From this page investigators may estimate the integration time and/or signal-to-noise for a spectroscopic observation for a selected telescope and instrument. These forms are Web interfaces to the IRAF package SPECTIME. (See the help page for details.)
This exposure time calculator is still being developed and compared to existing calibration data. We caution that many of the observing modes available at the telescope are not yet included in this simulator. Furthermore, some of the modes which are included have only been partially verified (e.g., in some cases extrapolations have had to be made). As modes are verified we will add them to the simulator. Questions or problems can be directed for now to firstname.lastname@example.org.
KPNO and CTIO also maintain other Web tools for helping investigators to determine spectroscopic needs and exposure times. See the KPNO tool at http://www.noao.edu/kpno/spectroscopy/spect.html for predicting a telescope/spectrograph combination based on input parameters. CTIO supports exposure time calculators for many of their optical spectrographs - see http://www.ctio.noao.edu/spectrographs/spectrographs.html. The Gemini Observatory maintains calculators for its facility instruments.