The PEPSI field lens assembly. It is a unit that contains the 4-lens field imager with smart dichroid coatings.
Latest dichroid lens unit CAD design.
An image of the echelle grating in the focus of the (white-pupil) collimator appears spread out in wavelength. In our case, this is in vertical direction. A slim field lens collects the "dispersed" light over a large field of view and redirects it optimally to the transfer collimators. We use this field lens also as our dichroid in order to split the light to the "blue" and the "red" arms. While the dichroid itself is just a smart coating on the inner plane-parallel surfaces of the field lens, the field lens itself is made up of 4 optical element. The outer two lenses are fixed and act as the immersion windows, the inner two can be exchanged with a sliding mechanism. The default cutoff wavelength will be 569nm, the second one is tbd. Because of the large number of optical surfaces, the field lens will be immersed in oil.
Part of the field-lens optics during first inspection in the clean room.