From the ISO mission planning, observing and data processing point of view the solar system targets differ from other astronomical targets. This is due to the fact that solar system objects have no fixed equatorial coordinates. Therefore solar system objects are identified in the ISO archive either by their names or the ISO Generic Number (IGN), which link them to ephemerides used in position calculation at the moment of the observation.
Observations of solar system targets usually required tracking of the specific objects, i.e. a spacecraft raster. As the spacecraft raster option was already used for tracking, there was no possibility to use the spacecraft raster mode also for observations on target. In a few cases (mainly calibration observations and some comet coma observations) the positions of the SSOs had been precalculated for a fixed time observation and ISO performed a raster on the precalculated sky position without using the tracking option.
The spacecraft raster limited the observations to objects with an apparent velocity of less than 2 arcmin/h with respect to equatorial coordinates.
There are several additional aspects specific to solar system targets (e.g. change of orbital parameters, eclipse of a moon around a planet, change of galactic background or stray radiation level, ...). Therefore solar system targets request in many cases a special treatment.
Not all SSOs have been observed in SSO tracking mode. In order to find these special observations one needs to know either the used target name or the OBSID/PROPID under which the programme has been performed. The following lists facilitate the search for all SSO observations in the ISO Data Archive.
The ISO pointing was usually within a few arcsec of the
specified coordinates. The "ISO Satellite and Data Manual" gives
in chapter 5.2 an absolute pointing error of approx. 2.5 arcsec
for the first half of the ISO mission and better than 2 arcsec
after revolution 452 (10-Feb-1997), values are 2 sigma for
the radius of the pointing cone.
But in case of SSOs the ephemeris were not always perfectly known at the time of the planning of the observations, especially for newly discovered comets. For those cases a independent checking of the ISO pointing (given in the IIPH file with one RA,Dec-entry every 0.5 sec) and the ISOcentric coordinates of the SSO is recommended.
Some ISO background measurements are problematic due to different reasons:
In these case independent methods to estimate the background can be necessary.
Thomas Müller, ISO Data Centre, 2 July 1999.