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Observing Strategies

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The recommendations and performance numbers given in this section are based on observations done during the commissioning and System Verification in August and September 2002. For the Hamamatsu CCDs, which have a higher sensitivity in the red, the considerations below still apply, however the exposure times may be ~1/3 shorter.


  • Recommended Exposure Times: Choosing the best exposure time for Nod & Shuffle observations is highly dependent on the stability of the current cloud cover conditions. There is a trade-off because shorter exposure times, while yielding more accurate sky-subtraction also lead to significantly increased overheads. During Commissioning and System Verification for Nod & Shuffle it was determined that an exposure time of A = 60sec gave acceptably low sky-subtraction residuals for photometric or nearly photometric (CC = 70%) and relatively stable sky conditions. For highly variable sky conditions or heavier cloud cover (CC = 90% or Any) an exposure time of A = 60sec gave unacceptably high residuals and we recommend a shorter exposure time, e.g. A = 30sec. 

  • Sky subtraction and Nod & Shuffle -vs- Classic GMOS Spectroscopy: Nod & Shuffle gives the possibility of eliminating systematics from the sky subtraction, even in the presence of very strong sky lines. The advantage of Nod & Shuffle is largest in the red, due to the many strong sky lines. By eliminating the systematics from the sky subtraction, Nod & Shuffle makes it possible to combine spectroscopic observations to reach total exposure times of many tenths of hours without being affected by systematic sky residuals. It has been demonstrated that with total exposure times of 28 hours it is possible to determine redshifts from absorption lines of objects as faint as i'(AB)=24.7 mag. For exposure times of a few hours and fairly bright objects, typically i' brighter than 20-21 mag, the advantage of Nod & Shuffle is smaller. However, if you are concerned about getting high signal-to-noise spectra and are studying faint absorption features, you may still want to consider Nod & Shuffle. Example longslit spectra taken with Nod & Shuffle illustrate these points.

  • GMOS Integration Time Calculators The GMOS North and South ITCs do not account for systematic uncertainties in the subtraction of bright skylines, and as such is quite easily adapted for estimating realistic sensitivities using Nod & Shuffle. All one needs to do is to set the sky aperture to 1 times the target aperture in the Analysis Method under Details of Observation instead of using the default sky aperture of 5 times the target aperture.

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