The Star Scanner is mounted on the spinning portion of the spacecraft called the “rotor”. The rotor spins at 18.9 degrees/second allowing the star scanner to swing around a full circle every 19 seconds. During each revolution, the field of view passes over or close to both the north and south celestial poles.
The star scanner is a telescope . Incoming starlight passes through a series of mirrors and lenses which focus the light onto a pair of slits. The second slit is tilted at 30 degrees with respect to the first slit. Starlight falls on the first slit and passes through to a light-sensitive vacuum tube. This tube senses the light and, if it is bright enough, the star scanner reports it to Galileo’s attitude control computer. A few tenths of a second later, the light hits the second slit and a second pulse is reported. Since the second slit is tilted, there is a difference in the time separation between the two pulses of light that corresponds to how far “up” or “down” the star is in the field of view (see image below). The two slits are wide enough to sweep out about a 10 degree swath of sky.
A software model showing the star scanner’s field of view (the two slits) which sweep towards the left. The circle is where stray light might cause the problem of washing out stars in the slits. In the example shown, both Jupiter and Io both are in the stray light field of view and the star scanner’s detector was powered down for a few seconds each time this happened. Image drawn by SOAP from The Aerospace Corp.
The attitude control computer is able to catalog up to six stars and remember the separation in time between the pulses as well as the separation in time between each star. When it finds a pattern in the sky that comes close to matching what is in its own catalog, it uses that pattern to calculate what direction the star scanner must be pointed. From this, it can then compute what direction the rest of the spacecraft is pointed. The pointing accuracy is better than 0.05 degrees.
To conserve space, the star scanner optical path is folded with a series of mirrors. Using the left figure above, one can trace a light ray. First it passes through a conical tube which contain a series of black baffles to reject off-axis light. The light then reflects off a pair of aluminum coated molybdenum mirrors and encounters a trio of glass lenses. These lenses focus the star light onto a pair of slits. A star in the center of the bore sight of the star scanner will (ideally) focus on the first slit. Any light passing through the slit then reflects off a “transfer mirror”. This mirror can be rotated to reflect the light to either one of the two light detectors (although the primary detector has always operated well and all star scanner data is from this sole unit).
These light detectors are photomultiplier tubes – essentially just light sensitive vacuum tubes. On the inside of each tube’s domed glass window is a metal that will kick off an electron in response to a photon of light. This electron passes through a series of 13 amplification grids in which there is very high voltage used to further accelerate any electrons flying around. Each grid will kick off several electrons for each one that hits it. After the 13 stages, there is a cascade of millions of electrons that are measured as an electrical current. The brighter the star, the more the current. The star scanner is pretty much linearly proportional in this way up to the brightness of Sirius. Note that if an electron coming in from the space environment is able to penetrate to the first stage, the star scanner will amplify it as if it had been generated by a photon of starlight. This is the source of the star scanner’s ability to measure “radiation”.
Power = 3.2 Watts continuous
30 VDC and 50 VAC @ 2.4 kHz square wave
Mass = 24.7 kg
Magnitude range : +3.42 to –1.46
Magnification = ~8.0x
Temperature = 10.3 +\- 2.1 deg C since entering Jovian orbit
Attitude accuracy = 0.87 mrad = 0.05 deg
Field of View (slit width) = 10.2 deg (perpendicular to sweep direction) by 230 arc seconds
Bore sight tilt angle with respect to spacecraft –Z axis (High Gain antenna direction) = 81.5 deg