ensure that the algorithms necessary to achieve the survey-specific science requirements are properly implemented. This includes the definition and implementation of quality control mechanisms.

Definition of derived data products and catalogues

The survey teams are expected to contribute to the development of source detection and characterisation methodology and should help define the top-level data products, including catalogues and meta-data such a quality flags, to be stored in the archive.

Data storage and accessibility

The survey teams are expected to help develop a model for storage and accessibility of the survey data in the Open Access archive. This includes considerations on the data volume and the required speed with which the data can be retrieved from tapes or hard disks. The possible need for reprocessing the data should be indicated with an estimate for the required computing power.

3. Commissioning (2013)

Commissioning of the integrated Apertif system (frontends, correlator, pipeline and archive) is expected to start in early 2013 and will take 6 to 8 months. Pilot observations for the surveys will be collected in this period and these data will remain proprietary to the survey teams until 12 months after the commissioning phase has ended.
The survey teams are expected to submit progress reports on a regular basis during the commissioning period.

4. Operations (2013-2017)

The surveys proper are expected to commence by late 2013 with a staged implementation, depending on the outcome of a survey-specific system readiness review. After commencement of the survey, its data will flow into the Open Access archive after a very brief period of quality assessment. Survey science teams are expected to be directly involved in the execution of the surveys on a near daily basis, and bear responsibility for the continuity of the data flow.

Technical specifications of Apertif

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
Aperture efficiency 0.75
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)

Design: Kuenst.    Development: Dripl.    © 2016 ASTRON