These remove part of the 'unused' light, outside the normal imaging area. The SBIG cameras do this, as do things like the Lumicon GEG, the Van-Slyke guider etc.. Advantage is it retains the ability
to correct for movements in the optical system, and costs nothing in
sensitivity. The 'downside', is that the selectable guide area, normally
has a small range of movement relative to the main CCD (for the SBIG system, none, unless you turn the whole camera), limiting the choice of guide stars. In the case of the SBIG system, filters also filter the light path to the guider, which can make guiding harder when using narrowband filters.