Extensive calculations of the radiative emission of an atmospheric pressure hydrogen arc in the temperature range from 8000 to 15000 K have been undertaken, which show that a partly ionized hydrogen arc plasma may serve as an intense and accurate spectral radiation standard in the ultraviolet. The principal features of the spectrum are: Between 1215 Å and 3600 Å the hydrogen arc emits only continuous radiation, dominated by the Balmer recombination continuum. Between 1215 Å and 930 Å the lower members of the Lyman series appear strongly Stark-broadened and optically thick in their central parts. The higher Lyman lines merge into a quasi-continuum, which leads into the very strong Lyman continuum. Between 930 and about 400 Å the continuum is optically thick, i.e., it represents blackbody radiation at typical arc temperatures of 12000 to 15000 K.

Using the principle of the ‘transition-radiation’, it is possible to construct a new light-source for the VUV. This principle states that an electromagnetic radiation is emitted, when an electric charge penetrates the surface of a metallic target. The essential advantages of such a source are that it emits a continuous spectrum in the whole region from 1000 to 3000 Å and that this radiation is emitted directly in the vacuum.

The synchrotron radiation from the Wisconsin 240 MeV electron storage ring was used to calibrate the absolute sensitivity of photoelectric detectors to radiant flux at wavelengths between 1100 and 3000 Å. The spectrum from an electron in the storage ring beam was theoretically predicted and the number of electrons in the beam determined exactly by observing the discrete decreases in radiant flux as single electrons were removed from the stored beam. Interference filters of measured transmission were used to limit the flux to the particular wavelength bandpasses desired (200–300 Å). Photomultipliers for use as laboratory standards of radiant flux, and more recently flight photometers for stellar observation via rockets, were calibrated against this standard.

Calibration for two rocket-borne spectrometers to observe the absolute intensity and the centre-to-limb variations of the Sun between 1500 Å and 2000 Å is described. The procedure of the calibration is divided into two parts, (a) the measurements of the transmission of the spectrometers, and (b) the absolute detector calibration.

The double dispersive spectrometer has been planned to be launched in September 1970, but it was postponed until the coming winter, and the single dispersive spectrometer will be launched in January 1971.

The first long wavelength radio astronomical observations from above the ionosphere were performed in the early 1960's using very short antennas on both sounding rockets and satellites. Although these exploratory measurements were sometimes troubled by perturbations arising in the local plasma enivronment and by spacecraft interference, they did develop the fundamental information and techniques on which to build more sophisticated low frequency radio astronomical systems. The first spacecraft designed specifically and exclusively for radio astronomical studies, Radio Astronomy Explorer 1, was launched on July 4, 1968. It carries two gravity-gradient stabilized, 229-m, travelling-wave V-antennas, a 37-m dipole antenna, and a number of radiometer systems to provide measurements over the 0.2 to 9.2 MHz frequency range (λ = 33 to 1500 m) with a time resolution of 0.5 sec and an absolute accuracy of about 25%. Although the observations may seem crude in some cases, when compared with those of ground-based telescopes, the RAE has afforded substantial improvement over the capabilities of the early flight experiments. Through the study of solar radio bursts down to 0.2 MHz, we are getting new information on the density, plasma velocity, and dynamics of coronal streamers out to distances of greater than 50 R⊙. From galactic continuum background maps at frequencies around 4 MHz, we are getting new information on the distribution of the ionized component of the interstellar medium. Cosmic noise background spectra measured down to ~ 0.5 MHz are providing new estimates of the interstellar flux of cosmic rays, of magnetic fields in the galactic halo, and of the nature of radiation from distant extragalactic radio sources.

A second RAE spacecraft is now being constructed for launch into a lunar orbit. This satellite will permit radio astronomical measurements at frequencies down to 30 kHz (λ = 10 km), will facilitate the use of the moon as an occulting disk for source position measurements, and will provide new information on the cislunar noise environment required to assess the feasibility of future lunar radio observatories. Studies are in progress on the feasibility and application of orbiting radio interferometers (baselines up to 5 km) and large orbiting filled-aperture telescopes (diameter ~ 1 km).

Most observed characteristics of type III bursts are accounted for by the ‘plasma hypothesis’ which implies that the radio energy is radiated from the coronal altitude where the local plasma frequency is close to the observing frequency and the index of refraction vanishes. Strong focusing is to be expected in the absence of scattering effects.