The FHiRE optical design is derived from the ESO HARPS spectrograph at the 3.6-m telescope on La Silla. The basic optical layout is a white pupil design (Baranne 1972)9. In a white pupil design, the beam is reimaged and then recollimated, placing a second pupil at the camera pupil. While the white pupil design appears odd at first, it offers several advantages for spectrograph performance. Vignetting is much reduced compared to conventional spectrographs. The camera aperture is kept small and aberrations are reduced.
The echelle grating can be used in quasi-Littrow, eliminating the anamorphic beam magnification. Using the echelle in a near-Littrow configuration is particularly important for an R4 echelle, as the efficiency drops rapidly with increasing camera-collimator angle. The cost of the white pupil design is three extra reflections, which can be minimized using high-reflectivity coatings. Generally the white pupil design offers higher overall throughput and better image quality than conventional designs for the same resolution.
Similar white pupil designs are common for modern high dispersion spectrographs. The HARPS spectrograph is itself based on the UVES (UV-Visual Echelle Spectrograph, Dekker et al. 2000)10 design for the VLT, and is similar to Tull's high resolution spectrograph on the Hobby Eberly Telescope. The HARPS design has been scaled down from a resolving power of 100,000 to 60,000, reducing both the beam diameter (and cost) and the optical fiber diameter for FHiRE.
The final optical design, optimized by Charles Harmer from a preliminary design by Sam Barden, is shown below. FHiRE is fed with an optical fiber from the WIYN Tip-Tilt Module (WTTM), and uses an R4 echelle grating and a VPH cross disperser. A single, all transmission camera forms the imaging element. The design also avoids any central obscuration, which reduces throughput in fiber spectrographs. To achieve the desired spectral coverage requires a mosaic of two 2k x 4k CCDs with 12 micron pixels.