As mentioned before, Canada’s, and more specifically the University of Calgary’s, contribution to the Viking satellite program was the Ultraviolet Auroral Imager. It was a fundamental part of the mission and to the progression of auroral research as it made dayside aurora accessible for analysis. It also proved the importance of auroral observations on other data. There were various considerations that influenced the design of the instrument:
Experience gained from previous imagers such as the one flown on ISIS-II.
Observations were made in the vacuum ultraviolet part of the spectrum where there was minimal solar interference. Auroral observations in the visible part of the spectrum were ruled out for both dayside and nightside areas of the oval because of the intense background coming from the earth and sunlit atmosphere.
A fast reflecting optical system with 25° and minimum number of optical elements was required. To achieve this, an f/1 inverse Cassegrain Burch camera involving two spherical mirrors was used which resulted in a spherical focal surface with its radius of curvature being 22.4 mm.
Separate imaging at the two most prominent UV auroral emissions, LBH and OI, was performed in order to derive information on the precipitating electrons responsible for their emissions from their relative intensities and spatial distributions.
Curved microchannel plate (MCP) intensifiers were required to conform to the spherical focal surface. Open intensifiers were used with the photocathode material deposited directly onto the front surface of the MCP. Because of the composition of the cathodes for the chosen wavelength ranges, the intensifier assemblies needed to be in a vacuum or dry nitrogen environment at all times.
Multistage MCPs presented many resolution lifetime problems so a single-stage MCP was utilized as its gain was more than adequate and avoided the problems that the multistage version faced. The intensifier was directly coupled to the detector with fibre optic blocks for optimal transfer efficiency and the fibre optic blocks also provided the desired mapping of the intensifier’s spherical surface to the plane of the CCD.
To achieve the desired sensitivity, one-second exposures were necessary from the spinning satellite. This was done by synchronously stepping the charge accumulating in the CCD with the image motion due to the satellite spin. This meant that the CCD had to be aligned with the spacecraft equatorial plane within ±0.2°.
Interactive control of the instrument pointing and image size was required and done to ensure the effective use of the available telemetry bandwidth and meet the scientific objectives. CCD exposure was triggered at any point during the spacecraft’s spin using a reference pulse from either the spacecraft limb or the sun sensors. Flexible CCD readout electronics allowed for the image dimensions to be shrunken or enlarged and to shift the field of view within the optical system’s bounds. These adjustments could all be made in nearly real-time by the instrument operator however, it required extensive viewing and planning at the ground telemetry site in Kiruna, Sweden.