  Ground-based thermal-infrared observations have a unique scientific
potential, but are also extremely challenging due to the need to accurately
subtract the high thermal background. Since the established techniques of
chopping and nodding need to be modified for observations with the future
mid-infrared ELT imager and spectrograph (METIS), we investigate the sources of
thermal background subtraction residuals. Our aim is to either remove or at
least minimise the need for nodding in order to increase the observing
efficiency for METIS. To this end we need to improve our knowledge about the
origin of chop residuals and devise observing methods to remove them most
efficiently, i.e. with the slowest possible nodding frequency. Thanks to
dedicated observations with VLT/VISIR and GranTeCan/CanariCam, we have
successfully traced the origin of three kinds of chopping residuals to (1) the
entrance window, (2) the spiders and (3) other warm emitters in the pupil, in
particular the VLT M3 mirror cell in its parking position. We conclude that, in
order to keep chopping residuals stable over a long time (and therefore allow
for slower nodding cycles), the pupil illumination needs to be kept constant,
i.e. (imaging) observations should be performed in pupil-stabilised, rather
than field-stabilised mode, with image de-rotation in the post-processing
pipeline. This is now foreseen as the default observing concept for all METIS
imaging modes.
