The global radio astronomical community is working towards the Square Kilometre Array (SKA), a radio telescope a hundred times bigger and better than any current radio telescope. One of the major improvements that the SKA will bring is that the field of view, i.e. the region of the sky that can be imaged in a single observation, will be much larger than what is possible now.
There are several ways of doing this, and 'Apertif' is a project that explores one of the technologies giving such a larger field of view, while also trying to exploit it for doing science. More concrete, Apertif aims to increase the field of view of the Westerbork Synthesis Radio Telescope (WSRT) with a factor 25, as is illustrated in the picture here. This remarkable performance gain is achieved by placing a receiver array in the focus of each parabolic dish of the WSRT, instead of the single receiver element that the current system employs. Such an array in the focus of a dish is called a focal-plane array (FPA).
The increase in field of view is comparable to the performance leap of optical telescope systems in the past when they made the transition from a single photometer in the telescope focus to large-format CCD detector arrays. Apart from studying the technology, because Apertif will be installed at an existing telescope, Apertif will make it possible to start doing some of the kinds of astronomy that SKA will do. So it is not only a technological project, but also, and mainly so, a science project.
The image on the right shows a prototype focal-plane array just before it got mounted in one of the WSRT dishes. One can clearly see the many elements of the FPA (there are 112 in this FPA). Each element receives signals from the sky and the field of view of each element has the same size as the field of view of the WSRT. In other words, the current WSRT system is equivalent with using only one of these elements. However, each element looks in a slightly different direction and sees a slightly different part of the sky. This means that by combining the signals of all elements a larger part of the sky can be observed. With Apertif, the WSRT can image an area on the sky about 25 times the size of the full moon.
The improvement of the field of view of the WSRT means that one can survey large areas on the sky much faster compared to the current system. A project that would take a century with the current WSRT will take only a few years with Apertif! This enormous improvement will enable all kinds of new types of astronomical research. Key science projects envisaged for Apertif are:
One of the great puzzles in current astronomy is that the rate at which new stars are born has been decreasing steadily for the last several billion years. It is not clear why this is happening, but it must be related to the amount of gas in and around galaxies. New stars form from gas so if fewer stars are born, this must be somehow connected to decreasing gas supplies. However, very little observational information is available about this. With Apertif it will be possible to determine the gas content of more than a million galaxies over this period of decreasing star formation and to study in detail what the role of dwindling gas supplies is in this process.
Apertif will make the WSRT a very effective fast-transient search machine. In general, the problem with arrays of radio telescopes for searching the sky to find new pulsars and transients is the limited field of view of the array beam. However, because the WSRT is a linear (i.e. purely east-west) array, combined with the fact that the spacings between the antennae of the array is regular (i.e. most of the dishes are 144 m apart, the WSRT+Apertif does not have this limitation. Using a dedicated, high-power "beam former" and transient-search compute cluster, Apertif can exploit the full field of view to search for fast transients and pulsars.
Because of the large field of view, the entire sky can be imaged with high spatial resolution and good sensitivity. With Apertif more than 10 million continuum sources will be detected, giving information about how star formation changed with cosmic time, and about how the population of active galaxies (i.e. galaxies with super massive black holes) evolves. An interesting aspect is, given the sensitivities of Apertif and LOFAR, that such a survey matches very well with similar surveys done with LOFAR. Most of the sources detected with LOFAR will also be detected with Apertif.
Because so many continuum sources will be detected, one can also study the magnetic fields in these sources, and such fields along the line of sight between these sources and us. This will allow astronomers to map the magnetic field in our own galaxy with unprecedented detail.
Currently, a prototype focal-plane array is mounted in one of the WSRT dishes. With this prototype, we can do all sorts of tests on performance, on how to make images using an FPA etc etc. In early 2008, this was used to make a spectroscopic observation of the nearby galaxy M31. The object on the left is the neutral hydrogen in M31 as seen with the Apertif prototype, while on the right is the image made with the full WSRT. Although the new image looks much worse compared to the old one, it still means an enormous improvement. The reason that the old image looks sharper is that all 14 WSRT dishes were used to make it, while for the new one with the FPA only one dish was used. The big improvement is that to make the new image, only one telescope pointings has to be used, while for the old image 163 pointings were used.
Just before the end of 2008, we managed to obtain the first interferometric results between the Apertif prototype and other WSRT dishes (equipped with the "old" receivers). The plot on the right shows the correlation signal (fringes) of a short observation of the source 3C286. The signal goes up and down because due to the earth's rotation, the path difference between the antennae changes. One can clearly see that for longer baselines this path difference changes faster (as it should of course).
Recently, the combination of the Apertif prototype with other dishes in the Westerbork array has borne even more fruit. By measuring the interferometric fringes on different baselines over the course of 12 hours, we have been able to achieve the first interferometric images using a focal plane array as one of the receiving elements. The first images used only a single FPA element, but we were soon able to form images which used all of the receiving elements in the FPA to form a compound beam - in this case, optimised to provide maximum signal-to-noise on the optical axis, but such compound beams can be formed to optimise other quantities of interest, such as the polarisation purity.
The image shown here is the first one to be formed using this new technology, and shows the field of the sources 3C343 and 3C343.1, the Castor and Pollux of the radio sky.