During the last six weeks, ISO's orbital geometry in the extended mission was such that it underwent eclipses with durations of more than double the design value for the two on-board batteries of some 80 mins. Additionally, during some of these days, marginal violations of the earth constraint on the pointing direction could not be avoided for some minutes each day as ISO went through perigee. Special operational measures were put into place for this period, including restrictions on pointing directions and on the number of instruments that could be used.
ISO has successfully come through this difficult period with better-than-expected battery and pointing performance and less-than-expected impacts from the unavoidable violation of the pointing constraints. It was also able to carry out many scientific observations of targets in the hitherto unavailable regions of Taurus and Orion.
From revolution 661 (7 September) to revolution 691 (7 October), the eclipse duration exceeded the design value of 80 mins. To conserve power in the period when the duration exceeded 140 mins (rev. 664, 10 September to rev. 678, 24 September), two of the instruments had to be switched off and no pointing changes were allowed during the eclipse. The longest eclipse, duration 166.5 mins, occurred on rev. 669 (15 September).
During this period, the performance of the two batteries was excellent with a less-than-expected depth of discharge and successfully reaching full charge each revolution. The pointing performance during eclipse was very stable and no drift about the telescope boresight was observed. This was achieved by using a second star (roll star) selected some 2 degrees away from the Guide Star. Hence, there was no pointing degradation for the scientific observations performed under eclipse conditions.
In addition to the eclipses, the orbital geometry meant that for a few minutes around perigee in the period from revolution 662 (8 September) to revolution 678 (24 September), there was no unrestricted pointing direction available to ISO. Even by driving the satellite to the extreme solar aspect angles of either 59.1 degrees or 120.9 degrees respectively, the Earth constraint still had to be violated marginally. The maximum was 2.3 degrees in revolution 673 (19 September). The effect of this violation was less than predicted, with the maximum temperature increase observed on the telescope upper baffle being just under 4 K. Nominal temperature were reached again within 30 mins.
Despite the restrictions in operations during this period as described above, ISO was able to carry out 100% of the first priority scientific observations in the Taurus and Orion regions. The overall completion rates for all priorities were 74% (Taurus) and 66% (Orion), leaving some 65 hours in each of these areas to be executed in the January to April 1998 visibility windows.
Martin F. Kessler, ISO Project Scientist, 16 October 1997.