Pupil slicer at high throughput for the EXtreme Precision Spectrograph (EXPRES) at the Lowell Discovery Telescope

The free-space pupil slicer of Yale University's EXtreme PREcision Spectrograph (EXPRES) at Lowell Observatory's Lowell Discovery Telescope (LDT) is placed in the beam conditioning sub-system of the spectrograph in between fiber feeds for an incoming science fiber at octagonal core dimensions of 66 µm, and a respective outgoing rectangular fiber at half the width and twice the height of the octagonal fiber with core dimensions of 132 x 33 µm2. At this location efficient slicing is accomplished between 380 nm and 680 nm because the near- and far-fields are swapped within a double-scrambler arrangement, thus providing a location to image the pupil. The resolution for EXPRES is ca. 150.000 (the spectrograph design without slicing has a resolution of 75.000), thus the pupil gets sliced two times, and the two images are injected into a rectangular fiber ca. 33x132 µm2 that matches the spectrograph slit at a f-number of 1/4. The pupil slicer provides a throughput of >85%, having only modest losses from reflections on the implemented, precision and miniaturized optics, as well as alignment errors, and the injection of light into the rectangular fiber. We report about the pupil slicer's design, integration and alignment features.