Introduction FUSE target visibility tool: VIZ v6.0

Following two reaction wheel failures in late 2001, the FUSE attitude control system was significantly modified to permit the continuation of the mission. The new hybrid pointing system uses the two remaining reaction wheels plus the three-axis magnetic torquer bars (MTB) to control the satellite's slewing and pointing. Because the MTBs are not strong enough to allow full torque authority for all combinations of time and target position, the instantaneously available sky is more restricted than in the nominal attitude control configuration. The FUSE project at JHU has developed a software tool ("VIZ") which allows users to evaluate the availability of any target for stable pointing control.

In 2003, a new attitude control system was uplinked to the spacecraft which included several improvements over earlier versions with respect to the sharing of the MTBs between attitude control and momentum un-loading for the reaction wheels. As of April 2004, several further improvements of the planning and scheduling software combined to significantly improve the FUSE sky availability. Primarily, these improvements involve the routine use of spacecraft offset ROLL angles as well as utilization of orbits with marginal attitude control for short periods of time during target occultation, to improve visibility windows, especially for targets in difficult to observe regions of the sky..

These algorithms are used for the "(DEFAULT) Mag torque" option on the "torque control" bar. The option "(NON-STANDARD sun/ram/moon only" may be useful to evaluate how robust a particular torque authority window is, as this option uses less conservative assumptions.

Several additional spacecraft and scheduling parameters in VIZ may also be modified by the user. These include the RAM zone avoidance angle, and the allowed range of beta angles and the excluded pole angle zone.. The defaults in VIZ reflect our best estimate of what the attitude control and scheduling parameters will be at the beginning of cycle 6.

Proposers who find that their target has less availability than the desired exposure time requires, may want to explore whether a more aggressive parameter set might provide sufficient availability to accomplish the desired observations. If non-default parameters are used to determine target visibility, these parameters must be cited and justified in the proposal.

In VIZ v6.0, the default setting "Show optimal windows only" has been introduced. This setting shows the longest VIZ windows that together comprise at least 75% of the total available time, while shorter intervals are not displayed. The total visibility show at the bottom of the page, however includes ALL visibility intervals. For simple investigations of total target visibility this option provides the most likely scheduling windows for the target. However, for time critical observations, ephemeris target, coordinated observations etc, this setting should be turned off.

When using the VIZ tool, note that:

1. The possible exposure time is only a fraction of the visibility period (~30%) since slews to and from the target, Earth occultations and SAA passes have to be taken into account.

2. The channel alignment for small windows cannot be guaranteed, since there is not sufficient time to thermalize and align prior to a science observation. Hence, for small windows, the short wavelength (SiC) throughput may be degraded. (LiF1 observations are unaffected.)

Still under study:

RAM avoidance angle: Default in VIZ: 10 degrees
At present (July 1, 2004), the RAM avoidance angle is 10°. Even at this angle, incoming atmospheric particles will still hit the baffles. However, at lower angles, some particles may strike the mirrors directly. Therefore, while we hope to be able to lower this angle further, such a decrease is contingent on a decrease Solar activity and will require significant testing before it can be implemented. As we evaluate the atmospheric density at the FUSE altitude with time and the possible impact on the optics, further reductions in the RAM avoidance angle may become possible.

For questions or comments, please contact FUSE User Support at fuse_support@pha.jhu.edu.