The Relationship between Image Dynamic Range and Antenna-based Gain Errors

Abstract

The ngVLA science requirements call for continuum image dynamic ranges of 45 dB and 35 dB at 8 and 27 GHz respectively. In interferometric aperture synthesis imaging, visibility amplitude and phase errors result in errors in the final images, limiting the dynamic range attained. In order to achieve the ngVLA dynamic range requirements it is necessary to limit the amplitude and phase errors to within appropriate levels. The relationship between the number of antennas N in an interferometric aperture synthesis array and the error in the constructed image has been previously derived and conventionally adopted to be a scaling of \approx N. In this short memo, we argue that this relationship may not hold everywhere in an image and derive the relationship in a more stringent limit. We derive a more stringent \sqrt N dependence as opposed to the previous, generally adopted N scaling and compare with existing simulations. This relationship is at the root of allowable amplitude and phase errors arising from practically every corrupting effect, e.g. antenna pointing, primary beam characteristics, tropospheric and ionospheric phase fluctuations and polarimetric imperfections, among others. Thus, it is central to ngVLA calibration requirements and strategies. We recommend the adoption of a conservative \sqrt N dependence as the basis to derive the requirements and to identify applicable strategies.