ISO Overview

The Infrared Space Observatory, a fully approved and funded project of the European Space Agency (ESA), is an astronomical satellite that will operate at wavelengths from 2.5 - 240 m. ISO will provide astronomers with a unique facility of unprecedented sensitivity for a detailed exploration of the universe ranging from objects in the solar system to the most distant extragalactic sources. In keeping with ISO's rôle as an observatory, the majority of its observing time will be made available to the general astronomical community via proposal submission and peer review.

The satellite essentially consists of a large cryostat, the payload module, containing about 2300 litres of superfluid helium to maintain the Ritchey-Chrétien telescope, the scientific instruments and the optical baffles at temperatures between 2K and 8K. The telescope has a 60-cm diameter primary mirror and is diffraction-limited at a wavelength of 5 m. A pointing accuracy of a few arc seconds is provided by a three-axis-stabilisation system consisting of reaction wheels, gyros and optical sensors. ISO will be launched in September 1995 by an Ariane 4 into an elliptical 24 hour orbit (apogee 70000km and perigee 1000km) and will be operational for at least 18 months.

The ISO scientific payload consists of four instruments: a camera (ISOCAM), an imaging photopolarimeter (ISOPHOT), a long wavelength spectrometer (LWS) and a short wavelength spectrometer (SWS). Each instrument has been built by an international consortium of scientific institutes using national funding and has been delivered to ESA for in-orbit operations. Each of the consortia is headed by a Principal Investigator (PI), who is the formal point of contact between ESA and the instrument team. The instruments view adjacent areas of the sky and switching between them will involve a repointing of the satellite. In principle, only one instrument will be operated at a time; however, when the camera is not the main instrument, it can be used in a so-called ``parallel'' mode to gain extra astronomical data. Whenever possible during satellite slews, the photopolarimeter will be operated in a so-called ``serendipity'' mode so as to make a partial sky survey at wavelengths around 200 m, a region not included on IRAS. As befits a true observatory, the individual instruments have been optimised to form a complete, complementary and versatile package.

In order to minimize straylight from bright infrared objects, ISO must never point close to the Sun, the Earth or Jupiter. These constraints mean that at any instant only 10 - 20% of the sky is visible for ISO. ISO's orbit precesses relatively slowly; thus, during its entire 18 month lifetime there will be areas of the sky permanently invisible. The location of these areas depend on the date and time of launch; for the planned launch in autumn, the area of zero visibility will lie in Orion and, for a launch in spring, in the Galactic Centre region. Thus, proposers must take these constraints into account at the time of their initial proposals.

During scientific use, the satellite will always be in real-time contact with its control centre in Villafranca near Madrid, Spain; however, ISO will be operated according to a detailed, pre-planned schedule in order to maximise the overall efficiency of the mission.

Examination of the scientific data will be carried out both on- and off-line. Within a few weeks of an observation being completed, observers will be supplied with a standard set of products from which they may make their astronomical analyses. The distributed products will contain: reformatted raw data; various levels of processed data; calibration data; and various auxiliary data (e.g. any relevant satellite information). The distribution method is either a 6250 bpi or DAT tape with the data stored in FITS format.