|Submitter:||Wim van Cappellen|
|Description:|| Our focal plane array work has reached an important milestone: We have successfully measured overlapping compound beams with a deep reflector system by using a dense Focal Plane Array (FPA) feed. |
A Westerbork telescope with the MFFE can measure only one (on-axis) beam per polarisation. But an FPA system has the potential to measure a number of beams simultaneously, enabling a wide and continuous field of view to be measured with a single telescope pointing.
Recent months, the DIGESTIF FPA demonstrator on the WSRT has been extended from 4 to 36 active elements. After measurements with single elements, we now managed to calibrate and combine 36 elements coherently into compound beams, overlapping each other at a -1.5 dB level. A drift scan has been performed through Cygnus A at 1420 MHz to measure the sensitivity of the beams. The above graph shows the measured Ae/Tsys ratio for the on-axis beam and several scanned beams.
The on-axis beam (peak at 0 deg) has an Ae/Tsys of 2. The beams with a small offset have a comparable sensitivity. As expected, the sensitivity drops at the edges of the field of view due to the limited size of the array. The compound beam sensitivity for negative offsets is lower than for positive offsets because the active elements are not symmetrically distributed over the array. This will be resolved when all elements become active.