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EXOSAT MISSION STATISTICS


At the time of the demise of EXOSAT on the night of April 9/10 1986, 1780 seperate observations had been performed. Of these, 1643 observations were the result of successful proposals made in response to the four announcements of opportunity for participation in the EXOSAT program. The remainder were calibration, performance verification, and spacecraft operations. The AO-1, 2, 3 and 4 programs were respectively 99%, 96%, 71%, and 9% complete. Approximately 10% of the observing time went to astronomers in countries that are not members of ESA. The final breakdown of the mission by nationality (as defined by the geographical location of the principal investigator's home institute) is as follows (one observing unit = 10,000 seconds):

Country No Approved
proposals
Time
approved
(units)
Time
completed
(units)
Pointings
approved
Pointings
performed
U.K. 65 232 1769 1323 722 603
Netherlands 33 136 970 689 429 334
F.R.G. 48 132 1097 821 319 268
Italy 33 84 436 344 217 191
France 13 33 192 124 63 49
U.S.A. 40 84 558 324 200 142
Denmark 1 6 56 56 21 21
Australia 4 7 28 25 11 10
Spain 3 5 20 7 8 4
Eire 1 1 11 11 2 2
Finland 1 1 10 10 2 2
Sweden 2 2 5 5 3 3
Turkey 1 1 3 3 2 2
Czechoslovakia 1 1 3 3 1 1
Sub-total 247 726 5158 3745 2001 1643
Cal/PV/Ops- --485- 137
Total---4230- 1780

The total elapsed time between detection of the first photon on 19 June 1983 and the last photon on 1986 April 9 was 8845 units, of which 5390 units were actually spent observing (including calibration and performance verification targets, but excluding slew time). On average, observation durations were ~25% longer than the completed time given in the table because of overheads involved in the planning and execution of observations.

The resulting operating efficiency was 61% of the total mission duration and 69% of the maximum usable time above the radiation belts (for the latter the remaining time was spent maneouvring). The time lost due to problems with the spacecraft and to high solar activity (causing unacceptably high background) was a very small fraction of the total mission duration (~3 weeks out of three years).

The flexibility of the EXOSAT mission operations was such that the program could be broken into and a new observation planned and executed within a few hours. In total ~150 observations of 59 objects were performed as targets of opportunity. Observations of ~50 sources - both targets of opportunity and already approved observations (e.g. recurrent novae, X-ray transients, active galactic nuclei) - were made after receiving alerts of their outbursts from other observatories.


P. Barr


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