|Boudewijn Hut and Hajee Pepping for the APERTIF team
| The hard work on the APERTIF correlator is paying off - on 8 October 2015, the first real-life fringe was measured on a celestial radio source, using two WSRT dishes! This is not only a great success from a technical perspective, but it also showed that such a complex system can be delivered on time.
In the weeks before reaching this milestone, the firmware (DESP) and commissioning teams cooperated seamlessly. The new correlator module was installed on a UniBoard FPGA, and step by step the optical fibers, new DDR3 memory, beamformer modules and receiver chains were included. This systematic approach allowed us to find (and squash) any bugs in an early stage.
The inset on the top right shows the installation of new DDR3 memory at a beamformer. This memory is used for the transpose operation (see daily image 19-05-2015). The data stream is then transmitted to the correlator by means of optical fibers. The bottom right inset shows the optical fiber interface on a correlator UniBoard.
The actual fringe is shown in the bottom left graph. It is measured using a single 12.207 kHz channel at 1.498 GHz. Two WSRT dishes on a 288 m East-West baseline were pointed at 3C147 for a period of 300 seconds, without any delay tracking. Also shown is a measurement towards the zenith, away from any strong source, where the correlation should be negligible, or rather noise-like. The measured variance of the correlation coefficient is indeed as expected.