PLATFORM DEFORMATION PHASE CORRECTION FOR THE AMiBA-13 COPLANAR INTERFEROMETER

Abstract

We present a new way to solve the platform deformation problem of co-planar interferometers. The platform of a co-planar interferometer can be deformed due to driving forces and gravity. A deformed platform will induce extra components into the geometric delay of each baseline, and change the phases of observed visibilities. The reconstructed images will also be diluted due to the errors of the phases. The platform deformations of The Yuan-Tseh Lee Array for Microwave Background Anisotropy (AMiBA) were modelled based on photogrammetry data with about 20 mount pointing positions. We then used the differential optical pointing error between two optical telescopes to fit the model parameters in the entire horizontal coordinate space. With the platform deformation model, we can predict the errors of the geometric phase delays due to platform deformation with given azimuth and elevation of the targets and calibrators. After correcting the phases of the radio point sources in the AMiBA interferometric data, we recover 50% − 70% flux loss due to phase errors. This allows us to restore more than 90% of a source flux. The method outlined in this work is not only applicable to the correction of deformation for other co-planar telescopes but also to single dish telescopes with deformation problems. This work also forms the basis of the upcoming science results of AMiBA-13.

AMiBA: CLUSTER SUNYAEV–ZEL’DOVICH EFFECT OBSERVATIONS WITH THE EXPANDED 13-ELEMENT ARRAY

“…Moreover, the deformation amplitude increases with lower elevation and larger platform hexpol rotations. The saddle pattern rotates with a roughly constant amplitude as the hexapod changes its azimuth pointing (Liao et al 2013;Huang et al 2008;Koch et al 2008). Relative to the neutral orientation (pointing toward zenith), the maximum normal deformation at the edge of the platform increases with decreasing elevation and can reach a value as high as 1.5 mm (Huang et al 2008;Koch et al 2008), or half of our observing wavelength.…”

“…All-Sky Radio Pointing Koch et al (2009) describe how the pointing model of the AMiBA hexapod mount was established with an optical telescope. Further taking into account the parametric model of the platform deformation developed by Liao et al (2013), we carefully rebuilt the pointing model by removing the tilt of the optical telescope due to the platform deformation in order to achieve a better pointing accuracy as required for the more extended AMiBA-13 array. We further observed a dozen radio sources, selected from the Australia Telescope Compact Array (ATCA) calibrator database 9 , that have a listed 3 mm flux density higher than 2 Jy and that are evenly distributed in 9 http://www.narrabri.atnf.csiro.au/calibrators/ Figure 6.…”

“…Therefore, in the current work, the band-smearing effect remains a systematic factor that reduces the peak flux of a point source by up to 10 % at the lowest elevation of el = 40 • . Liao et al (2013) summarize in detail the method that we use to measure and model the platform deformation. Moreover, they demonstrate its effectiveness when applied to planets and a few radio point sources.…”