The selection of the correct gain (”LOW” or ”HIGH”) is driven by source flux estimates given by the observer. The switch to low gain is required for the flux limits given in Table 5.3. Each observation – chop-nod or scan-map – can be repeated several times, driven by the observer-specified repetition factor, to increase the depth of the observation, the sensitivity scaling with the inverse of the on-target integration time (number of readouts).
Table 5.3. PACS bolometer readout saturation levels (high-gain setting)
Point source [Jy]
This standard ADC gain of the bolometers allows photometry on a large flux density dynamical range, from the mJy level up to about 200-1000 Jy, before the brightest pixel saturates (ADC saturation and not the detector). Hence this standard gain shall be appropriate for almost all types of scientific observations. However for very bright sources, such as planets or stars in star forming regions, a low bias gain could be needed.
|The low-gain setting increases the flux dynamic range by a factor less than 2, at the expense of losing sensitivity at low flux levels, as the noise is not properly sampled anymore with the low-gain, due to the coarser digitalization.|
|The low-gain shall be used with caution and under exceptional circumstances only. If the low-gain is selected it applies to both the red and the blue channel.|