The internal calibration device was designed to provide
a reproducible, but not absolute, calibration
reference. Calibration of the SW channel required source
temperatures up to 350 K which are difficult to fit in the low
thermal dissipation budget of ISO. To achieve the required emission,
a small resistor was mounted on a thin Kapton film. The low thermal
conductivity of the Kapton film prevented heat losses towards the
ISOCAM base plate and ensured that almost all the energy would be
radiated. This emitter fed, through a small input hole, an
integrating sphere mounted on the selection wheel. In the
calibration mode, the output hole of the integrating sphere took
the place of the field mirror. The brightness uniformity was better
than 1% over the unvignetted circular 3
field of view of ISO.
Two spheres were included in the system, one for each channel, with
entrance holes adapted to the flux required for each array. The
temperature of the emitter could vary in the range 150 K to 350 K.
This design had two limitations. From the point of view of the optical design, the beam aperture of the ICD was defined only by the aperture stop at the filter location. It was therefore different from the ISO telescope f/15 beam. Nevertheless, SW flat-fields were obtained using the ICD (see Section 4.5). The second limitation was due to the presence of some low level emission bands from the Kapton film support which was heated by conduction. They can be seen at long wavelengths in the spectrum of the calibration device. This precluded the usage of this system as an absolute flux calibration reference. However, it was used for flat-fielding the SW array and for trend analysis (see Section 3.7.3). The ICD was also used to `flash' the detectors after leaving the Van Allen radiation belts or after a saturation event. A large current pulse was sent to the IR emitters at the beginning of a calibration sequence to speed up the heating of the emitter before setting the current to its required stable value. To avoid saturation of the detectors during this heating period, the filter wheel was set on a blind (dark) position. Therefore, it was not possible to make a calibration of the arrays without going for a short time to a dark position.
