National Aeronautics and Space Administration

THE EXOSAT COORDINATED EXPERIENCE

The importance of studying astrophysical objects across a number of difference wavelength bands has been long recognized, but such observations can be difficult to obtain, mainly because they require the synchronization of the various observing windows of orbiting satellites and ground based facilities. The characteristics of the EXOSAT mission made it particularly suitable for taking part in multi-wavelength campaigns. The single most important feature in this was the long orbital period of EXOSAT (90hr) which, for the first time, considerably reduced the number of constraints on the X-ray observations. In spite of the relative flexibility of the EXOSAT mission, many coordinated campaigns still proved difficult to organise and execute. In this article we review the statistics of the EXOSAT coordinated campaigns, comment on the various problems encountered and give suggestions as to how future planning of multiwavelength campaigns can be improved.

The deep orbit of EXOSAT meant that, unlike spacecraft in low Earth orbit, most of the celestial sphere was free from Earth occultation, so simultaneous observations with groundbased observatories were relatively straightforward. Out of a total of 1643 observations made as part of the AO program a total of 503 were coordinated with other astronomical facilities, or 31% of the total. The percentage of coordinated observations rose steadily throughout the mission. The coordination with different facilities can be broken down as follows:

A total of 409 observations were coordinated in one band, while the remainder were coordinated with two or more facilities as follows: 68 in two bands, 22 in three bands, and 4 in four bands.

Coordinating observations with other facilities proved to be the single most important factor driving the EXOSAT schedule. It also posed the greatest problems for mission planning. Other observatories produced their schedules up to six months in advance with the intervals covered by each often out of phase with each other. Once a facility had allocated time it was very difficult to get things changed e.g. a problem that occured several times was that WE time would be allocated on a particular European or US shift, but the source would be visible from a ground based facility out of phase with that shift. In these and other cases where scheduling clashes occured, rearrangements to accommodate the various schedules proved very difficult. There were even some cases of different multi- wavelength campaigns using different telescopes, but with EXOSAT as the common denominator, being scheduled at the same time!

Ground-based, and space-based, observatories usually have very different observing windows. While EXOSAT had the capability to observe all targets at angles greater than 70 degrees from the sun, this was after a few months operations restricted to between 90 and 130 degrees because of thermal and other constraints on operation of the telescopes as well as to minimize the use of attitude control gas. This meant that many stars near the ecliptic equator could only be observed for half the night. For this reason it is important to keep the observing constraints of future satellites as flexible as possible.

In December 1984, a meeting was held at Villafranca attended by the schedulers of EXOSAT, IUE and ESO, and agreement was reached for more joint consultation before the production of timelines. A rapport was also reached with the scheduler of the VLA. Direct contact between these various observatories proved to be very important for the optimisation of observing schedules. However, for most other facilities - for instance, ground-based telescopes in the USA, Australia and the Canaries - liason was possible only through the principal investigator, PI, and in many cases the EXOSAT PI was not the same person responsible for the ground-based observation. This led to another link in an increasingly unwieldy chain of communication.

Another problem that often occured was that a PI would not be allocated telescope time at some of the facilities. Towards the end of the EXOSAT mission coordinated observations were approved only on the condition that the other observatories also approved the time. Because the lifetime of EXOSAT was limited, the review process for EXOSAT proposals was highly competitive and envitably in some cases requests for observing time for multiwavelength campaigns was turned down. In several cases the telescope time for the other facilities was still approved, and the EXOSAT project scientist then came under considerable pressure to allocate observation time. In many cases this time was granted, but it was not the best way to review the scientific merit of such campaigns.

While overall the multi-wavelength programs which EXOSAT participated in were highly successful, it seems worthwhile to pass on some general comments from the EXOSAT experience that may help future coordination between ground and space based facilities:

1. Orbiting astronomical observatories operate best in deep EXOSAT-type orbits. This makes coordination with other facilities much simpler, by removing the 100 min periodic interuption of data by earth occultation and passage through the earths radiation belts. It also allows direct communication with the spacecraft which can give rapid response to unexpected events.

2. All the major observatories should arrange their proposal deadlines and scheduling intervals to coincide.

3. If astronomers are trying to obtain time simultaneously on three or four different telescopes then it might be simpler if the proposal was not reviewed three or four times by specialists in different wavebands, but rather only once by specialists who have a good grasp of the multiwavelength issues. This would avoid the frustration of not all the observatories participating in programs. It would also give help to observing committees in judging the overall merit of a multiwavelength proposal.

The case for trying to improve the way in which multi- wavelength campaigns are organized is being emphasized by a series of workshops and meetings specifically to discuss these issues. At the most recent of these organized by the Space Astronomy and Astrophysics Group at the Los Alamos National Laboratory in Taos New Mexico in August 1987 many of the above points were discussed and it was proposed that representatives from the major world observatories should be brought together to discuss them further. Please contact Dr France Cordova, D436, Los Alamos, NM 87545, USA for further information. Hopefully through such discussion the success of EXOSAT in raising the multiwavelength expectations of astronomers, will be carried forward to future orbiting facilites.

N.E. White
P. Barr