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  • 1
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    Meter-wavelength VLBI. I - Cassiopeia A (1974)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: Very-long-baseline interferometric observations of the supernova remnant Cassiopeia A, at 74 MHz with a 12,000-wavelength baseline and at 111 MHz with a 18,500-wavelength baseline, are reported. The fringe amplitudes are strongly varying on a time scale of about 15 to 30 minutes, which is attributed to much the same complex structure as that observed at higher frequencies, plus one other compact source. Due to the poor (u, nu)-plane coverage, the location of the extra source can not be isolated unambiguously, but possibilities are suggested. The source must lie outside the supernova remnant shell, possibly associated with a concentration of emission north of the shell, or lying outside the gap in the northeastern side of the shell. The flux and spectral index deduced for the compact source depend on the assumed size, with a range of 100 Jy and 500 Jy at 74 MHz. If the source is associated with the supernova explosion, it must have been traveling at least 5000 km/sec.
    Keywords: ASTRONOMY
    Type: Astronomical Journal; 79; Nov. 197
    Format: text
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  • 2
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    Aperture synthesis using orbiting telescopes (1985)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: A study was carried out to determine the feasibility, with current technology, of performing aperture synthesis using two telescopes orbiting the earth in coordinated orbits separated by approximately 10 m to 1 km. The objective was to determine whether there is a practical alternative to a very large, deployed, servo-controlled submillimeter telescope (i.e., the Large Deployable Reflector) for obtaining high-resolution submillimeter images of astronomical sources. It is found that suitable classes of orbits exist which can provide good UV coverage over the entire sky and the real-time correlation of wideband signals can be performed in orbit using current technology. The most difficult task appears to be the real-time determination of the orientation of the baseline vector in a stable coordinate system. A plausible scheme has been identified for the determination of an arbitrary direction to within 0.003 arcsec in an astrometric coordinate system. This scheme not only makes submillimeter interferometric image reconstruction possible but should also have numerous other applications.
    Keywords: ASTRONOMY
    Type: Radio Science (ISSN 0048-6604); 20; 1105-111
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  • 3
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    Radiometric correction of atmospheric path length fluctuations in interferometric experiments (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: To support very long baseline interferometric experiments, a system has been developed for estimating atmospheric water vapor path delay. The system consists of dual microwave radiometers, one operating at 20.7 GHz and the other at 31.4 GHz. The measured atmospheric brightness temperatures at these two frequencies yield the estimate of the precipitable water present in both vapor and droplets. To determine the accuracy of the system, a series of observations were undertaken, comparing the outputs of two water vapor radiometers with the phase variation observed with two connected elements of the very large array (VLA). The results show that: (1) water vapor fluctuations dominate the residual VLA phase and (2) the microwave radiometers can measure and correct these effects. The rms phase error after correction is typically 15 deg at a wavelength of 6 cm, corresponding to an uncertainty in the path delay of 0.25 cm. The residual uncertainty is consistent with the stability of the microwave radiometer but is still considerably larger than the stability of the VLA. The technique is less successful under conditions of heavy cloud.
    Keywords: ASTRONOMY
    Type: Radio Science (ISSN 0048-6604); 19; 411-422
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  • 4
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    Meter-wavelength VLBI. III - Pulsars (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Observations of pulsars, especially the Crab Nebula pulsar, made in very long baseline interferometry (VLBI) experiments are discussed. Based on a crude 144 MHz visibility curve which is consistent with a Gaussian brightness distribution, the measured visibilities at 196, 111, and 74 MHz were interpreted to yield apparent angular diameters (at half-power) of about 0.03 sec, 0.07 sec, and 0.18 sec, respectively. These sizes scale approximately as wavelength-squared, and the 74 MHz size agrees with recent observations using interplanetary scintillation techniques. The total flux densities lie on the extrapolation from higher frequencies of the pulsing flux densities. Variations in the total flux density up to 25 per cent were observed. A lack of fine structure other than the pulsar in the nebula is indicated by the simple visibility curves. The pulse shapes are similar to single-dish measurements at 196 MHz but reveal a steady, nonpulsing component at 111 MHz. The ratio of pulsing to total power was approximately equal to one-half but varied with time. It was found that four strong, low-dispersion pulsars were only slightly resolved.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal; 207; Aug. 1
    Format: text
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  • 5
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    Meter-wavelength VLBI. II - The observations (1975)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Observations made during a series of meter-wavelength very-long-baseline (VLBI) experiments conducted during 1971-1973 are reported. A wide variety of objects was observed, including many extragalactic sources known to have compact components, several strong pulsars (especially the Crab nebula pulsar), and a selection of supernova remnants. The experiments are discussed in detail, and tests made to check the consistency of the results are described. The measured correlated flux densities are presented for each source as a function of interferometer baseline and observing frequency. An assessment is made of the measurement errors and of the possibility of confusion within the interferometer beam. The results show that all sources except the pulsars, which are known to have intrinsically small sizes, are resolved on the longest baseline. We also conclude that no supernova remnants except the Crab nebula and Cassiopeia A gave evidence of compact components.
    Keywords: ASTRONOMY
    Type: Astronomical Journal; 80; Nov. 197
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  • 6
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    A high declination search at 8 GHz for compact radio sources (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: With the Haystack-NRAO interferometer (baseline length of 20 million wavelengths at 3.8 cm) 37 sources were observed whose declinations were above 50 deg. Seven of these sources have compact cores with diameters smaller than 5 milliarcsec and with correlated flux densities greater than about 0.5 Jy; the remaining sources have no cores with flux densities above about 0.3 Jy, the sensitivity limit of the interferometer. Two of the sources with detected compact cores, 4C 67.05 and 3C 418, were also observed with longer-baseline interferometers; the diameter of the core of 4C 67.05 was estimated to be smaller than 1 milliarcsec and that of 3C 418 to be smaller than 0.4 milliarcsec. All diameter estimates were based on an assumed circular Gaussian distribution of radio brightness and refer to the contour with brightness density e to the -1/2 power times that of the center. Positions for the detected sources were also obtained from the interferometric data, the uncertainty in these coordinate estimates ranging from 0.04 to 0.6 arcsec. The compact core detected in 3C 390.3 was found to lie near the center of this extended (approximately 4 arcmin in diameter) double radio source and to be coincident to within 1 arcsec with an N galaxy previously identified with 3C 390.3.
    Keywords: ASTRONOMY
    Type: Astronomical Journal; 83; June 197
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