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Old GRACES Exposure Time Estimation

Content owned by kchiboucas

This page describes the deprecated GRACES ITC written in IDL for reference only. The new ITC written in Python should now be used instead.

List of critical bugs identified in the IDL ITC script:

  • severely underestimated IQ signal losses
  • overestimated blue signal, underestimated red signal, and a missing red-most order in 1-fiber mode
  • incorrect signal distribution to individual pixels in the saturation test
  • no gain factored in for the saturation test
  • cubic spline interpolation occasionally produced negative S/N for certain templates
  • interactively-entered blackbody temperature was saved as an exponent value and caused the IDL script to crash
  • incorrect sky contribution to noise for the 2-fiber mode
  • pre-2017A read noise values

The GRACES Integration Time Calculator (ITC) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for all the available GRACES configurations and any kind of observing conditions.

The current ITC is an IDL script that can be downloaded HERE. To install it, simply untar the file (please choose the location wisely). Then, open IDL (assuming yu have a access to a licence) and get to the directory where the file ITCv1.4.tar was untarred. Then type:



The script will prompt you a series of questions before it produces the plot showing the S/N estimation. All the parameters (target parameters, observing conditions, etc.) can be given as inputs. For more details, type:

IDL> ITC,/help

You should also have the IDL Astronomy User's Library

The plot shows the signal-to-noise ratio as a function of wavelength per (physical) pixel. If you want to know the results for the optical resolution element, or for the interpolated pixels after spectra extraction, this is how to do it:
Signal-to-noise ratio per optical resolution element - The optical resolution element is 1.74 pix for the 1-fiber mode and 2.88 pix for the 2-fiber mode. Open the file .dat, and plot the signal-to-noise curve multiplied by the square-root of the size of the optical resolution element in pixels.
Signal-to-noise ratio per interpolated pixels - Once the spectrum is extracted, the steps between each wavelength is 0.63 pix. To get the signal-to-noise ratio for each of these interpolated pixel, you need to multiply the curve by the square-root of 0.63.

If you do not have access to an IDL licence, you can use the current sensitivity curves to deduce the estimated exposure time for your target. Finally, you can always contact the GRACES team by emailing for more extended help.

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

Old GRACES Exposure Time Estimation | Gemini Observatory


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