Introduction

Brad Holden of UCO/Lick Observatory has created a web-based exposure time calculator for HIRES that is recommended for stellar sources. Given the observer's slit width, CCD binning, exposure time, seeing conditions and airmass, the tool will generate the resulting S/N in the extracted spectrum as a function of wavelength. The observer can select from a number of different template sources (e.g. main sequence stars of various spectral types, galaxies, planetary nebulae, etc) to optimize the desired S/N estimate. The target is assumed to be a point source having a PSF FWHM as specified by the seeing parameter. This calculator can be found here:

Brad Holden's HIRES Exposure Time Calculator