Fig. 6-37. Simplified Diagram of a V Slit Star Sensor detector. This portion is known as the instantaneous field of view (IFOV). The image definition device may be either a reticle consisting of one or more transparent slits etched on an otherwise opaque plate, or an image dissector tube in which the IFOV electronically scans the FOV. The detector transforms the optical signal (i.e., whatever light is not blocked by the image definition device) into an electrical signal. The most frequently used detector is a photomultiplier. Solid-state detectors are also commonly employed, but they are usually noisier than photomultipliers. Finally, the electronics assembly filters the amplified signal received from the photomultiplier and performs many functions specific to the particular star sensor.

Star scanners used on spinning spacecraft are the simplest of all star sensors because they have no moving parts. The image definition device employed by this type of sensor consists of a slit configuration, such as the V slit arrangement shown in Fig. 6-37. The spacecraft rotation causes the sensor to scan the celestial sphere. As the star image on the focal plane passes a slit, the star is sensed by the detector. If the amplified optical signal passed from the detector to the electronics assembly is above a threshold value, then a pulse is generated by the electronics signifying the star's presence. The accuracy of this sensor is related to the width of the slits and is typically on the order of 0.5 to 30 arc-minutes, although more accurate models exist. Star scanners have been used successfully on several missions, including the OSO and SAS series. Table 6-5 lists the characteristics of several typical star scanners. The OSO-8 star scanner is further described in Section 6.4.2.

The interpretation of star scanner measurements becomes increasingly difficult as spacecraft motion deviates from a non-nutating, uniformly spinning rigid body. For example, data from the SAS-3 star scanner is useful only during the constant spin rate portions of the mission. The nominal spin rate at 1 rpo (approximately 0.07 deg/sec) is at the lower range for successful interpretation of star scanner data. ProBwms of noise and the generation of false star crossing signals are greater at this spin rate than at 2 or 3 rpo. Interpretation of the SAS-3 star scanner data is virtually impossible during the portion of the mission when the spin rate changes rapidly.

Gimbaled star trackers, illustrated in Fig. 6-38, are commonly used when the spacecraft must operate at a variety of attitudes. This type of tracker has a very

Table 6-5. Parameters for Representative Star Scanners
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