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Exposure time estimation and Overheads

There is no Integration Time Calculator (ITC) available for GRACES. A zero point will be given after commissioning in May 2014.

In the meantime, it is possible to roughly estimate the required exposure time following this procedure.


  1. Open the ESPaDOnS' exposure time calculator (EETC).
  2. Under Calculator options, Use the Olapa 1 amp detetor and select the other options at your own taste.
  3. Under Source model, enter the target's V magnitude and play with different values of effective temperature. The EETC assumes photometric conditions. To correct for worst conditions, please refer to Gemini observing constraints for could cover and modify the V magnitude accordingly. For instance, for CC70%, you must add 0.3 mag to the V magnitude.
  4. To guess the Observing conditions to use, please refer to Gemini observing constraints for image quality for the seeing. For the Sky brightness, the three choices of Moon phases can be considered as equivalent to sky backgrounds of 20%, 50/80% (depending on the distance to the moon you choose) and Any.
  5. Under Instrument configuration, choose object+sky or object only for the Observing Mode (see more details about spectroscopic modes here). To choose a CCD readout speed, please refer to the Detector characteristics page.
  6. Click on the Apply button. 
  7. The EETC's estimation has to be calibrated for GRACES. 

To level the EETC's result to GRACES forecasted performance, consider the following factors:

  • The forecasted relative performance (i.e. flux ratio) between ESPaDOnS and GRACES is given here.
  • The difference in spectral resolution should not make a big difference in the result, since both GRACES and ESPaDOnS are sharing the same grating and use image slicers.
  • Make sure to be on the safe side when estimating the total requested time. We would recommand at this point to overshoot the aimed S/N until we measure GRACES throughput.


When preparing a proposal using GRACES, PIs have to include Overheads in their estimation of the total requested time per target.