Beam squint and Stokes V with off-axis feeds

Radio telescopes with off-axis feeds, such as the (E)VLA, suffer from "beam squint" in which the two orthogonal circular polarizations sampled have different pointing centers on the sky. Its effects are weak near the beam center but become increasingly important towards the edge of the antenna power pattern where gains in the two polarizations at a given sky position are significantly different. This effect has limited VLA measurements of circular polarization (Stokes V) and introduced dynamic range limiting, wide-field artifacts in images made in Stokes I. We present an adaptation of the visibility-based deconvolution CLEAN method that can correct this defect "on the fly" while imaging, correcting as well the associated self-calibration. We present two examples of this technique using the procedure "Squint" within the Obit package which allows wide-field imaging in Stokes V and reduced artifacts in Stokes I. We discuss the residual errors in these examples as well as a scheme for future correction of some of these errors. This technique can be generalized to implement temporally- and spatially-variable corrections, such as pointing and cross-polarization leakage errors.Comment: 9 pages, 6 figures (five of them double), to appear in Astronomy & Astrophysics (accepted: May 9, 2008). High-resolution versions of the figures (gzipped, tar,gzipped) can be downloaded from http://www.cv.nrao.edu/~juson/technical/squint/squint_figures.g

The Position of Sgr A∗^* at the Galactic Center

The absolute position of the compact radio source at the dynamical center of the Galaxy, Sgr A$^*$, was known only to an accuracy of $0.2''$ in spite of its accurate location with respect to near-IR stellar sources to within 30 milliarcsecond (mas). To remedy this poor positional accuracy, we have selected 15 high-resolution, high-frequency VLA observations of Sgr A$^*$ carried out in the last 13 years and determined the weighted average position with the average epoch 1992.4 to be at $\alpha$, $\delta$[1950] = $17^{\rm h} 42^{\rm m}$ 29\dsec3076$\pm0.0007$, $-28^\circ 59^\prime 18.484\pm0.014^{\prime\prime}$, or $\alpha$, $\delta$[2000] = $17^{\rm h} 45^{\rm m}$ 40\dsec0383$\pm0.0007$, $-29^\circ 00^\prime 28.069\pm0.014^{\prime\prime}$ which agrees with earlier published values to within the $0.2''$ error bars of the earlier measurements. An accurate absolute position of Sgr A$^*$ can be useful for its identification with sources at other wavelengths, particularly, in soft and hard X-rays with implications for the models of a massive black hole at the Galactic center.Comment: 11 pages, one figure and one table. ApJL (in press

A Data Exchange Standard for Optical (Visible/IR) Interferometry

This paper describes the OI Exchange Format, a standard for exchanging calibrated data from optical (visible/infrared) stellar interferometers. The standard is based on the Flexible Image Transport System (FITS), and supports storage of the optical interferometric observables including squared visibility and closure phase -- data products not included in radio interferometry standards such as UV-FITS. The format has already gained the support of most currently-operating optical interferometer projects, including COAST, NPOI, IOTA, CHARA, VLTI, PTI, and the Keck Interferometer, and is endorsed by the IAU Working Group on Optical Interferometry. Software is available for reading, writing and merging OI Exchange Format files.Comment: 26 pages, 1 figur

Source structure corrections to the geodetic very long baseline interferometry observables

Why source structure affects VLBI observations and how source structure corrupts the geodetic observables are discussed. Evidence is presented which shows that corrections for source structure are possible and necessary for centimeter level accuracy VLBI measurements