Cautionary notes

Although great care has been given to deriving the numbers in this document, still they have to be treated with some caution :

  • Definite values for α1 and α2 have to be determined during the commissioning of LOFAR.

  • The sensitivity numbers have been calculated taking a representative out of the plane value for the sky brightness. However, the sky brightness varies with factors of a few and this needs to be taken into account. Furthermore, the very bright galactic plane will contribute quite a lot of power through side and, in some cases, grating lobes increasing the effective visibility noise.

  • The inner sidelobes of a (HBA) station need to be suppressed to reduce scattered sidelobe noise. This will be done by tapering the station beam which has the drawback that the sensitivity will be reduced by 30-50%. The tapering will result in a larger station beam, leading to an increase in survey speed. Note however, that tapering will always result in a loss of survey speed.

  • The sensitivity of a phased array like LOFAR is less at low elevation due a number of reasons. These include (i) smaller gains of the dipoles, (ii) reduced projected collecting area of the phased array, (iii) longer paths length through the ionosphere, (iv) larger separation of the ionospheric line of sight of the calibrator sources. Considering the first two points currently sensitivity values for a pointing at 45 degrees has been used. The latter two points imply that remaining calibration errors will be larger at low elevations.

  • One of the most challenging aspects of deep LOFAR observations is to obtain the needed dynamic range of 10000 to 1. Residual phase errors in solving for the ionosphere and the beams are likely to limit the dynamic range. To what extent needs to be determined during commissioning.

Design: Kuenst.    Development: Dripl.    © 2015 ASTRON