Station housing

The largest part of the GLIP consists of the station housing with the observing dome and instrument mount points. A Housing for ALIS (HALIS) must provide protection against the harsh climate: temperatures ranging from $-45^\circ{}C$ to $+30^\circ{}C$ (worst case), strong winds and intense solar radiation. The housing should also be easy to transport and assemble at distant locations. A special problem for an imaging station is the roof, where a large opening is required. On the prototype structure the roof was built in a traditional way, but the demand for a circular hole for the dome made this design complicated and ended up heavy and difficult to keep water-proof. Based on this experience, an improved roof was designed as described below.

The optical system could be said to start with the observing window. From the optical point of view, no window at all apart from the front lens itself would be the best solution. However in this case the imager must either be fixed, or the entire instrument housing must be mounted in a positioning system. The latter solution would be difficult to realise at a reasonable cost in the harsh climate. This fact, combined with the requirement of a pointable camera, led to the compromise of a half-spherical plexi-glass dome. Such domes have long been in wide use for auroral observations. The transmittance curve of the plexi-glass is shown in Figure A.2.

Figure A.2: Transmittance curves $T_{dome}(\lambda)$ for Plexi-glass Röhm GS2458 where $\lambda$ is the wavelength in nm. Curve 1 corresponds to the transmission of a 3 mm thick Plexiglas sample obtained by Röhm, Darmstadt. Curve 2 has been measured by J. Stegman, MISU, Stockholm for a sample of 4 mm thickness. [After Steen, 1987]

The bottom part of the HALIS is an iron frame (2 m $\times$ 2 m $\times$ 0.75 m) firmly anchored in such a way that the frame rests $\approx 0.5$ m above the ground. In this way the wind can blow under the floor, preventing snow from piling up around it, while also stopping some animals from reaching the station house (Figure A.3).

Figure A.3: The GLIP in Tjautjas (4) showing how snow is prevented from building up around the station.

This design requires a well-insulated floor.

A quadratic structure of dimensions 2 m $\times$ 2 m $\times$ 1.5 m is attached onto the iron frame by means of four bolts. The structure is made of two layers of construction plywood, separated by a 5 cm layer of thermal insulation. The door is 1 m $\times$ 1.2 m.

Inside the structure the imager-stand, consisting of four iron rods, is directly attached to the corners of the iron frame. The imager is thus not affected by vibrations in the structure caused by strong winds, etc.

The removable roof is made of plastic reinforced with glass-fibres and thermal insulation. The roof makes a smooth and water-proof transition from the square 2 m $\times$ 2 m cross-section of the structure to the circular 1 m diameter observing hole where the dome is mounted.

The semi-half-spherical observing dome is designed with a sleeve in such a way that the interface to the roof is water-tight (Figure A.4).

Figure A.4: Detail of the GLIP dome, showing the water-tight interface between the dome and the roof.

The HALIS can be transported and mounted at a distant location in less than one working day by 2-4 persons, using a simple trailer for road transportation, and assistance from a tractor or helicopter for the final move. The removable roof simplifies transportation and enables easy installation of large equipment modules into the station.

In total ten^A.1 HALIS of the design described above have been built for ALIS; eight are deployed on site and two of the structures are temporarily loaned out to ESRANGE. Some of the GLIPs have been on site for about ten years, without any major problems. A repainting of the plywood surfaces each decade seems appropriate to prolong the lifetime of the station.

Another experience gained from the early campaigns, (when two manned intensified CCD-cameras were operated at the Abisko and Nikkaluokta stations on a temporary basis), is that the GLIPs are not well-suited for manned operation. This is mainly due to condensation on the inner side of the dome, resulting from the humans in the station, and from frequent use of the door. After leaving the station, defrosting the dome might take up to 15 min depending on weather conditions. Therefore visits to the stations should be avoided close to measurement periods.