A new digital all-sky camera

An all-sky camera has been operating in Kiruna since the International Geophysical Year in 1957 [Stoffregen, 1962]. In 1977 the camera was replaced with a new more automated camera [Hyppönen et al., 1974]. This camera is still in operation, but a replacement was strongly desired as the data storage medium is 16 mm colour film, a medium soon to become obsolete, as well as expensive to develop and copy and difficult to digitise, and thus complicated to make available and distribute to the scientific community and general public. In Finland all 16 mm all-sky cameras have already been replaced by advanced new cameras with filter-wheels and narrow-band interference filters similar to ALIS, but with all-sky optics and intensified CCDs [Syrjäsuo, 2001]. Furthermore, a non-intensified spectroscopic all-sky imager is described by Ejiri et al. [1999] (see also Section 1.1). Here an approach is investigated based on a commercial digital colour camera with replaceable standard optics. Such a camera can produce digital data of better, or at least similar quality, to the 16 mm colour films produced by the old camera. After testing a number of cameras, most were found to have too poor sensitivity or other problems. Eventually one camera with acceptable signal-levels for auroral imaging was found.

This camera (Fuji FinePix S1Pro, cost $\approx 30000$ SEK) was equipped with a Nikon Nikkor 8 mm 1:2.8 objective-lens giving almost all-sky field-of-view (about $180^{\circ} \times 112^{\circ}$). Figure C.1 (Left) shows the prototype camera mounted in a dome of the optics lab in Kiruna.

Figure C.1: Left: The prototype new digital all-sky camera (Fuji FinePix S1Pro) mounted on a tripod. The lens is a Nikon Nikkor 8 mm 1:2.8 giving almost all-sky field-of-view. Right: Block diagram of the new all-sky camera. The camera is controlled from a PC (CAMERA CTRL). The images (quick-looks and full-resolution images) are then downloaded to a data archive with a web-server. An UPS provides backup power. The camera is mounted in a heated insulated box with a defrosting device for the front-lens. Using two computers increases reliability and accessibility of the data-archive.

The final version will be mounted in an insulated box together with all necessary equipment such as heaters, defrosters, etc.

The camera is controlled by a camera control computer (Figure C.1), and the image data is transferred to an archiving computer, where it is made available to the world-wide web. Monitoring and control of the camera can be done remotely using an ordinary web browser. A data storage medium suitable for long-term storage will be selected later, for example writable CDs or DVDs, etc. A sample images appears on the in Figure C.2. The large

Figure C.2: Example image from the new all-sky camera prototype 2001-03-19 19:10 UTC. A digital version of this image can be downloaded from: http://www.irf.se/~urban/28am

auroral image on the cover is also from this camera. This prototype camera eventually developed severe reliability problems related to its internal firmware, the USB interface (used for communication with the PC) and possibly also the shutter. Therefore the manufacturer suggested that it should be replaced by a newer model (Fuji FinePix S2Pro). This model has an IEEE-1394 interface (aka. ``Firewire'') instead of USB. As the new all-sky cameras are intended to operate continuously at least during the hours of darkness, exposing on average one image per minute, a major concern for both cameras tested is the long-term mechanical reliability of the shutter. So far no major problems have been encountered after a couple of months of operation. However it is highly desirable to have a spare camera in order to avoid data-gaps during maintenance operations. This work is to a large extent carried out by Arne Moström (programming) and Torbjörn Lövgren (photographer). For further information see http://www.irf.se/data.html