It is evident that the different instruments that are fed by ISLiD have individual requirements, which makes it necessary to sort the instruments in a priority list – not in the sense of ranking their scientific importance for the mission, but rather from the point of view of their intrinsic challenges which they impose on the optical design of ISLiD.
Since all instruments were designed to operate at their individual diffraction limit, which is set by the telescope aperture and the specific working wavelength of the instrument, the most critical instrument path is imposed by the SuFI instrument, which aims at a diffraction limit at 214 nm. Also the wavelength bandwidth of SuFI is by far the broadest, spanning almost a factor of two.
It is evident that diffraction-limited performance at these short wavelengths can only be achieved if the number of optical surfaces is kept to a minimum. To this end SuFI is completely incorporated into ISLiD, making the SuFI image path an intrinsic part for all instruments. All instrument feeds make use of the SuFI path, which is designed to be diffraction limited at 214 nm, and is therefore uncritical for the longer wavelengths. The UV light is separated after the beam stabilization, and close to SuFI focus, mitigating the disturbing influence of the beam-splitter plates. After the separation of the UV light, each of the remaining instruments is fed by a dedicated optical feed providing the required magnification, image, and pupil position. Since CWS and IMaX are operating in the visible range and can be considered as monochromatic, this can best be achieved by dedicated lens optics.
ISLiD was developped and built by the MPS.