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.

  • Related to the point above, please note that below approximately 30 degrees elevation the sensitivity drops significantly such that the Sun becomes the only viable target for interferometric observation below about 10 degrees elevation.

    Commissioning observations have managed successful imaging of a target at -7 degrees declination, but imaging is not straightforward and the following points need to be noted:

    • The thermal noise cannot be attained at these declinations;
    • Short baselines have to be flagged;
    • Some additional flagging of data may be required.

Furthermore, the shorter length of time that such targets are above a useable horizon can severely limit the u-v coverage attainable.  Therefore for interferometric observations, -5 degrees declination should be regarded as a lower limit and targets should preferably be above the celestial equator.  Proposers wishing to image targets below the celestial equator are expected to justify that their observing programme can attain the sensitivity and/or u-v coverage required.

Design: Kuenst.    Development: Dripl.    © 2022 ASTRON