Fiber-fed High Resolution Echelle

The optical design of FHiRE is a traditional "white pupil echelle" (see Baranne 1972) similar to the ESO HARPS spectrograph. The white pupil design reduces vignetting, and minimizes optical aberrations. It also minimizes the camera aperture as well but with the cost of additional optical reflections. The design of FHiRE originated from Sam Barden with significant further refinements by Charles Harmer (NOAO). Ming Liang (NOAO) redesigned the FHiRE camera reducing the number of optical elements from 8 to 6. The drawback is a significant change in focus with wavelength but this is almost linear so that it is mitigated by tilting the detector focal plane. Liang's design included a glued triplet that was very sensitive to temperature change due to the differing coefficients of thermal expansion for the triplet elements. Pierce (UW) "broke" the FHiRE triplet by replacing the glue with an index matching fluid, with a slight improvement to the design, but more importantly removing the danger that the large seasonal temperature changes at WIRO could put the camera at risk. Light enters the instrument via a 50-micron, multi-mode optical fiber located at the focus of the first of a pair of off-axis paraboloids that form a collimated beam for the Iodine cell. The output f-number of the fiber will be f/4. The second paraboloid then forms an intermediate focus as this beam is directed toward one side of the collimator mirror. The collimator has a focal length of 390mm and also shares this focus so that the light reflecting from this mirror it is parallel, i.e., collimated, with a diameter of 98 mm when it reaches the echelle grating. FHiRE uses an R4 echelle with 31.6 grooves/mm at a near Littrow condition so that the angle of incidence is 76 degrees. The central blaze order is 113 corresponding to a wavelength of 538 nm. The grating is slightly tilted (5-deg.) so that the dispersed light returns to the collimator at this same angle which forms a secondary focus slightly displaced from the first one where a pupil mirror and slit are located. The pupil mirror directs the light back to the collimator for the third time after which it emerges as a set of collimated beams, one for each spectral order. A mirror located near the Iodine cell folds the beam so that it enters a volume phase holographic (VPH) grating oriented 90-deg to the echelle in order to "cross disperse", or separate, the spectral orders. The VPH grating is located immediately in front of the FHiRE camera. The camera has a focal length of 257 mm and produces excellent images over a slightly tilted focal plane. The spot sizes are 2.2 pixels over the full 2048 x 4096 CCD detector (12 micron pixels) with 69 orders resulting in a wavelength coverage of 390 nm - 680 nm, orders 90 - 158, with a resolution of 60,000.