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The purpose of this note is to summarise tile rationale behind the Observatory's planning for the final, critical phase of the mission. EXOSAT, through frugal use of the available fuel and assuming a successful perigee raising strategy, should continue to operate until about October 1986. The current observation program which comprises targets selected from AO-3 should be largely complete ( ~85%) by March 1986 when the AO-4 program Would commence.

Given the uncertainties surrounding the end of flight operations and the obvious need to minimise the expenditure of fuel, a certain number of mission planning constraints will be imposed. The constraints are summarised as follows:

- Severe reduction of the number of simultaneous, coordinated or phase-locked observations.

- Reduced revisit capability for short exposures.

- All AO-4 selected targets will be categorised in priority according to recommendations of the Committee on Observation Proposal Selection (COPS) as follows:

1. Essential observations which must be performed prior to the end of the mission.

2. Desirable observations which should be performed provided they satisfy an optimised manoeuvre strategy.

3. Observations which will be performed only if time allows or which fit into an optimised manoeuvre strategy.

With this approach, the Observatory should be able to complete the AO-3 program and the vast majority of tile AO-4 (priority 1 & 2) program before October 1986.

The situation with the grating also requires some clarification. The 1000 line/mm grating on the LE1 telescope has not been used since a serious mechanical malfunction occurred early in the mission (ref. Express No. 1 p.3). It should be stressed that the failure left the grating at an angle of 45 degrees to the X-ray beam, ie. stuck half-way in - the worst of both worlds, useless for spectroscopy, impossible for imaging, and extremely difficult for photometric observations except on the brightest of sources. Only by careful control of the drive mechanism were we literally able to drag the grating out of the X-ray beam, although there is no evidence that it is completely orthogonal to tile optical axis. The Agency is currently evaluating the following aspects related to grating observations:

1. The grating insertion procedure.

2. The number of possible grating targets on which the grating could obtain reasonable spectra (brightness criteria) based on the AO-1 to AO-3 observation programs.

3. Assessment of the typical exposures required on each of these possible targets.

4. Assessment of the impact a possible grating observation program would have on the LE photometric/imaging observations and the overall schedule.

5. Assessment of the best insertion time given the celestial constraints imposed on key grating targets.

6. Impact of any grating program on the overall fuel usage.

For AO-4 the grating was not offered in the over-all instrument complement available to observers. This resulted from major uncertainties in the success of the grating insertion procedure as well as a large number of incomplete AO-1 to AO-3 observations. Note that if a possible attempt to place the grating in the beam were successful, it is likely to be irreversible. In addition, those observers with incomplete grating observations to date can naturally expect their observations to be performed (subject to celestial or mission planning contraints), should the grating become available.

Finally, let us look towards the future. Currently, the Agency is in the initial phase of development of the high-throughput X-ray Spectroscopy Mission. This is a cornerstone of the future ESA Science programme which represents the next major step in the Agency's high energy astrophysics program. While ROSAT and Astro-C will naturally fill an important gap for high energy astrophysics in the late 1980's, one of the next truly worldclass observatories in which the general astronomer can actively participate will be this mission. The experience gained by the ESA team in working with the world-wide user community which has developed out of the EXOSAT program will certainly be utilised in our next big stride. I hope you, the user, will be able to participate and take a step forward with us.

A. Peacock

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