synopsis: residual systematic uncertainty which is a fixed fraction of the background emission, the correction accuracy is better than 2%.
limitations and applicability:
Only applied to C100 and C200 chopped observations, the other detectors
(PHT-P and S) show chopper vignetting/offset of less than 2%. Consequently,
faint sources that are observed in chopped mode which are fainter than 2%
of the background emission can have more than 100% systematic uncertainty.
description:
The chopper vignetting/offset is corrected by scaling the on- and
off-target signals with a factor measured in-orbit.
purpose correction:
In case of a perfectly flat sky, the chopper offset/vignetting introduces a
non-zero signal difference between on- and off-target signals. This effect
causes a systematic error or even a non-detection of the source when the
difference signal becomes zero. The effect is larger for bigger chop throws.
uncertainty/noise introduced:
The C200 observations in chopped mode are the most affected by this effect.
The residual photometric uncertainty due to chopper offset/vignetting is
better than 2% and scales directly with the level of the background.
E.g if the background is 1 Jy in the pixel or aperture, then the systematic
uncertainty is better than 20 mJy/beam.
auxillary data:
Cal-G files: PP1VIGN, PP2VIGN, PP3VIGN, PC1VIGN, PC2VIGN
no uncertainties are supplied in the Cal G file.