8.1 ISOCAM Astrometry

Under the worst circumstances, ISOCAM's coordinate accuracy is given by , although typical measurements are much better than that, as reported in Section 4.11, for example. The overall error is decomposed as follows:

• Repositioning accuracy of the lens and filter wheels. The penalty is flat-fielding and minor coordinate errors typically less than 2 pixels.

• There is a small displacement of the ISOCAM optical axis in some filters and CVF positions, leading to coordinate errors of up to one pixel.

• Optical distortion, leading to celestial position and flux errors at off-axis positions.

• Very infrequent ISO pointing-system errors.