|Resolving power (3.3 pixel)||60,000|
|Spectral coverage (fixed, complete)||390-680 nm|
|S/N ratio per resolution element||300 in 20 m at V=9|
|Radial velocity precision (initial)||100 m s-1|
|RV precision (with upgrades)||5 m s-1|
|Expected vsini precision||1 km s-1|
|Expected relative abundance precision
(excl. systematic errors)
FHiRE performance goals are summarized in the above table.
FHiRE is designed to produce high-quality spectra of relatively bright sources efficiently, to allow large sample sizes for surveys and statistical studies of particular groups of stars. Design goals include not only throughput, but stability and symmetry in line profiles.
A resolving power of R=60,000 (3.3 pixels) has been selected to resolve the thermal line widths of solar temperature and cooler stars. This resolution is optimal for most abundance and velocity studies, without paying the price of higher resolution in both throughput and observing time. High dispersion optical spectrographs in operation on moderate-to-large telescopes today are optimized for resolving power typically between 30,000 and 100,000. The upper end of this resolution range is useful for detailed studies of line profiles, including interstellar lines, while the lower end is commonly chosen for spectroscopy of faint sources near the limiting magnitude of a telescope, compromising the integrity of the spectrum to achieve better throughput. FHiRE is optimized for high dispersion survey programs, to provide sufficient resolution for precise radial velocities, projected rotational velocities, abundances, and variability studies.
The resolution of FHiRE will allow the derivation of relative abundances to better than 0.03 dex (decimal exponent), and the determination of projected rotational velocities of better than 1 km s-1. The precision of radial velocity measurements will depend on the degree of calibration, but with simple, standard calibrations, the precision will be better than 100 m s-1. A gas absorption cell will be incorporated into the design (but not implemented in the initial configuration), and will allow velocity accuracies of better than 5 m s-1 once implemented. The spectrograph will be designed with a simultaneous comparison source to monitor the position of the spectrum during exposures.
The spectral coverage will be fixed to be complete from shortward of the Ca II H and K lines at 390 nm to longward of the Li I doublet at 670 nm. This spectral range covers a significant range of astrophysical interest. This region is rich in diagnostic features in the stellar temperature range, including several Balmer lines, and numerous key Fraunhofer lines.