D. Routledge

The synthesis telescope at the Dominion Radio Astrophysical Observatory

We describe an aperture synthesis radio tele- scope optimized for studies of the Galactic interstellar medium (ISM), providing the ability to image extended structures with high angular resolution over wide elds. The telescope produces images of atomic hydrogen emis- sion using the 21-cm H i spectral line, and, simultaneously, continuum emission in two bands centred at 1420 MHz and 408 MHz, including linearly polarized emission at 1420 MHz, with synthesized beams of 1 0 and 3:4 0 at the respective frequencies. A full synthesis can achieve a con- tinuum sensitivity (rms) of 0.28 mJy/beam at 1420 MHz and 3.8 mJy/beam at 408 MHz, and the 256-channel H i spectrometer has an rms sensitivity of 3.5B 0:5 sin K per channel, for total spectrometer bandwidth B MHz and declination . The tuning range of the telescope permits studies of Galactic and nearby extragalactic objects. The array uses 9 m antennas, which provide very wide elds of view of 3.1 and 9.6 (at the 10% level), at the two frequencies, and also allow data to be gathered on short baselines, yielding extremely good sensitivity to extended structure. Single-antenna data are also routinely incorpo- rated into images to ensure complete coverage of emission on all angular scales down to the resolution limit. In this paper we describe the telescope and its receiver and corre- lator systems in detail, together with calibration and ob- serving strategies that make this instrument an ecient survey machine.

Ground radiation scattered from feed support struts: A significant source of noise in paraboloidal antennas

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.

Aperture synthesis polarimetry: Application to the Dominion Radio Astrophysical Observatory synthesis telescope

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 difficult, 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°. 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°.

The far sidelobes and noise temperature of a small paraboloidal antenna used for radio astronomy

Measurements have been made of the radiation pattern of a symmetrical, prime-focus paraboloidal antenna which is used as a radio telescope at 1420 MHz. A transmitter was placed on a nearby hilltop, and the test antenna, used as a receiver, was driven through the range of directions permitted by its mounting; about 55% of the radiation pattern was accessible to measurement. The main beam, near sidelobes, and spillover lobes have been measured, and the conical sidelobes generated by scattering from the feed support struts are clearly seen. The effective temperature of the ground at 1420 MHz has been determined by radiometry. By convolving the radiation pattern with the ground temperature distribution the antenna temperature has been calculated and the contribution from various parts of the radiation pattern estimated. Of a measured total antenna temperature of 26.8 K with the antenna pointing at the zenith, the following contributions can be accounted for: cosmic microwave background (2.7 K), galactic emission (1.0 K), atmospheric emission (2.0 K), direct spillover from the ground into the feed (8.0 K), leakage through the reflector mesh (5.9 K), and diffraction around the reflector rim (0.6 K). It is concluded that ground radiation scattered from the feed support struts into the aperture is a significant contributor to antenna noise, and 5.8 ± 2.5 K has been attributed to this cause. Of the total noise from the ground, 1.1 K is contributed by hills surrounding the site; most of this enters the antenna through the sidelobes generated by the feed support struts. On a completely flat site, strut scattering would contribute about 4.7 K.

Design and results for a 345 GHz sis focal plane array using planar technology

We describe a focal plane array using antenna elements and SIS junctions photo-etched on a quartz substrate. The etched antenna is of a new strip-line type in which a two-dimensional corrugated horn is phase-corrected with a strip-line lens. Good beam efficiency is indicated by the measured antenna patterns. A DSB receiver temperature of 225 K was obtained with a number of imperfections still present.

Integrated Josephson effect suppression electromagnets for sis receivers

We describe a method for integrating a Josephson current suppression electromagnet along with a superconductor-insulator-superconductor (SIS) mixer. In this technique, circuitry already in place for matching the SIS mixing device with the antenna is used as the conductor for an electromagnet. Reduced cost and size are among the advantages achieved by elimination of external magnets.

Techniques in Small Area SIS NbN Junction Manufacture

This paper discusses progress made in the development of techniques for manufacture of submicron area superconductor/insulator/superconductor junctions executed in niobium nitride. The junctions are meant for use as mixers in integrated planar multi-beam array receivers in the 100 to 800 GHz frequency range. Sputtered thin film NbN/MgO/NbN metallized sandwiches are plasma etched into mesa junctions. Standard 1.5 μm UV lithography is used to deposit the smallest possible junctions. Selective over-etching of the upper NbN layer, with MgO acting as the etch-stop, further reduces the area from that produced by the lithography. The smallest junctions produced so far have areas of 3 μm2, but areas as small as 0.2 μm2 appear possible.