Observing constraints during Eclipse Season

1. Introduction

ISO will experience these long eclipses during this particular period because the earth-sun-ISO alignment will occur when the satellite is farther from perigee, hence traveling slower, than has previously been the case. Thus it will remain in the Earth's shadow, without illumination on the solar panels, for a longer period of time. The basic geometry is shown in a sketch of the eclipse geometry.

ISO planning took into account periods of eclipse, but the batteries were designed for periods without solar-panel illumination not exceeding 45 minutes. The originally planned 18 month lifetime of ISO would not have extended into this "long eclipse season", during which the eclipses will at some times exceed 140 minutes. In order to survive this period, a number of special conditions have to be placed on spacecraft operations.

These conditions include additional constraints on the solar aspect angle (normally 90 +/- 30 degrees), constraints on the number of instruments that can be used and constraints on satellite movement -- slews, microslews, nodding and tracking. There are also additional restrictions on the overall pointing direction which can be used. Pointing will be somewhat degraded during the eclipse because the fine sun sensor cannot be used.

As the eclipse season progresses, the eclipses occur later during the science window, as detailed in the plot of the eclipse duration.

2. Observing constraints

• The solar aspect angle constraint is as for normal observations.
• RA is constrained and must be between about 0h and 13h.
• all 4 instruments can be used, but in practice LWS and CAM are favored due to their relative insensitivity to the degraded pointing during the eclipse.
• The only spacecraft movements allowed are changes of instrument or aperture -- no rasters may be executed nor targets changed. SSOs cannot be tracked. Therefore a single source must be observed. In practice only staring mode observations of this source will be made during the period beginning 30 minutes before the eclipse and continuing through the eclipse.
• The satellite must be pointed in its eclipse position 30 minutes before eclipse entry.
• During the second period of short eclipses no observation can be scheduled after the eclipse because it ends near the end of the science window.

During the longer eclipses (exceeding 140 minutes and occurring in revolutions 664 to 678 inclusive) in addition to the above, the following more stringent conditions must be met:

• The solar aspect angle outside of the eclipse is constrained to 90 +/- 28 degrees. The solar aspect angle at the time eclipse is entered must be between 90 and 114 degrees. The observability database used by the "ISO Target Visibility Tool" takes the 90+/-28 degree constraint into account.
• Additional constraints to the Right Ascension and declination apply. These vary with revolution, but roughly constrain RA to be about 3h to 6h and dec to be below about 60 degrees.
• Only 2 instruments can be powered on, the other 2 must be switched OFF before the eclipse begins. These two instruments will NOT be powered back on after the eclipse exit. The eclipses occur near the end of the science window. CAM observations are favored during the long eclipses and either PHT or SWS may be used to perform special calibrations which are not pointing sensitive. After revolution 665 LWS is no longer favorable for science observations because the eclipse occurs late in the revolution when LWS is normally not scheduled due to high glitch rates.
• No observations will be planned after eclipse exit during the latter portion of this period (after 672).
• NO SPACECRAFT MOVEMENT is permitted. Thus a single, non-rastering AOT that does not involve any changes of aperture or virtual aperture must be used. This essentially mandates a single staring mode observation must be made, beginning 30 minutes before the eclipse begins. In practice, for science observations, this means a single AOT since changes of AOT incorporate pointing verification.

3. How will the eclipse season affect the average observer?

Pointing will be somewhat degraded during the eclipse itself and for 15 minutes afterward because the Fine Sun Sensor cannot be used. It is expected that the pointing accuracy will be +/-2 arcseconds but it could be up to +/-11 arcseconds if guide star choice is poor. For this reason LWS and CAM observations are preferred during and immediately after the eclipses, even for the short eclipses when all four instruments remain powered on.

There are additional visibility constraints during the eclipses that will affect scheduling. The most important of these is incorporated in the Visibility Tool, but the detailed constraints during the eclipses themselves are not. Further, the eclipse itself must be scheduled as a fixed time observation, and the observations must be hand-selected for feasibility. As many science observations as possible will be scheduled, but few suitable candidates were found -- mostly very long CAM 04 scans and long LWS integrations. A number of special calibrations are also planned by SWS and PHT. The rest of the schedule outside the eclipses themselves will be formulated as is normally done. However, since efficiency of scheduling encourages keeping slews short, the eclipse viewing constraints will have some effect on the choice of sky covered during the eclipse season. As the eclipse season coincides with the period of visibility for Taurus-Orion this will make scheduling somewhat of a challenge! As always, all reasonable attempts will be made to schedule all observations of Grade > 3 in regions of sky which are disappearing forever, but ISO cannot guarantee that any particular observation will be performed, even during nominal operation.