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There is a remotely driven three position slide which mechanism contains options for the blue fold. One of these positions is usually 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 3 ; and described in more detail in Appendix C . There are three interchangeable slides for this slide mechanism; the procedure for changing these is described in Section 8.2.3. 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 above. 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.