Pointing performance

Pointing and tracking will be performed on guide stars using the two star trackers mounted outside the cryostat. The guide stars have apparent visual magnitudes between 2 and 8 and are distributed over the entire sky. For each observing target there is at least one guide star in the field of view ( ) of the CCD detector inside the operational star tracker. The misalignment between the operational star tracker and the telescope boresight is periodically measured and corrected using a star sensor located on the optical axis of the telescope.

The standard ISO observing mode is a 3-axis-stabilized pointing at a selected target. This permits observations of a target for a period up to several hours. Tab. 1 gives the required and the actual in-orbit values for:

1. the Relative Pointing Error (RPE): the angular separation between the instantaneous absolute orientation of the satellite fixed axis at some time t and a reference axis defined over 30 sec;
2. the Absolute Pointing Drift (APD): the angular separation between the short time average (barycentre of the actual pointing during some time interval) and a similar pointing at a later time;
3. the Absolute Pointing Error (APE): the angular separation between the commanded direction and the instantaneous actual direction;
4. the Average Measurement Accuracy (AMA): the angular separation between the actual and the measured orientation of the satellite fixd axis defined instantaneously over a time interval.

The values given are 2 values for the radius of the cone, i.e. the angular separation between the actual and the commanded pointing direction is within these limits 95% of the time.

The influence of the higher pointing performance is discussed in Sect. 13.2.

Table 1: Pointing performance, assuming one calibration per revolution (values are 2 , halfcone)