Integral Field Spectroscopy (IFS) provides a spectrum simultaneously for each spatial sample of an extended, two-dimensional field. Basically, an IFS is located in the focal plane of a telescope and is composed by an Integral Field Unit (IFU or image slicer) and a spectrograph. The IFU acts as a coupler between the telescope and the spectrograph by reformatting optically a rectangular field into a quasi-continuous pseudo-slit located at the entrance focal plane of the spectrograph. Therefore, the light from each pseudo-slit is dispersed to form spectra on the detector and a spectrum can be obtained simultaneously for each spatial sample within the IFU field. The principle of an image slicer is presented in Fig.1.

Figure 1 - The principle of an image slicer. The slicer mirror array, located at the image plane of the telescope, divides the entrance field of view (FOV) and re-mages the telescope exit pupils along a line on the pupil mirrors. Each pupil mirror then re-images its corresponding slice of the entrance FOV on its corresponding slit mirror located at the spectrograph's focal plane (slit plane). The reformatted FOV acts as the entrance slit in the spectrograph where all the slices are aligned as a pseudo long slit.

An image slicer system is usually composed of a slicer mirror array associated with rows of pupil mirrors and slit mirrors. These components are formed by a segmented assembly of several tilted, and spherical or flat mirrors. Making use of ZEMAX, the classical modelling method consists in using the multi-configuration mode. However, the use of such a mode implies that each mirror is independently computed compared with each other. Furthermore, such classical modelling is time-consuming because ZEMAX has to compute a large number of parameters (i.e. curvatures, tilts, decenters for each sub-mirror) and configurations (one by sub-mirror) to consider the whole instrument.

By using the User Defined Surface (UDS)-DLL ZEMAX capability, I present an easier method to simulate segmented surfaces (slicer mirror array and rows of mirrors). For details of how to compile a user-defined surface, please see this article.