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Basic Concepts

The capability of GMOS on Gemini to perform Nod & Shuffle operations is not one of the observing modes for which the instrument was originally designed and developed. The capability was proposed, developed, implemented and tested as an enhancement to the existing GMOS-N instrument from late 2001 to September 2002 by the GMOS Nod & Shuffle Commissioning Team. Below we give a brief overview of the Nod & Shuffle concept - for more information please see "Microslit Nod-Shuffle Spectroscopy: A Technique for Achieving Very High Densities of Spectra", Karl Glazebrook and Joss Bland-Hawthorn, 2001, PASP, 113, 197.

 

Nod & Shuffle allows the accurate subtraction of night sky emission by frequently nodding the telescope pointing between an object position and a sky position while simultaneously shuffling the charge on the CCD detectors between science and storage (unilluminated) regions. This is similar to the common practice in near-infrared astronomy of nodding or beam-switching, with the difference that the detector is not read out after each nod as is the case for background-limited near-infrared observations which do not suffer from read noise. The resulting image produced by the Nod & Shuffle technique contains two spectra obtained quasi-simultaneously through each slit in the focal plane mask - one of the object and one of the sky. Although these spectra are stored in different regions of the CCD detector, they were obtained with exactly the same pixels through identical optical paths. The effects of pixel response (flat-field), fringing, irregularities in the slit, and temporal variations in the sky cancel out when one subtracts the sky spectrum from the object spectrum. For long exposures, one can realize a factor of 10 improvement in the systematic uncertainties associated with subtraction of bright sky lines, especially in the red (600-1000 nm) where such errors typically dominate over photon or read noise. There is a trade-off, however: the noise in sky-subtracted Nod & Shuffle spectra is higher (by up to a factor of sqrt(2)) compared to classically sky-subtracted spectra using a very long slit. Thus Nod & Shuffle is not for every GMOS spectroscopy program.

 

Because with Nod & Shuffle one no longer derives the sky from regions adjacent to the object, one can use significantly shorter slits than in classical MOS spectroscopy which typically uses 5-10 arcsec long slitlets. In the limit where the slits have the same length as the object size (or the seeing disk), the density of these "microslits" can be 5-10 times higher than in the normal MOS mode. This can be particularly advantageous when performing MOS spectroscopy of crowded fields. However, because part of the detector is used for charge storage and therefore can not be illuminated, one necessarily loses between half and two-thirds of the GMOS field of view with Nod & Shuffle, depending on the Nod & Shuffle layout and shuffling mode selected. A further disadvantage of Nod & Shuffle observing is the increased overheads, described elsewhere.

The spectral images produced by Nod & Shuffle are, naturally, different from those produced by classical MOS spectroscopy and the data reduction is also different. Additional tasks to handle the reduction of Nod & Shuffle spectral data are included in the current releases of Gemini IRAF scripts, so these should not be considered a disadvantage.