Physics of auroral phenomena : proceedings of the 33rd Annual seminar, Apatity, 02 - 05 March, 2010 / [ed.: A.G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 206 с. : ил.

F. Sigemes al al. Figure 1. Left: The StellaCam II from WATEC Inc. with its manual controller. Right: The CS-mount fisheye lens from Fujinon with a focal length of 1.4mm to 3.1mm at an aperture of F/1.4. 3. Experimental setup The camera was mounted under a lm diameter transparent Plexiglas dome at KHO. The idea was to position the camera in a geomagnetic reference frame with the image axis up as North and East to the left. The camera was also tilted towards the South in order to look up the local magnetic field line (~8 degrees off the vertical). But as shown below, it turned out that the tilt was off by 2 degrees and the azimuth by 12 degrees. As a consequence, it is decided to use the geographic frame as reference. Fig. 2 shows an example image taken at 07:32:25 UT. Weak red colored dayside aurora with an average intensity of about 3 kR is seen in the zenith (not visible to the human eye). The unit Rayleigh, R, is used by both the space- and the astronomical community [Baker and Romick, 1976]. The effect of scattered sun and moon light is seen to the South-East and North-East, respectively. They show up as intensified regions on the horizon where the camera is over exposed. To find the exact orientation or attitude of the camera, it is necessary to calibrate the image by the use of stars. 13 stars are identified in Fig. 2 by the use of the software AstroViewer. The elevation and azimuth values for each star are found by the open source program called Stellarium. The procedure is given in 3 steps. First we conduct a translation in order to move the coordinate system to the center of the image. The x-axis points to the right and the у-axis up, respectively. The image of the stars is now in the xy-plane with z equal to the negative focal length of the all-sky lens. Secondly, each star coordinate is rotated towards Geographic North and tilted to the South. Thirdly, the star azimuth values are calculated from the image coordinates and compared to the one found by Stellarium. The procedure is then repeated as a function rotation. A tilt o f 10 to the South and a rotation of 19 degrees to the North-West are found by optimizing the least squared error. The results are displayed in Fig. 3. The results are as expected. The calculated azimuth and the corresponding almanac values are one to one with a standard deviation less than 5 degree. The radial center pixel position of a point in the image may be fitted by the function R & 2 x / x sin ( 1 ) 4 J where f = 1.7mm is the focal length and 9 is the angle between any object and the optical axis of the lens. Eq. (1) is known as the mapping function of a typical equal area or equisolid angle fisheye lens. The image appears like a mirror image on a ball [Hill, 1926]. 42