ALFA Precursor Observations of the Pilot Region


These observations were carried out between December 13 and January 3. Observers were Christy Bredeson, Deshpande, Topasi Gosh, Emmanuel Momjian and Chris Salter.

Postscript file of the Observing Proposal Cover Sheet
Postscript file of the Body of the Observing Proposal

The ALFA Precursor observations were conducted using the Meridian scanning procedure proposed for GALFACTS.  The feed is driven northward and southward along the Meridian at a rate of 1.53 degrees per siderial minute.  The rotation of the feeds system is set to a position angle of zero, thereby producing seven equally-spaced scan tracks on the sky separated by 1.83 arc minutes.


Meridian Scan Geometry

Figure 1. Scanning pattern of the ALFA feeds a scan driving north along the meridian.


The data processing pipeline for the GALFACTS project is described in the proposal.  The pipeline is being developed using the precuror data which were taken over a 100 MHz bandwidth with 256 spectral channels using the WAPPS.  The complex gains of the x and y linear polarisation paths in each spectral channel are calibrated using a pulsed cal signal.  The northbound and southbound scans are then "basketweaved" into raw Stokes image cubes.  Band-averaged raw images in Stokes I and Q are shown in Figure 2.


Arecibo and Bonn Stokes I and Q images
Figure 2. Raw Stokes I and Q images averaged over an 80 MHz band from the Arecibo GALFACTS precuror observations (top image in each panel) and the Bonn Mid-latitude Survey (Uyaniker, et al. 1999, A.&A. Supp. 138, 31) (bottom image in each panel).  The angular resolution of the Arecibo images is about 3.5 arcminutes.

Channel-to-channel offets and Stokes Q, U and V leakage terms are calibrated with strong unpolarised sources using an "empirical calibration" process which assumes that along the Meridian the these parameters are functions of declination only.   Figure 3. shows the resultant measured spectro-polarimetric properties for the highy polarised source in the Arecibo image at ra, dec = 07 08, 11 33.

spectropolarimetry


Figure 3. Spectro-polarimetry of a highly polarised source in the GALFACTS precursor observations. 
The plot shows the fractional polarized intensity (P) and circular polarisation (V/I), as well as the polarisation
position angle, as a function of wavelength squared across the band. The average slope of the position
angle yields the rotation measure.  The average value of the circular polarisation is 0.1%.