HEAD> Aspect Reconstruction

Aspect Reconstruction

The aspect reconstruction is reasonably satisfactory for the majority of the observations: when the Z star tracker (the one aligned with the NFI) is in use, the attitude reconstruction achieves a relative error of the order of 30 arcsec. Larger deviations (less or similar to about 2 arcmin) may be present when only one X or Y star trackers is in use or when no star tracker is active. This applies only in the few minutes before and after any Earth occultation, due to the Z star tracker being active only at elevation angles higher than 14 degrees. Observers who wants to use the best possible aspect reconstruction should screen their data for the periods in the orbit when the elevation angle is smaller than 14 degrees.

A small number observations have been carried out without the use of the Z star tracker, due to the lack of suitable guide stars in the Z star tracker FOV. The attitude reconstruction software dealing with these special cases is currently being improved by the AOCS team. We hope to provide for these cases an acceptable (relative error better than 1 arcmin) aspect reconstruction in a short time.

The Attitude files contain the information on which was the star tracker configuration at each time. This information can be accessed looking at the parameter STR_CONF. In the SAXDAS hk fits files this information is stored, as all the other attitude data in the third extention of the housekeeping files.

The meaning of this parameter is as follows:

0 = NO ST in use
1 = -X ST in use
2 = Y ST in use
3 = -X & Y ST in use
4 = Z ST in use
5 = -X & Z ST in use
6 = Y & Z ST in use<
7 = Quadra solar sensor mode, data aquired when this mode is active are not to be used for scientific purposes

The figure gives an example of attitude reconstruction. Note the jumps in corrispondence of changes in the star tracker configuration.

Please send questions/problem reports to: helpdesk@sax.sdc.asi.it