Due to the Earth constraint and the fact that the ISO orbit precesses very
little during the in-orbit lifetime, there is a region in the sky which
is not accessible for observations during the entire mission. This region is
called the `hole'.
The position of the `hole' is determined by the position of the Earth when ISO is launched. Since the launch was midnight 17 November 1995, the 'hole' is centered on Right Ascension = 7 hours 40 minutes, Declination = 0 degrees (Tab. 2).
Figure 7: Map of the sky visibility for the entire mission. The Right Ascension and Declination correspond to epoch 2000 and the map is presented in Mercator projection. Each number represents the visibility of the skybin in percentage of the total time this bin is visible divided by the total time of the maximum possible duration of the mission, i.e. from revolution 13 to 950.
Figure 8: As Fig. 7, but for the remaining time of the mission, assuming a liftetime of 24 months; covering revolutions 382 to 777, between 2 December 1996 and 31 December 1997.
Table 2 gives an overview of the location of the `hole'.
The sky is divided up into sky bins of 10 by 10 degrees each. In Appendix E details on the visibility for each sky bin can be found.
| Location of the hole (J2000) | ||||||
|
| Declination [] | Right Ascension [h m s] | ||||
| entire | -35 |  | -25 | 07:30:00.0 | | 08:14:59.9 |
| mission | -15 | | -5 | 07:20:00.0 | | 07:59:59.9 |
| -5 | | +5 | 07:20:00.0 | | 07:59:59.9 | |
| +5 | | +15 | 07:20:00.0 | | 07:59:59.9 | |
| +15 | | +25 | 07:45:52.9 | | 08:28:14.0 | |
| +25 | | +35 | 07:30:00.0 | | 08:14:59.9 | |
| rest of | -65 | | -55 | 08:00:00.0 | | 11:59:59.9 |
| mission | -55 | | -45 | 07:00:00.0 | | 11:59:59.9 |
| -45 | | -35 | 06:51:25.7 | | 12:51:25.6 | |
| -35 | | -25 | 06:45:00.0 | | 12:44:59.9 | |
| -25 | | -15 | 07:03:31.8 | | 12:42:21.1 | |
| -15 | | -5 | 06:40:00.0 | | 13:19:59.9 | |
| -5 | | +5 | 06:40:00.0 | | 13:19:59.9 | |
| +5 | | +15 | 06:40:00.0 | | 13:19:59.9 | |
| and+5 | | +15 | 20:40:00.0 | | 21:59:59.9 Jupiter! | |
| +15 | | +25 | 07:03:31.8 | | 12:42:21.1 | |
| +25 | | +35 | 06:45:00.0 | | 12:44:59.9 | |
| +35 | | +45 | 06:51:25.7 | | 12:51:25.6 | |
| +45 | | +55 | 07:00:00.0 | | 12:59:59.9 | |
| +55 | | +65 | 08:00:00.0 | | 11:59:59.9 | |
Table 2: Approximate locations in Right Ascension and Declination (J2000) of the visibility `hole'. Upper part refers to entire mission assuming 30 months, lower part refers to the remaining time of the mission assuming 24 months - relevant for this Supplemental Call.
A sky bin is evaluated to be unusable for observations if it is visible for less than 3 hours in a revolution.
Due to the actual launch date and time and the expected longer lifetime of the mission, there are a number of skybins, which became visible or will become visible later in the mission. The hole for the entire mission is, in the most optimistic case of a maximum lifetime of 30 months reduced the a strip of skybins (see Fig. 7 and Tab. 2).
The size of the hole after this Supplemental Call is bigger, since a number of skybins became already inacessible (see Tab. 2).
This Supplemental Call (see SUPPLEMENTAL CALL FOR OBSERVING PROPOSALS - POLICIES AND PROCEDURES) is issued eight months after launch. The observations recommended by the OTAC can start as early as December 1996, which is 12 months after the launch. Fig. 8 illustrates the visibility of the sky for the time period relevant to the Supplemental Call.
