3.6 Parallel Mode

ISOCAM was in its so-called `parallel' mode (Siebenmorgen et al. 1996, [54]; 2000, [56]) while observations were performed with other ISO instruments and during satellite slews. In this mode the sky was observed between 12$^{\prime}$ and 17$^{\prime}$ away from the prime target observed by another instrument. The allowed configurations during parallel mode were as follows: LW2 (6.7$\mu$m), 6 $^{\prime \prime }$ pfov and gain 2; LW4 (6.0$\mu$m), 6 $^{\prime \prime }$ pfov and gain 2; LW3 (14.3$\mu$m), 1.5 $^{\prime \prime }$ pfov and gain 1; CVF position at 15$\mu$m, 1.5 $^{\prime \prime }$ pfov and gain 1, or dark position. These different configurations corresponded to different levels of sensitivity and were chosen as a function of the risk of saturation of the camera during any given prime-instrument TDT. For example, in the LW2, 6 $^{\prime \prime }$ parallel mode configuration, a star brighter than about 0.5Jy in the IRAS 12 micron band would saturate the detector.

After the observing timeline for an ISO revolution was planned, the offset positions where ISOCAM would observe in parallel mode were checked for saturation by the CAM Instrument Dedicated Team (CIDT), using the IRAS database. If a danger of saturation existed, a change of configuration was scheduled for manual uplink during the revolution.

As ISOCAM was also measuring during slews which could not be screened in advance for saturations, it was also decided to use specific parallel mode configurations for different regions of sky. For observations further than 10$^\circ$ from the galactic plane, the default parallel mode configuration (hardwired in the uplink system) was the one that used the LW2 filter. For observations within 10$^\circ$ of the galactic plane, the default parallel mode configuration (also hardwired) was the one that used the LW4 filter. For restricted locations, other parallel configurations were either observer selected or manually imposed by the instrument team before target acquisition, or before observation termination (see below). For $\vert {\rm b} \vert < 1^\circ$ and $\vert{\rm l} \vert <60^\circ$ the LW3 option was used. This configuration was also used in the revolutions on which systematic calibration measurements were performed. For observations within 1 degree from the Galactic Centre, where the risk of saturation was even higher, the CVF 15$\mu$m mode was used. If none of these modes would prevent a saturation, the camera was put in a dark position.

When an ISOCAM observation was scheduled, the instrument arrived on the source in one of the relatively sensitive default parallel mode configurations and it was put back in parallel mode at the end of the AOT, before leaving the source. Therefore there was a risk that the ISOCAM detector would be saturated in the parallel mode configuration even if the source was perfectly safe in the requested configuration for observing. In order to allow observations of relatively bright sources to coexist with the parallel mode, the observer could control the instrument set-up before and after the actual observation. This was done by adding to the beginning and to the end of an observation or concatenated chain of observations, dummy observations on safe off-target positions, where the desired observing configuration could be obtained.