Non-Coplanar Baselines

The procedures by which nearly all images are made in Fourier synthesis imaging are based on the assumption that all the coherence measurements are made in a plane. This is strictly true for E-W interferometers, but is false for the VLA, with the single exception of snapshots. Analysis of the problem shows that the errors associated with the assumption of a planar array increase quadratically with angle from the phase-tracking center. Serious errors result if the product of the angular offset in radians times the angular offset in synthesized beams exceeds unity. The effects are especially severe at the 90 cm and 400 cm bands, where standard two-dimensional imaging can be done only for D-configuration data. This effect is noticeable at $\lambda$20 cm in certain instances, but can be safely ignored at shorter wavelengths.

A solution to the problem of imaging wide-field data taken with non-coplanar arrays is well known, and has been implemented in the AIPS program IMAGR. This program can now correctly image up to 512 subfields, sufficient to handle observations in the B, C, and D configurations. We expect that as computer performance continues to improve, full imaging of even A configuration data will soon be practical. Refer to the help file for this program, or consult with Rick Perley or Frazer Owen, for advice. More computationally efficient imaging with non-coplanar baselines is being investigated, such as the ``W-projection'' method; see EVLA Memo 67 for more details.