4.2 Responsivity non-linearities

The AOT logic tries to perform an FCS measurement for which the power on the detector is close to the power on the sky. This is done because the responsivity has to good approximation a fixed value only when the power on the detector is in the same range within factors a few. For larger variations in illumination, responsivity non-linearities can become a major source of uncertainty in the flux calibration. Typical observations affected by non-linearities are:

• multi-aperture photometry. If the source is not point-like then going from a small (e.g. 52'') to a large aperture (of 180'') can easily give a factor 10 increase in power on the detector. Note that the FCS is always tuned according to the brightest flux.
• multi-filter photometry. For example the 11.3 and the 11.5 m filters have a difference in transmission of a factor 5. On the other hand, targets with steep spectra (like stars) can also cause a high dymanic range in power on the detector.
• maps with high dynamic range. Usually the FCS is tuned accouding to the flux of the brightest source in the map, therefore the photometry of the weaker targets in the map will be biased. The morphology of bright extended sources will be distorted.