This remarkable GMOS capability applies techniques adopted for infrared-to-optical spectroscopy, whereby the sky is sampled with the same pixels used to observe the science target. The Nod & Shuffle (N&S) observation mode was not designed or developed as one of Gemini’s original capabilities. The concept was first invented, and implemented on the AAT, by Glazebrook & Bland-Hawthorn (2001). Soon after it was implemented on GMOS-N, by the GMOS Nod & Shuffle Commissioning Team.
The principal advantages of observing with N&S include:
♦ Improved sky subtraction
♦ Potential increased density of slits
However, Nod-and-Shuffle observations are prone to:
♦ Create increased observing overheads
♦ Potential decreased field of view
Nod & Shuffle is, at its core, a simultaneous:
♦ Frequent nodding of the telescope pointing between an object position and a sky position.
♦ Shuffling the charge on the CCD detectors between science (object target) and storage (unilluminated) regions.
Compared to common NIR (near-infrared) practices, N&S is similar to nodding or beam-switching. They differ in that the detector is not read out after each nod, unlike background-limited NIR observations which do not have significant read noise.
As a result, the image produced contains two spectra obtained quasi-simultaneously through each slit in the focal plane mask – one of the object and one of the sky. The spectra are stored separately in their respective regions of the CCD detectors, although they are obtained with exactly identical pixels and optical paths.
By subtracting the sky spectrum from the object spectrum, the effects of pixel response (flat-field), fringing, irregularities in the slit, and temporal variations in the sky are cancelled out. Due to this, a factor of ~ 10 in improvement is possible in systematic uncertainties (for long exposures), where the uncertainties associated with subtraction typically dominate over read noise.
This mode no longer requires sky derivation from regions adjacent to the object; as a result, significantly shorter slits can be used in contrast to classical MOS spectroscopy. The density of “microslits” can be up to 10 times higher, a particularly advantageous thing when performing spectroscopy of crowded fields (as is the case with GOGREEN).