The ISAAC collimator is used to focus the slit mask wheel onto the detector. The collimator has a different optimal position for each combination of different elements in the beam (e.g. each instrument mode and filter). All the optimal positions for SW imaging are fairly similar, as are all the positions for LW imaging, however there is quite a difference between the SW and LW positions.
The ISAAC collimator is mechanically unreliable and often gets stuck. This has the effect of increasing the image size and ellipticity in imaging, and of increasing and distorting the full width half max (fwhm) of the spatial profile and increasing the line tilt in spectroscopy.
In imaging we can reduce the effects of the bad collimator position by using the telescope to focus. In spectroscopy this is not possible as we need the object to be in focus on the slit. Therefore when the collimator is stuck we aim to put it in a reasonably good position for spectroscopy and then use the telescope to focus for imaging if necessary. This works well for SW imaging. In LW imaging we never get quite as good IQ using this method as we do when the collimator is working.