Southern hemisphere observations of ultra-violet radiation from celestial objects I. Experimental techniques and rocket payload technology

Abstract

An experiment designed to measure the ultra-violet irradiances of stars from above the atmosphere of the Earth is described. A number of photometers were mounted on a Skylark vertical sounding rocket which was fired from Woomera, South Australia at 12.10 h U. T. on 1 May 1961. Each photometer consisted of a photomultiplier tube and a device to limit the field of view. The spectrum sensitivity of the photometers was determined by the silica windows and gold cathodes of the photomultiplier tubes and was equivalent to measure­ment at 1950 Å with 400 Å bandwidth. The photometers used logarithmic amplifiers to give a wide dynamic range. The sky was scanned by the angular motion of the rocket vehicle and the signals recorded by the photometers were transmitted to the ground over a radio telemetry link. Devices for measuring the attitude of the rocket relative to the lunar and geomagnetic vectors were mounted on the vehicle. One lunar sensor is described in detail in part II of the paper where the analysis of the motion is considered. The angular motion is shown to be one of roll a t 93.70°/s and precession around a cone of 49° semi-angle with a period of 178 s. The identification of signals with 22 stars and with the Moon is described in part III . An ultra-violet index is defined and the values found in this experiment are compared with the predictions of a number of stellar model atmospheres. This comparison, which is independent of the absolute intensity calibration of the photometers, shows that the models for early B stars over-estimate the ultra-violet irradiances. The absolute calibration shows that this over-estimate occurs for stars of spectrum classes O and B and is particularly marked for B 0 and B 1 stars. The irradiance of a star of zero magnitude at 1950Å is found to be 640 photons cm -2 s -1 Å -1 with an uncertainty of a factor of 1.5. The distribution of stars by apparent ultra-violet magnitude is discussed. Observations of the Moon are interpreted to yield a value for the lunar albedo of 3.3 x 10 -3 at 2200 Å.