Introduction to the verification systems

Instruments

The development of Aperture Array (AA) instruments at ASTRON is distributed over two SKA related 'projects'. The first is the Mid-Frequency AA (MFAA), second is the Low-Frequency AA (LFAA), for which LOFAR is a pathfinder. Both frequency ranges are progressing through a range of verification systems, in short AAVS. The intention of the verification system is to demonstrate the technical suitability of the technology for the science that will be done with SKA. The larger the demonstrator, the more it will look and feel like an actual instrument.

EMBRACE (MFAA AAVS1)

Since March 2012 there are roughly bi-monthly meetings with the EMBRACE team. We discuss the observations for technical and astronomical verification, and are working towards a final verification plan. The meetings are resulting in a detailed verification plan which will be distributed within ASTRON mid-July. The connection with the French EMBRACE team has been tightened, the intention is to run verification tests on both stations to corroborate results.
Since late last year, large improvements have been made with the EMBRACE at WSRT. The tracking is now working properly, as has been demonstrated by a 9-hour pulsar observation. In addition, the two beams are now using entirely different signal paths, and are properly calibrated. There is an ongoing effort to produce a data format which is easily accessible for astronomers (MS format).
The list of tests focuses on demonstrating reliability of the system and repeatability of the observations. It includes beam pattern tests and bandpass observations.

EMMA (MFAA AAVS2)

It is obvious that EMBRACE has severe limitations in demonstrating the use of MFAA for SKA. We need a next step, an instrument which is science capable, has full polarization and a larger bandwidth. This instrument will be an interferometer, and is momentarily named EMMA.
After setting up a baseline design for EMMA, the costing is being discussed. Though EMMA will be a radio astronomy instrument, it will also develop the use of green energy and renewable materials. This makes it possible to acquire funding from an ERC Synergy grant, and combine this with an NWO-groot request. If both are accepted, a total of 20ME would be available, but that would not meet current cost estimates, which lie around 30ME.

Major issues to be discussed in this context are:
- the need for continuous or large bandwidth (500MHz or more): is this a hard requirement, and can we achieve it, given the RFI situation on site?
- the need for 1.4GHz: can EMMA provide new science at that frequency or should we decide to focus on 300-1000MHz instead?
- the location of EMMA: northern hemisphere (Moura) or southern hemisphere (South Africa)?

These issues will be addressed at the upcoming EMMA workshop in ASTRON.

LFAA

The LFAA development is still in the stage of testing and verifying the basic antenna design. Two antennae types are being considered, a log-periodic and a large Vivaldi element. Both have clear pro's and con's, and are currently being tested in a small array of 9 or 16 elements (AAVS0).
The next step will be to build a small multi-element demonstrator to study the behavior of each element in a small interferometer (this will be AAVS1). The interferometer will consist of 256 elements, divided over either 4x64 of 16x16 stations. In order to demonstrate imaging capabilities more stations are prefered, but the size of the demonstrator will determine if the sensitivity is enough to beat the confusion noise for such a small instrument.
Finally, the last demonstrator (AAVS2) will be the size of a single SKA-low station, roughly 11,000 elements. This can be comfortably divided over 22 stations of 500 elements each, giving the AAVS2 demonstrator roughly the size and sensitivity of the Dutch LOFAR core. The maximum baseline length is an important cost factor though, so in practice the AAVS2 may be much smaller, though its sensitivity will be comparable to the LOFAR HBA core, and for the lower frequencies even better.

Dual band discussion

During a recent meeting at ASTRON it was discussed how the LFAA developments should continue, lacking clear guidelines from SPO. There is still a discussion on a dual band option, and it was proposed that this could include the MFAA developments, which are tending towards lower frequencies as well. The frequency range for LFAA could be considered to range from 50 to 300 MHz, and the MFAA could then cover 300 to 1200 MHz. This would meet requests from the scientific community to have frequencies below 70MHz, and optimize the costing for MFAA (which becomes significantly cheaper at lower frequencies).