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When deciding on what slitwidth to use, the starting point is to determine the size of the 'matched slit'. As discussed in section 11.1, this is the slitwidth which corresponds to the spatial resolution of the detector; in the case of a CCD, it is the slitwidth which projects to two detector pixels. The matched slit is therefore the minimum slitwidth which is properly sampled at the detector; using a larger slitwidth will degrade the instrumental resolution, whilst using a smaller slitwidth will result in the line profiles not being properly sampled. The matched slit with different detectors is summarised in Tab. 19. The figures in arcseconds have been calculated assuming the derotator is in the beam, and assuming no focal modifier lenses are being used.
In conditions of poor seeing it may be sensible to open up the slit, trading poorer resolution for improved throughput. For example, the IPCS-II can provide higher spectral resolution than the CCD, but at the cost of increased slit losses; if spectral resolution is less important than throughput, it may be sensible to use a 1 arcsecond slit with the IPCS-II.