Self-noise in interferometers - Radio and infrared

Abstract

view Abstract Citations (45) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Self-noise in interferometers - Radio and infrared Kulkarni, Shrinivas R. Abstract The quality of images obtained from modern radio synthesis arrays is, under the best conditions, limited only by the noise generated by the receiving circuitry and extraneous radiation from a variety of background sources. We present a complete theory of the noise in a synthesis image valid for a source of arbitrary strength. The analysis presented here gives a deeper understanding of coherence theory as applied to astronomical imaging. In the limit of faint sources, we recover the standard estimates of noise in a synthesis image. In the opposite limit of a strong source we show that the noise in the synthesis image is dominated by self-noise, or the noise generated by the source signal itself. We extend our theory to imaging by the use of closure phase or the bispectrum. Application of our theory to large, low-noise arrays like the VLA, the planned Indian GMRT, or VLIB show that a fair number of sources are bright enough that self-noise is an important source of noise. We present formal expressions of self-noise in synthesis maps and show that the distribution of self-noise is not uniform across the map. We suggest that some of the best VLBI maps, with noise approaching the thermal noise, may in fact be limited by self-noise. We also show that there is a bias in the standard definition of the bispectrum phasor and hence the closure phase as well. Fortunately, this bias is negligible. Finally, we resolve some of the conceptual difficulties associated with the hybrid mapping procedure and suggest contrary to the established procedure that all closure phases carry information and there are no "basic" closure phases. In particular, we suggest that at low signal levels, characteristic of infrared interferometers, it is best to fit the model to all the closure phases and fringe amplitudes. Publication: The Astronomical Journal Pub Date: September 1989 DOI: 10.1086/115202 Bibcode: 1989AJ.....98.1112K Keywords: Infrared Interferometers; Noise Generators; Radio Interferometers; Signal To Noise Ratios; Synthetic Apertures; Covariance; Random Variables; Variance (Statistics); Very Large Array (Vla); Very Long Base Interferometry; Astronomy full text sources ADS |