Design Considerations for Two CCD-based Star Sensors

Solid-state image sensors such as charge-coupled devices (CCD) are being considered as the detectors in future star sensors required for the high-accuracy attitude control of spacecraft. Such detectors are adequate in respect of their electro-optical properties, and offer a fundamental advantage in geometrical stability in addition to their obvious practical advantages over other types of image scanner. However, imperfections inherent in present semiconductor processing technology necessitate complex calibration procedures if their full performance is to be realized. Other properties, such as the small image formats, complicate the optical design. Measures taken to overcome such design difficulties must be considered in relation to their impact on the strict weight, size, power and reliability specifications imposed on star sensors. These problems are illustrated with reference to two specific requirements: for a general-purpose star sensor with 1 arc second accuracy and for a 10 to 30 arc second Polaris sensor.