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.
Table: ISIS Dichroic Filters
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.