The Low-Resolution DRAO Survey of Polarized Emission at 1.4 GHz

- A New Polarization Survey of the Northern Sky -

Max-Planck Institut für Radioastronomie, Bonn, Germany
Dominion Radio Astrophysical Observatory, Penticton, Canada

Introduction | Description | Publications | Data

Observing and Calibration Scheme

Observations were carried out by drift scanning the sky with the telescope stationary at the meridian to keep ground and stray radiation constant. This way, 392 drift scans during night time were obtained in 17 months observing time. Remaining time-variability of the system temperature was corrected by software processing.

For the absolute calibration of Stokes U and Q the Leiden/Dwingeloo polarization survey (Spoelstra 1972a,b, 1977) has been used. This survey provides about 1700 absolutely calibrated pointings north of 0 degree declination. We used these data as reference values for the determination of the system parameters of our instrument and the determination of zero-levels in U and Q. For the extrapolation of the zero-levels below 0 degree declination profiles of ground radiation were measured. Finally, the brightness temperature scale has been attached to the Effelsberg scale. Total intensity has been calibrated using the Stockert 1.4 GHz survey (Reich 1982; Reich & Reich 1986; survey available here).

Spoelstra, T. A. Th., 1972A&AS....5..205S, ADS
Spoelstra, T. A. Th., 1972A&AS....7..169S, ADS
Spoelstra, T. A. Th., 1977BICDS..13...66S, ADS
Reich, W., 1982A&AS...48..219R, ADS
Reich, P.; Reich, W., 1986A&AS...63..205R, ADS

Survey Parameters

The integration time per area on the sky depends on declination, because of the observing method. Drift scans are fully sampled along right ascension. The separation in declination varies between 0.25 and 2.5 degree.
Integration time per 15 arc min in right ascension 60 s
Observing periods 1.) Nov 2002 - May 2003
2.) Jun 2004 - Mar 2005
RMS-noise (Stokes U and Q) 12 mK
Systematic errors < 50 mK
Declination range -29 to +90 deg
Fully sampled area 41.7%

Telescope Parameters

The uncooled receiver of the DRAO 26-m telescope consists of a corrugated feed and a dual polarization coupler providing linear (X, Y) polarization components. A quadrature hybrid is used to form circular (R, L) out of linear polarization components. The circular components are amplified by two low-noise amplifiers. The analog IF-polarimeter, which is of the type used on the Effelsberg 100-m telescope (e.g. Junkes et al. 1987; Uyaniker et al. 1998) , provides four output channels: two total power channels (RR, LL) and two cross-products (RL, LR).

Junkes, N.; Fuerst, E.; Reich, W., 1987A&AS...69..451J, ADS
Uyaniker, B.; Fuerst, E.; Reich, W.; et al., 1998A&AS..132..401U, ADS

Telescope coordinates -119:37.2, +49:19.2
Antenna diameter 25.6 m
HPBW 36 arc min
Aperture efficiency 55%
Bandwidth 12 MHz (10 MHz)
Pointing accuracy < 1 arc min
System temperature 125 K

A brief description of the Penticton 26-m Telescope (J. Galt, 1999)
Picture of the Telescope
Block diagram of the receiver used for the survey

Errors in Stokes U and Q

Based on the correlation of the DRAO polarization survey with the Leiden/Dwingeloo survey and measurements of the northern celestial pole the final rms-noise of the survey calculates to 12 mK in Stokes U and Q.

A correction of instrumental polarization caused by side lobes is not intended. Side lobe polarization is noticeable in the U and Q maps to radii of about 1.5 degree around strong point sources with total intensities of  > 4 K, and towards a 2.5 degree wide stripe around the Galactic plane at l < 50 degree. We estimate the instrumental polarization of the first side lobe to be < 6% at maximum. For diffuse extended emission the effect of side lobes is most likely negligible, because their contributions over a large area around the main beam average close to zero.

Systematic errors are introduced by the separation of sky emission and system temperature fluctuations. In case of intense polarized structures elongated along right ascension this error may be < 50 mK, depending on the shape, intensity and coverage of the structure. At declinations < 0 degree a systematic base level error of < 50 mK may be caused by uncertainties in the ground profiles, which should be highest at the lowest declination.