The primary function of the two batteries on board IUE is to provide spacecraft power during the semi-annual Earth shadow seasons. Observations at high or low Beta angles may also discharge the batteries. In order to prolong the life of the batteries and the IUE mission lifetime, the number of times that the batteries can be discharged per year for normal science operations is very limited. In most cases, proper scheduling of your observing program can preclude the need to operate at high or low Betas. However, battery discharge may be necessary in some circumstances, such as time-critical observations or targets of opportunity. It is necessary that approval to discharge the batteries be obtained form the Project Scientist prior to your arrival at GSFC. The general guidelines for battery discharge are reviewed below (also see Appendix A). Please refer to the articles in the NASA IUE Newsletter (Kalinowski 1983 and Sonneborn 1983) for further details.
In any 12-month period, the batteries may be discharged below 22.5 Volts only 36 times (24 times for GSFC and 12 for VILSPA). Of these 36 discharges, the batteries may reach the `red-line' limit (20.6 Volts) only 12 times (8 for GSFC and 4 for VILSPA). If a red-line limit is reached, the spacecraft must be promptly slewed to a power-positive attitude to allow the batteries to recharge. Depending on the initial depth of discharge, the recharging period may take up to 16 hours. This could restrict observing plans for the remainder of the shift and the next observer's shift as well.
The rate of discharge depends upon the level of observing activity. Reading images from the cameras consumes more power than exposing. The camera deck heaters, which are used to control the focus at low Betas, draw about twice as much current as reading a camera. Spacecraft rangings draw even more power than the deck heaters. Although rangings normally only last about 10 minutes, they can rapidly drain the batteries down to the 22.5 Volt limit if the spacecraft is at a high or low Beta. An observing plan for exposing one camera with a primary mirror heater turned on is an example of a minimum level of observing activity. Exposing one camera with both primary mirror heaters turned on, while reading the other camera once per hour, is an example of a high level of activity. For observations at high Beta angles the spacecraft will be put in a lower power configuration(again, with prior approval from the Project) which extends the power positive Beta range.
Since the solar arrays are gradually degrading with time, the actual Beta limits at which battery discharge is likely to occur are also gradually changing. Predictions of future power positive regions are given by Jackson (1991). The current range of power positive Betas is somewhat better than previously predicted, since the two-gyro/FSS system uses slightly less power than the former three-gyro system. The letter you receive with your skymap will give the current Beta limits for the month of your observations.
Twice a year, for about three weeks in late summer and winter, the IUE's orbit carries it through the Earth's shadow once each day. During the shadow passages, which may last as long as 80 minutes, the batteries lack sufficient power to permit observations or maneuvers. On exiting shadow, normal operations resume, although battery discharge is not permitted. During the July-August 1991 shadow season observations were confined to Beta angles between about 45 and 95 degrees.