MultiView High Precision VLBI Astrometry at Low Frequencies

Abstract

The arrival of the Square Kilometer Array (SKA) will revitalise all aspects of Very Long Baseline Interferometry (VLBI) astronomy at the lower frequencies. In the last decade there have been huge strides towards routinely achieving high precision VLBI astrometry at frequencies dominated by the tropospheric contributions, most notably at 22GHz, using advanced phase referencing techniques. Nevertheless to increase the capability for high precision astrometric measurements at low radio frequencies (<8 GHz) an effective calibration strategy of the systematic ionospheric propagation effects that is widely applicable is required. Observations at low frequencies are dominated by distinct direction dependent ionospheric propagation errors, which place a very tight limit on the angular separation of a suitable phase referencing calibrator.The MultiView technique holds the key to the compensation of atmospheric spatial-structure errors, by using observations of multiple calibrators and 2-D interpolation. In this paper we present the first demonstration of the power of MultiView using three calibrators, several degrees from the target, along with a comparative study of the astrometric accuracy between MultiView and phasereferencing techniques. arXiv:1612.02554v1 [astro-ph.IM] 8 Dec 2016-2 -MultiView calibration provides an order of magnitude improvement in astrometry with respect to conventional phase referencing, achieving ∼ 100micro-arcseconds astrometry errors in a single epoch of observations, effectively reaching the thermal noise limit.MultiView will achieve its full potential with the enhanced sensitivity and multibeam capabilities of SKA and the pathfinders, which will enable simultaneous observations of the target and calibrators. Our demonstration indicates that the 10 micro-arcseconds goal of astrometry at 1.6GHz using VLBI with SKA is feasible using the MultiView technique.

High-Precision Astrometric Millimeter Very Long Baseline Interferometry Using a New Method for Multi-Frequency Calibration

“…This multi-frequency approach is one of two methods we are currently developing to improve astrometric calibration of the ionospheric contributions, the other being to use multiple calibrators around the target and to solve for the spatial structure of the atmosphere, which we call MultiView (Rioja et al , 2016. This method, which requires more calibrators rather than less, simultaneously solves for a 2D Ionospheric and Tropospheric phase screen over the array, and will be extremely suitable for VLBI stations with multiple beams, such as SKA and ASKAP.…”

Astrometry of OH/IR Stars Using 1612 MHz Hydroxyl Masers. I. Annual Parallaxes of WX Psc and OH138.0+7.2

“…This makes it possible to fully compensate for the spatial static component of the ionosphere, even with calibrators several degrees away. We demonstrated this technique in Rioja et al (2017), achieving complete atmospheric mitigation and effectively reaching the thermal noise limit in our measurements. We plan to further develop and test these techniques to support our mentioned science goals.…”

“…For the target scans, antennas were pointed halfway between the OH maser and C ib positions, except for Epochs I-II of the WX Psc sessions, where the pointing center was the OH maser source. Rioja et al 2017, but not related to this work). The on-source time on the target was decreased to 40%; the switching cycle to C 1 (and two other calibrators) was decreased to 5 min and the IF bandwidths were increased to 8 MHz (256 Mbps recording rate), but keeping the same spectral resolution.…”

“…We are therefore working on developing an alternative multi-calibrator approach (Rioja et al 2009;Dodson et al 2013) that can fully remove even the effects of the static ionosphere and make our astrometry completely free of systematic errors. This technique, known as MultiView, will be demonstrated in a forthcoming paper (Rioja et al 2017); while our current focus centers on exploring the limits of in-beam maser astrometry at low frequencies.…”

(Ultra) Precise Astrometry today and tomorrow, with Next-generation Observatories

“…The MultiView calibration method offers the potential to achieve precision astrometry for many targets at the low frequency regime dominated by ionospheric disturbances. MultiView VLBI has been reviewed elsewhere [7] (see also [23] and [24]), and here we only summarize the method and benefits. It consists of using observations of (at least) three calibrators surrounding the target, and uses a 2D interpolation in the visibility domain to provide corrections of the spatial atmospheric distortions along the direction of the target.…”