The 4MOST secondary guider imaging system

The 4-meter Multi-Object Spectroscopic Telescope (4MOST), wide-field, high-multiplex spectroscopic survey facility will enable the simultaneous spectroscopy of up to 2400 targets within a 2.5° diameter field of view. A secondary guider system optical relay and data production description is described. The role of this guider is for fine rotational and target alignment corrections. The output of the 12 times 7 fibres are arranged such that the CCD detector can be read out in continuous read mode. The position of all fibres are illuminated at their spectrograph end and measured using a camera system at the positioner end. For the secondary guide bundles, only the central fibre is illuminated. A notch filter is used in the image relay in such a way that that the back illumination light is reflected from back illumination fibres to illuminate only the central fibres of each guide probe. This allows on-sky guiding while the fibres are being positioned.