Introduction

The Nod-and-Shuffle spectroscopic observing mode was first brought to our attention by Karl Glazebrook who helped implement this mode on the LDSS++ spectrograph at the Anglo Australian Observatory (Glazebrook et al. 1998, AAO Newsletter, 84, 9; Glazebrook 1998, AAO Newsletter, 87, 11). The technique involves shuffling the charge on the CCD array (without reading it out) synchronized with nodding the telescope between object and sky in order to measure the sky nearly simultaneously with the object measurements, through the same slitlets and on the same pixels. Subtraction of the near-simultaneous sky spectra from the object spectra results in excellent sky and fringe subtraction (even through ragged slitlets), allowing for spectroscopy near the shot-noise limit of the sky. The penalty in this mode is two-fold: the noise in the sky is increased by $\sqrt{2}$ and the observing time overhead is larger by the factor of 2 due to the extra time spent observing the sky as well as by the time spent offsetting the telescope.

The nod-and-shuffle (hereafter N&S) mode necessitates a successful coupling between the CCD control software, the data acquisition software, and the telescope control software. In this document, we describe the modifications made to the CCD control software, ICE, and the telescope offset control software in order to implement the nod-and-shuffle mode at the Mayall and the tests carried out during the run. We then describe possible ways in which the overhead may be reduced and outline tasks that need to be carried out before our next T&E run. We conclude by briefly describing 3 operational modes that would greatly enhance the scientific capability of the Mayall.