The high-latitude supernova remnant (SNR) DA 530 (G93.3]6.9), apparently a typical shell remnant, has highly polarized radio continuum emission and a very uniform circumferential magnetic Ðeld. We present new radio continuum (408 and 1420 MHz) and H I line observations, made with the Dominion Radio Astrophysical Observatory Synthesis Telescope, and we have made the Ðrst detection of X-ray emission from the SNR, using the ROSAT Position-Sensitive Proportional Counter. The SNR lies within a shell of H I, possibly created by an earlier stellar wind, whose kinematic distance is nominally 2.5 kpc but whose actual distance may be larger. The X-ray emission is extremely faint. A Raymond-Smith ionization-equilibrium model Ðts the data and suggests a very low density, D0.05 cm~3, consistent with the occurrence of the supernova in a stellar wind cavity, but this model yields an explosion energy 100 times lower than the accepted value. A nonequilibrium shock model, incorporating a range of ionization timescales, is able to give more realistic physical parameters for the supernova remnant. On the balance of the evidence, we place DA 530 at a distance of 3.5 kpc, the largest distance permitted by the H I observations, where it lies 420 pc above the Galactic plane. The explosion, probably a Type Ia supernova, in a low-density cavity has resulted in weak X-ray emission and slow evolution. The explosion energy was 3.9 ] 1050 ergs and the age is D5000 years. The remnant, having swept up 3.9in an M _ ambient density of D0.01 cm~3, is only now in the adiabatic phase, and this explains the absence of detected optical emission. Despite the low ambient density the efficiency of generation of synchrotron radio emission is D0.4%, higher than in some historical SNRs. The ratio of radio to X-ray Ñux is about 100 times that for the remnant of SN 1006, which has comparable radio continuum properties. The very uniform magnetic Ðeld is not explained. DA 530 joins a small group of remnants at high Galactic latitude with unusual features, perhaps resulting from low ambient densities. Inhomogeneous nonequilibrium ionization models may be required for the interpretation of the X-ray emission from many other older SNRs.

Abstract. Aperture synthesis is a powerful technique for imaging the radio sky and can be used to make images in all four Stokes parameters, providing a complete measurement of the polarization state of the received radiation. In centimeter-wavelength continuum astronomy the received signals are generally partially linearly polarized, with a negligibly small fraction of circular polarization. For this application the preferred antenna configuration receives both left-and right-hand circular polarization. In this paper the effects of nonideal antenna performance are analyzed, and calibration and data correction procedures are described which allow precise measurement of the four Stokes parameters. Three levels of data correction are identified. Level 1, complex channel gain correction, is the standard calibration of amplitude and gain required in every synthesis telescope. Level 2, orthogonality correction, makes the instrument appear as a set of interferometers, identical to the level of approximation involved, which have two orthogonal, but not precisely circular, polarizations. Level 3 correction converts the telescope into an array of identical antennas with exactly circular polarization. Level 3 correction gives the most accurate polarimetry; for this level of correction the polarization characteristics of one antenna, which is used as a reference, must be determined. In the absence of a precise determination of the polarization characteristics of the reference antenna, a measurement which may be very ditficult, there is no value in proceeding to Level 3 correction. If the cross-hand contamination of the antennas is less than about 15%, then only level 1 and level 2 corrections are needed to achieve a sensitivity to polarized emission of 1% of the total intensity, an accuracy of measurement of polarized intensity of better than 10% and a measurement of the position angle of linearly polarized emission better than 5 ø. The implementation of polarimetry at 1420 MHz on the Dominion Radio Astrophysical Observatory synthesis telescope, Penticton, British Columbia, Canada, is described, and instrument performance to the above specifications is demonstrated. Observations of the supernova remnant DA530 are presented, demonstrating a usable field of view for polarimetry of at least 1.5 ø.

The struts supporting the feed or subreflector of a symmetrical paraboloidal antenna generate sidelobes around cones of wide opening angle by scattering energy from the plane wave leaving the reflector. On the basis of simple assumptions about these “scatter cone” sidelobes, an approximate formula is derived to predict their level relative to the main beam; it is tested against published measurements. The noise added to the antenna when the scatter cone sidelobes receive radiation from the ground is calculated, and this mechanism is seen as a significant contributor to antenna noise. The use of struts of triangular cross section in place of circular ones redirects sidelobe energy away from the back hemisphere of the radiation pattern to the front hemisphere. For most antenna pointings these sidelobes will not strike the ground, and antenna noise temperature is likely to be reduced. Radiometric measurements at 1420 MHz have been made using a 9‐m antenna equipped with struts of various cross sections and sizes. These measurements have been used to isolate the strut contribution to antenna noise, and have verified that this contribution can be reduced by using triangular struts. The strut contribution to antenna noise is calculated as a function of zenith angle of the main beam of the antenna. Triangular struts are superior to circular ones at all main‐beam zenith angles. When three struts are used, the upright Y configuration of the tripod is better than the inverted Y.