Noise in the Deep

Given the novel nature of the Multibeam System and the active scanning mode in which data is taken there was no certainty that the noise would decrease with integration time as predicted. We have carried out an experimental survey to a depth of 12.5 x HIPASS, i.e. 5,600 secs per beam, in a 4 x 8 degree region of Centaurus, taking all the data at night. The signal to noise in most parts of the frame (Fig. 1) increases with

throughout. The main sources of noise appear to be background continuum objects and characteristic long-wavelength ripples (caused by standing waves between the focus box and the surface of the dish) which look nothing like real 21-cm sources. Thus there is hope of `cleaning up' such cubes and going deeper still.

Figure 1: A portion of the Deep Cen survey cube with velocity in the horizontal direction, declination upwards. Note the Galactic HI emission at zero velocity on the left, continuum sources at declinations of -26 and -33, a real source at declination -30 and velocity around 4000 km s^-1, edge noise at the top and bottom and the long baseline ripple with period 1000 km s^-1.

© Copyright Astronomical Society of Australia 1997