There is a remotely driven three position slide mechanism which contains
options for the blue fold.
One of these positions is clear; the others contain any of: a folding
prism; a 45 degree mirror; or any of a number of dichroic filters, which are
listed in Table 
. There are five
interchangeable slides for this slide mechanism. If the folding prism is used then the
blue arm of ISIS will require a substantial refocus, in the sense that the
blue collimator position must be increased by about 26000 microns compared with
the value for a mirror or dichroic.
If a dichroic filter is in the beam the optical thickness of the material
introduces a focus offset for the red arm or FOS. The focus offset for the
red arm, in microns, is tabulated in Table . The value of the red collimator
position is always higher if a dichroic is in the beam than if the red
fold position is clear.
The 7500 Å dichroic gives only a 2 arcminute unvignetted field along the slit, and is generally used for stellar observations.
The red fold consists of a remotely driven two position slide; one position is clear while the other contains a silver mirror with a reflective stack overcoat to send light into the red arm.
