Wide Field Imaging (WFI)

Background:

Radio interferometric imaging has developed over the past five decades successfully exploiting 2-dimensional (2D) flat Fourier plane (u,v), flat sky (x,y) approximations. While this has been satisfactory for the small fields of view (FOVs) and relatively short physical interferometer baselines used in the past, and can be an exact solution for interferometers which are one dimensional (1D) or truly 2D coplanar, the approximation fails for larger fields of view with non-coplanar arrays (eg. the VLA). In these cases sources away from the center of the image (usually coincident with the center of the FOV) become increasingly distorted and are no longer described by a single instrumental point spread function (PSF). This means that deconvolution (e.g., CLEANing) becomes increasingly inaccurate and the sidelobes of confusing sources can no longer be properly removed. This severely limits the dynamic range of the observation, leaves uncorrectable distortions in the sources of interest, and precludes achieving thermal noise limiting sensitivities.

Because low frequency observations have large FOVs, require physically long baselines for good resolution, and have a high density of bright sources on the sky, these difficulties become severe for low frequency WFI. The VLA suffers some distortions even at 5000 and 1400 MHz in its widest configurations, while 333 MHz (P-band) suffers significant distortions in all configurations. Furthermore planned larger, low frequency, non-coplanar ground arrays and fully 3D space arrays (see Low Frequency Radio Astronomy (LFRA)) cannot image effectively without solving this problem.

Approach: