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:
|
Observing facility | No of
observations | Percentage of total |
|
Optical | 303 | 18% |
IUE | 188 | 11% |
Radio | 63 | 4% |
IR (ground-based) | 41 | 3% |
Other1 | 20 | 1% |
|
1. Including: IRAS, Tenma, Voyager, balloon flights, and gravitational-wave
detectors.
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.
4. In many more EXOSAT observations it would have been scientifically rewarding
to have obtained simultaneous optical photometry of relatively bright objects
(brighter than 18th magnitude), even though the prime purpose of the
observation might not have been a multiwavelength campaign. However, many
observers did not bother because a coordinated observation is so difficult to
arrange. Because of this there have been several proposals to put optical
photometers on future X-ray missions (e.g. XMM) to provide such information
(similar perhaps to the FES on the IUE), or even to fly optical, uv and X-ray
telescopes on one satellite. However the cost of putting anything on a
spacecraft is expensive and requires very long lead times. It would be
much quicker and considerably cheaper if one or two of the large
optical facilities set aside a modest aperture telescope (0.7 - 1
meter) equipped with a CCD camera and a high speed photometer, that
had as a first priority the support of observations made by orbiting
observatories. The proposal process for such a telescope could be
linked to those of the orbiting facilities.
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
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