The camera positioning system

As the imagers are using moderate fields-of-view (Figure 2.3), a Camera Positioning System (CPS) is required in order to be able to configure ALIS for various imaging situations. For example, when running in a tomographic mode, observation of a common volume centred on an auroral arc is desired, while maximum sky coverage, or tracking of the foot-point of a spacecraft might be the objective of other observational modes. When observing for example HF-pump enhanced airglow (Section 6.4), the cameras need to point towards the anticipated region of enhanced airglow above the EISCAT-Heating facility in Tromsø

Initially a traditional azimuth/elevation drive with rotation in azimuth and elevation was considered, but it was found that such a drive would be difficult and expensive to build given the size and weight of the imager and the available space at the station. There is also a cable wrap problem associated with the azimuth drive, where the drive would have to rotate back $360^{\circ}$ in certain imaging situations. Because of this, a solution consisting of three frames mounted inside one another was selected.

The two innermost frames are mounted on perpendicular axes. The ALIS imager is attached to the innermost frame (Figure 3.10). Stepping

Figure 3.10: Bottom view of the camera positioning system at ALIS station 4 (Tjautjas). North is downward and east is to the left in the figure. Azimuth ($a_{\phi}$) is $0^{\circ}$ towards north and $90^{\circ}$ towards east, as indicated by the arrow in the lower left part of the picture. Zenith angle ( $z_{\theta}$) is $0^{\circ}$ in zenith and increases as the optical axis of the lens is lowered towards the horizon (decrease in elevation). $\alpha$ is zero in zenith and positive when tilting eastward. $\beta$ is zero in zenith and positive southward (for $\alpha=0$). The back end of the CCD camera head of an ALIS imager (ccdcam1) is mounted in the innermost frame, rotating about the $\beta$-axis. This frame is in turn mounted in the middle frame, rotating about the perpendicular $\alpha$-axis, which, in turn, is attached to the outermost fixed frame. The stepping motors and gear boxes for the axes are seen in the lower middle, and right side of the photograph. Incremental angular encoders are at the opposite side of each axis. A part of the camera control unit is seen at the lower right-hand side. The unit is mounted on one of the four iron legs supporting the outermost frame. The plywood ring, together with the counter-weight at the bottom of the innermost frame serve as an emergency-stopping device for out-of-bounds CPS movements.

motors are connected to one end of each axis via a worm-gear assembly, and an angular encoder is attached to the other end. The worm-gear assembly gives an automatic brake when the stepping motors are disengaged, but also introduces a small lag. The innermost axis is denoted $\alpha$ and the outer axis $\beta$. In this way it is possible to reach any region on the sky, keeping the image rotations small and without wrapping the cables. Also the cable lengths can be kept short (about 0.5 m ) and the imager is accessible for maintenance and fairly easy to remove. A worst-case repositioning typically takes less than 10 s, and the accuracy is $0.01^\circ$ as long as the stepping motors are engaged.

The CPS Control unit (CPC) is a similar electronics unit to the FWC (Figure 3.9) and contains the power supply and electronics for the CPS. Both the FWC and the CPC are in turn controlled by the NIPU (Section 2.2.2). Conversion of set azimuth, $a_{\phi}$, and zenith-angle, $z_{\theta}$, of the optical axis, into $\alpha$ and $\beta$ values is automatically done by the NIPU software according to these equations:

$$\alpha = \arctan (\sin a_{\phi}\:\tan z_{\theta})$$ (3.46)

$$\beta = -\arcsin (\cos a_{\phi}\: \sin z_{\theta})$$ (3.47)

Azimuth is defined as zero towards North and $90^{\circ}$ towards east. Zenith angle is zero in zenith and increases towards the horizon, (i.e. $z_{\theta}= 90^{\circ} -h_{\theta}$, where $h_{\theta}$ is the elevation). It is worth remembering that the azimuth value is undefined for small zenith angles. The maximum zenith angle is typically about $54^{\circ}$.

To simplify the use of ALIS, a set of standard pre-programmed positions for various typical observing requirements are available (Table 3.6 and Figure 3.11).

Table 3.6: ALIS preset camera positions. The columns give azimuth and zenith angles for the six stations. See also Figure 3.11.

1 KRN		2 MER		3 SIL		4 TJA		5 ABK		6 NIL
$a_{\phi}$	$z_{\theta}$	$a_{\phi}$	$z_{\theta}$	$a_{\phi}$	$z_{\theta}$	$a_{\phi}$	$z_{\theta}$	$a_{\phi}$	$z_{\theta}$	$a_{\phi}$	$z_{\theta}$
core -- Centred on a volume above Kiruna
$0^{\circ}$	$0^{\circ}$	$298^{\circ}$	$24^{\circ}$	$249^{\circ}$	$28^{\circ}$	$346^{\circ}$	$20^{\circ}$	$130^{\circ}$	$24^{\circ}$	$90^{\circ}$	$20^{\circ}$
eiscat -- Optimised for field-aligned EISCAT studies
$0^{\circ}$	$39^{\circ}$	$348^{\circ}$	$42^{\circ}$	$350^{\circ}$	$32^{\circ}$	$0^{\circ}$	$42^{\circ}$	$20^{\circ}$	$35^{\circ}$	$15^{\circ}$	$42^{\circ}$
E-W -- Optimise east-west field-of-view
$0^{\circ}$	$39^{\circ}$	$0^{\circ}$	$15^{\circ}$	$0^{\circ}$	$32^{\circ}$	$0^{\circ}$	$42^{\circ}$	$0^{\circ}$	$35^{\circ}$	$0^{\circ}$	$15^{\circ}$
heating -- Optimised for HF-pump enhanced airglow
$346^{\circ}$	$37^{\circ}$	$340^{\circ}$	$40^{\circ}$	$330^{\circ}$	$37^{\circ}$	$345^{\circ}$	$44^{\circ}$	$5^{\circ}$	$25^{\circ}$	$5^{\circ}$	$40^{\circ}$
mag-Z -- Local magnetic zenith
$180^{\circ}$	$12^{\circ}$	$180^{\circ}$	$12^{\circ}$	$180^{\circ}$	$12^{\circ}$	$180^{\circ}$	$12^{\circ}$	$180^{\circ}$	$12^{\circ}$	$180^{\circ}$	$12^{\circ}$
north -- Centred on a volume north of Kiruna
$0^{\circ}$	$30^{\circ}$	$330^{\circ}$	$33^{\circ}$	$311^{\circ}$	$25^{\circ}$	$355^{\circ}$	$33^{\circ}$	$85^{\circ}$	$22^{\circ}$	$44^{\circ}$	$26^{\circ}$
south -- Centred on a volume south of Kiruna
$180^{\circ}$	$30^{\circ}$	$250^{\circ}$	$23^{\circ}$	$215^{\circ}$	$28^{\circ}$	$260^{\circ}$	$7^{\circ}$	$148^{\circ}$	$31^{\circ}$	$135^{\circ}$	$23^{\circ}$
surv -- Optimised for maximum field-of-view
$225^{\circ}$	$20^{\circ}$	$100^{\circ}$	$14^{\circ}$	$45^{\circ}$	$12^{\circ}$	$115^{\circ}$	$15^{\circ}$	$30^{\circ}$	$14^{\circ}$	$270^{\circ}$	$20^{\circ}$

Figure 3.11: Projections to 110 km for the preset camera positions of Table 3.6. The axes are in km west-east (X) and north-south (Y) relative to Kiruna.

The present CPS has been in use since 1993. Since being put into operation the CPS appears to be reliable and it has worked for years without any major problems. Worm-gear adjustment due to mechanical wear, as well as some angular encoder failures, probably due to an aging laser-diode in the angular encoder, have been encountered so far. Future CPS units will need a slight re-design of the hardware and a micro-controller replacing the obsolete NIPU. The existing devices will need upgraded electronics including new angular encoders as part of normal system maintenance.