Practical Use of the Whole-Sky Kinematography

Abstract

As cameras for photographing the clouds we have the theodolite kinematograph and other various sorts of cameras. But when we use the lenses of the ordinary focal length, only 50° of the sky can be rendered and so the photography is limited to some extent. Moreover, if we intend to render 150° or so, the photographic plate must have the diameter of 3 feet, and for 180°, infinitely large plate is needed. This is a matter of impossibility. The improved camera, to photograph the clouds of the whole sky in one plate by using a special lens, devised to converge the incident ray to about 90° before it comes into the lens, is the whole-sky camera of Mr. Robin Hill. When we use this camera, all the objects in the whole sky, which can be regarded as a semisphere, can be photographed in a plate of 3.1×4.1 inches (8×10.5cm). The study of the clouds using this has been carried out by many persons, but 1 devised the connection of the whole-sky lens with the kinematograph to see the states of the ever changing whole sky clouds in a roll of moving picture. The focal length must be prolonged to catch the image of the whole sky on the standard film. For this purpose 1 set two concave lenses in the midway and fitted them adequately, and succeeded, in making a practical whole-sky kinematograph. After the accomplishment of this camera, we took photographs of the change of the clouds of the whole sky at the time of the solar eclipse of May 9' 1948, at Wakkanai in Hokkaido and at Tateno in Ibaragi Prefecture. The investigation of the change of the clouds then photographed is shown in another report, so that 1 will only refer to the structure and method of measuring with regard to this camera.
In Fig. 1, the whole-sky lens, the concave lenses, the lens of the camera and the photographic plate are the main parts. And when we take photographs directing the lens axis to the zenith, we can get the image of the clouds of the whole sky. The sunlight may give rise to halation on fine days, so to prevent it the sunshade as shown in Fig, 5 is used, and an automatic apparatus is attached to turn the shade following the sun. Besides, as an automatic apparatus to know the movement and the change of the clouds, by means of exposures at every five seconds, the sprocket-type electrical automatic apparatus, shown in Fig. 5, is used. For the measurement of the direction and velocity of clouds, a scale as shown in Fig. 7 is set on the screen, and adjusting the scale as the clouds should trace the curve, one end of the curve shows the azimuth of the aloud direction.
Suppose that the cloud-point which was at the intersection P₁ on the curve at time t₁ came to the intersection p₂ at time t₂, then the average velocity of cloud can be obtained by the formula H(tan θ₂±tan θ₁)/t₂-t₁, where H is the height of the cloud, and must be obtained by another method; θ₁ and θ₂ are the angles from the zenith at p₁ and p₂ respectively. The scale used is the curve obtained by photographing a grating, which consists of lines that intersect at right angles at each 10° in the sky, by the whole-sky camera.
To obtain the cloud-amount, the scale as shown in Fig. 8 is used. This scale consists of concentric circles spread from the centre to the circumference, and each round bands have the same areas, and is separated in eight directions. The sum of the cloud-amount is obtained, when we measure the amount in each section. The whole-sky camera of Mr. Robin Hill was equidistant, and had the form φ/tan θ=const. In our camera, however, sin φ/tan θ=const, because the concave lenses are not specially designed. φ is the angle of incidence and θ is that of refraction.